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LIBPNG(3)                  Library Functions Manual                  LIBPNG(3)

NAME
       libpng - Portable Network Graphics (PNG) Reference Library 1.6.39

SYNOPSIS
       #include <png.h>

       png_uint_32 png_access_version_number (void);

       void png_benign_error (png_structp png_ptr, png_const_charp error);

       void png_build_grayscale_palette (int bit_depth, png_colorp palette);

       png_voidp png_calloc (png_structp png_ptr, png_alloc_size_t size);

       void  png_chunk_benign_error  (png_structp png_ptr, png_const_charp er-
       ror);

       void png_chunk_error (png_structp png_ptr, png_const_charp error);

       void png_chunk_warning (png_structp png_ptr, png_const_charp message);

       void png_convert_from_struct_tm  (png_timep  ptime,  struct  tm  FAR  *
       ttime);

       void png_convert_from_time_t (png_timep ptime, time_t ttime);

       png_charp   png_convert_to_rfc1123   (png_structp   png_ptr,  png_timep
       ptime);

       png_infop png_create_info_struct (png_structp png_ptr);

       png_structp   png_create_read_struct   (png_const_charp   user_png_ver,
       png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

       png_structp   png_create_read_struct_2  (png_const_charp  user_png_ver,
       png_voidp error_ptr,  png_error_ptr  error_fn,  png_error_ptr  warn_fn,
       png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

       png_structp   png_create_write_struct   (png_const_charp  user_png_ver,
       png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

       png_structp  png_create_write_struct_2  (png_const_charp  user_png_ver,
       png_voidp  error_ptr,  png_error_ptr  error_fn,  png_error_ptr warn_fn,
       png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

       void  png_data_freer  (png_structp  png_ptr,  png_infop  info_ptr,  int
       freer, png_uint_32 mask);

       void    png_destroy_info_struct    (png_structp   png_ptr,   png_infopp
       info_ptr_ptr);

       void  png_destroy_read_struct  (png_structpp  png_ptr_ptr,   png_infopp
       info_ptr_ptr, png_infopp end_info_ptr_ptr);

       void  png_destroy_write_struct  (png_structpp  png_ptr_ptr,  png_infopp
       info_ptr_ptr);

       void png_err (png_structp png_ptr);

       void png_error (png_structp png_ptr, png_const_charp error);

       void png_free (png_structp png_ptr, png_voidp ptr);

       void png_free_chunk_list (png_structp png_ptr);

       void png_free_default (png_structp png_ptr, png_voidp ptr);

       void png_free_data (png_structp png_ptr, png_infop info_ptr, int num);

       png_byte png_get_bit_depth (png_const_structp png_ptr,  png_const_infop
       info_ptr);

       png_uint_32    png_get_bKGD   (png_const_structp   png_ptr,   png_infop
       info_ptr, png_color_16p *background);

       png_byte png_get_channels (png_const_structp  png_ptr,  png_const_infop
       info_ptr);

       png_uint_32  png_get_cHRM  (png_const_structp  png_ptr, png_const_infop
       info_ptr, double  *white_x,  double  *white_y,  double  *red_x,  double
       *red_y,  double  *green_x,  double  *green_y,  double  *blue_x,  double
       *blue_y);

       png_uint_32     png_get_cHRM_fixed     (png_const_structp      png_ptr,
       png_const_infop  info_ptr,  png_uint_32 *white_x, png_uint_32 *white_y,
       png_uint_32   *red_x,   png_uint_32   *red_y,   png_uint_32   *green_x,
       png_uint_32 *green_y, png_uint_32 *blue_x, png_uint_32 *blue_y);

       png_uint_32   png_get_cHRM_XYZ  (png_structp  png_ptr,  png_const_infop
       info_ptr, double *red_X, double *red_Y, double *red_Z, double *green_X,
       double  *green_Y, double *green_Z, double *blue_X, double *blue_Y, dou-
       ble *blue_Z);

       png_uint_32 png_get_cHRM_XYZ_fixed (png_structp png_ptr,  png_const_in-
       fop  info_ptr,  png_fixed_point *int_red_X, png_fixed_point *int_red_Y,
       png_fixed_point     *int_red_Z,      png_fixed_point      *int_green_X,
       png_fixed_point     *int_green_Y,     png_fixed_point     *int_green_Z,
       png_fixed_point     *int_blue_X,      png_fixed_point      *int_blue_Y,
       png_fixed_point *int_blue_Z);

       png_uint_32 png_get_chunk_cache_max (png_const_structp png_ptr);

       png_alloc_size_t png_get_chunk_malloc_max (png_const_structp png_ptr);

       png_byte png_get_color_type (png_const_structp png_ptr, png_const_infop
       info_ptr);

       png_uint_32     png_get_compression_buffer_size      (png_const_structp
       png_ptr);

       png_byte     png_get_compression_type    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_byte png_get_copyright (png_const_structp png_ptr);

       png_uint_32 png_get_current_row_number (png_const_structp);

       png_byte png_get_current_pass_number (png_const_structp);

       png_voidp png_get_error_ptr (png_const_structp png_ptr);

       png_byte png_get_filter_type (png_const_structp png_ptr,  png_const_in-
       fop info_ptr);

       png_uint_32  png_get_gAMA  (png_const_structp  png_ptr, png_const_infop
       info_ptr, double *file_gamma);

       png_uint_32     png_get_gAMA_fixed     (png_const_structp      png_ptr,
       png_const_infop info_ptr, png_uint_32 *int_file_gamma);

       png_byte png_get_header_ver (png_const_structp png_ptr);

       png_byte png_get_header_version (png_const_structp png_ptr);

       png_uint_32  png_get_eXIf  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_bytep *exif);

       png_uint_32 png_get_eXIf_1 (png_const_structp png_ptr,  png_const_infop
       info_ptr, png_unit_32 *num_exif, png_bytep *exif);

       png_uint_32  png_get_hIST  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_uint_16p *hist);

       png_uint_32 png_get_iCCP  (png_const_structp  png_ptr,  png_const_infop
       info_ptr,  png_charpp  name, int *compression_type, png_bytepp profile,
       png_uint_32 *proflen);

       png_uint_32  png_get_IHDR  (png_structp  png_ptr,  png_infop  info_ptr,
       png_uint_32   *width,   png_uint_32   *height,   int   *bit_depth,  int
       *color_type, int  *interlace_type,  int  *compression_type,  int  *fil-
       ter_type);

       png_uint_32     png_get_image_height     (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32     png_get_image_width     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_int_32 png_get_int_32 (png_bytep buf);

       png_byte     png_get_interlace_type     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_uint_32 png_get_io_chunk_type (png_const_structp png_ptr);

       png_voidp png_get_io_ptr (png_structp png_ptr);

       png_uint_32 png_get_io_state (png_structp png_ptr);

       png_byte png_get_libpng_ver (png_const_structp png_ptr);

       int  png_get_palette_max(png_const_structp   png_ptr,   png_const_infop
       info_ptr);

       png_voidp png_get_mem_ptr (png_const_structp png_ptr);

       png_uint_32  png_get_oFFs  (png_const_structp  png_ptr, png_const_infop
       info_ptr,   png_uint_32   *offset_x,   png_uint_32    *offset_y,    int
       *unit_type);

       png_uint_32  png_get_pCAL  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_charp  *purpose,  png_int_32  *X0,  png_int_32  *X1,  int
       *type, int *nparams, png_charp *units, png_charpp *params);

       png_uint_32  png_get_pHYs  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int *unit_type);

       float    png_get_pixel_aspect_ratio     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_pHYs_dpi (png_const_structp png_ptr, png_const_in-
       fop info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int *unit_type);

       png_fixed_point   png_get_pixel_aspect_ratio_fixed   (png_const_structp
       png_ptr, png_const_infop info_ptr);

       png_uint_32    png_get_pixels_per_inch    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_pixels_per_meter   (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_voidp png_get_progressive_ptr (png_const_structp png_ptr);

       png_uint_32  png_get_PLTE  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_colorp *palette, int *num_palette);

       png_byte png_get_rgb_to_gray_status (png_const_structp png_ptr);

       png_uint_32 png_get_rowbytes (png_const_structp png_ptr,  png_const_in-
       fop info_ptr);

       png_bytepp  png_get_rows  (png_const_structp  png_ptr,  png_const_infop
       info_ptr);

       png_uint_32   png_get_sBIT   (png_const_structp   png_ptr,    png_infop
       info_ptr, png_color_8p *sig_bit);

       void png_get_sCAL (png_const_structp png_ptr, png_const_infop info_ptr,
       int* unit, double* width, double* height);

       void  png_get_sCAL_fixed  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, int* unit, png_fixed_pointp width, png_fixed_pointp height);

       void   png_get_sCAL_s   (png_const_structp   png_ptr,   png_const_infop
       info_ptr, int* unit, png_charpp width, png_charpp height);

       png_bytep  png_get_signature  (png_const_structp   png_ptr,   png_infop
       info_ptr);

       png_uint_32  png_get_sPLT  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_spalette_p *splt_ptr);

       png_uint_32 png_get_sRGB  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, int *file_srgb_intent);

       png_uint_32  png_get_text  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_textp *text_ptr, int *num_text);

       png_uint_32   png_get_tIME   (png_const_structp   png_ptr,    png_infop
       info_ptr, png_timep *mod_time);

       png_uint_32    png_get_tRNS   (png_const_structp   png_ptr,   png_infop
       info_ptr,  png_bytep  *trans_alpha,   int   *num_trans,   png_color_16p
       *trans_color);

       /* This function is really an inline macro. */

       png_uint_16 png_get_uint_16 (png_bytep buf);

       png_uint_32 png_get_uint_31 (png_structp png_ptr, png_bytep buf);

       /* This function is really an inline macro. */

       png_uint_32 png_get_uint_32 (png_bytep buf);

       png_uint_32    png_get_unknown_chunks    (png_const_structp    png_ptr,
       png_const_infop info_ptr, png_unknown_chunkpp unknowns);

       png_voidp png_get_user_chunk_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_user_height_max (png_const_structp png_ptr);

       png_voidp png_get_user_transform_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_user_width_max (png_const_structp png_ptr);

       png_uint_32 png_get_valid (png_const_structp  png_ptr,  png_const_infop
       info_ptr, png_uint_32 flag);

       float png_get_x_offset_inches (png_const_structp png_ptr, png_const_in-
       fop info_ptr);

       png_fixed_point  png_get_x_offset_inches_fixed  (png_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_x_offset_microns    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_x_offset_pixels    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_x_pixels_per_inch   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_x_pixels_per_meter   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       float png_get_y_offset_inches (png_const_structp png_ptr, png_const_in-
       fop info_ptr);

       png_fixed_point  png_get_y_offset_inches_fixed  (png_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_y_offset_microns    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_y_offset_pixels    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_y_pixels_per_inch   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_y_pixels_per_meter   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

       int   png_image_begin_read_from_file   (png_imagep  image,  const  char
       *file_name);

       int png_image_begin_read_from_stdio (png_imagep image, FILE* file);

       int,      png_image_begin_read_from_memory      (png_imagep      image,
       png_const_voidp memory, size_t size);

       int  png_image_finish_read  (png_imagep  image,  png_colorp background,
       void *buffer, png_int_32 row_stride, void *colormap);

       void png_image_free (png_imagep image);

       int png_image_write_to_file (png_imagep image, const  char  *file,  int
       convert_to_8bit,  const void *buffer, png_int_32 row_stride, void *col-
       ormap);

       int png_image_write_to_memory (png_imagep image, void *memory,  png_al-
       loc_size_t  *  PNG_RESTRICT  memory_bytes,  int convert_to_8_bit, const
       void *buffer, png_int_32 row_stride, const void *colormap);

       int png_image_write_to_stdio (png_imagep image, FILE  *file,  int  con-
       vert_to_8_bit,  const  void  *buffer, png_int_32 row_stride, void *col-
       ormap);

       void      png_info_init_3      (png_infopp       info_ptr,       size_t
       png_info_struct_size);

       void png_init_io (png_structp png_ptr, FILE *fp);

       void png_longjmp (png_structp png_ptr, int val);

       png_voidp png_malloc (png_structp png_ptr, png_alloc_size_t size);

       png_voidp  png_malloc_default  (png_structp  png_ptr,  png_alloc_size_t
       size);

       png_voidp png_malloc_warn (png_structp png_ptr, png_alloc_size_t size);

       png_uint_32 png_permit_mng_features (png_structp  png_ptr,  png_uint_32
       mng_features_permitted);

       void   png_process_data   (png_structp   png_ptr,  png_infop  info_ptr,
       png_bytep buffer, size_t buffer_size);

       size_t png_process_data_pause (png_structp png_ptr, int save);

       png_uint_32 png_process_data_skip (png_structp png_ptr);

       void  png_progressive_combine_row   (png_structp   png_ptr,   png_bytep
       old_row, png_bytep new_row);

       void png_read_end (png_structp png_ptr, png_infop info_ptr);

       void png_read_image (png_structp png_ptr, png_bytepp image);

       void png_read_info (png_structp png_ptr, png_infop info_ptr);

       void  png_read_png (png_structp png_ptr, png_infop info_ptr, int trans-
       forms, png_voidp params);

       void png_read_row (png_structp png_ptr, png_bytep row,  png_bytep  dis-
       play_row);

       void  png_read_rows  (png_structp  png_ptr,  png_bytepp row, png_bytepp
       display_row, png_uint_32 num_rows);

       void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

       int png_reset_zstream (png_structp png_ptr);

       void png_save_int_32 (png_bytep buf, png_int_32 i);

       void png_save_uint_16 (png_bytep buf, unsigned int i);

       void png_save_uint_32 (png_bytep buf, png_uint_32 i);

       void png_set_add_alpha (png_structp png_ptr,  png_uint_32  filler,  int
       flags);

       void  png_set_alpha_mode  (png_structp  png_ptr,  int mode, double out-
       put_gamma);

       void   png_set_alpha_mode_fixed   (png_structp   png_ptr,   int   mode,
       png_fixed_point output_gamma);

       void   png_set_background  (png_structp  png_ptr,  png_color_16p  back-
       ground_color, int background_gamma_code, int need_expand, double  back-
       ground_gamma);

       void png_set_background_fixed (png_structp png_ptr, png_color_16p back-
       ground_color, int background_gamma_code, int  need_expand,  png_uint_32
       background_gamma);

       void png_set_benign_errors (png_structp png_ptr, int allowed);

       void png_set_bgr (png_structp png_ptr);

       void    png_set_bKGD    (png_structp   png_ptr,   png_infop   info_ptr,
       png_color_16p background);

       void png_set_check_for_invalid_index  (png_structrp  png_ptr,  int  al-
       lowed);

       void  png_set_cHRM  (png_structp  png_ptr,  png_infop  info_ptr, double
       white_x, double white_y, double red_x, double  red_y,  double  green_x,
       double green_y, double blue_x, double blue_y);

       void   png_set_cHRM_fixed  (png_structp  png_ptr,  png_infop  info_ptr,
       png_uint_32   white_x,   png_uint_32   white_y,   png_uint_32    red_x,
       png_uint_32    red_y,   png_uint_32   green_x,   png_uint_32   green_y,
       png_uint_32 blue_x, png_uint_32 blue_y);

       void png_set_cHRM_XYZ (png_structp png_ptr, png_infop info_ptr,  double
       red_X, double red_Y, double red_Z, double green_X, double green_Y, dou-
       ble green_Z, double blue_X, double blue_Y, double blue_Z);

       void png_set_cHRM_XYZ_fixed (png_structp png_ptr,  png_infop  info_ptr,
       png_fixed_point  int_red_X,  png_fixed_point int_red_Y, png_fixed_point
       int_red_Z, png_fixed_point  int_green_X,  png_fixed_point  int_green_Y,
       png_fixed_point      int_green_Z,      png_fixed_point      int_blue_X,
       png_fixed_point int_blue_Y, png_fixed_point int_blue_Z);

       void   png_set_chunk_cache_max   (png_structp   png_ptr,    png_uint_32
       user_chunk_cache_max);

       void png_set_compression_level (png_structp png_ptr, int level);

       void    png_set_compression_mem_level    (png_structp    png_ptr,   int
       mem_level);

       void png_set_compression_method (png_structp png_ptr, int method);

       void png_set_compression_strategy (png_structp png_ptr, int strategy);

       void png_set_compression_window_bits  (png_structp  png_ptr,  int  win-
       dow_bits);

       void  png_set_crc_action (png_structp png_ptr, int crit_action, int an-
       cil_action);

       void  png_set_error_fn  (png_structp  png_ptr,   png_voidp   error_ptr,
       png_error_ptr error_fn, png_error_ptr warning_fn);

       void png_set_expand (png_structp png_ptr);

       void png_set_expand_16 (png_structp png_ptr);

       void png_set_expand_gray_1_2_4_to_8 (png_structp png_ptr);

       void  png_set_filler  (png_structp  png_ptr,  png_uint_32  filler,  int
       flags);

       void png_set_filter (png_structp png_ptr, int method, int filters);

       void  png_set_filter_heuristics  (png_structp  png_ptr,   int   heuris-
       tic_method,  int  num_weights,  png_doublep filter_weights, png_doublep
       filter_costs);

       void png_set_filter_heuristics_fixed (png_structp png_ptr, int  heuris-
       tic_method,    int   num_weights,   png_fixed_point_p   filter_weights,
       png_fixed_point_p filter_costs);

       void png_set_flush (png_structp png_ptr, int nrows);

       void png_set_gamma (png_structp png_ptr,  double  screen_gamma,  double
       default_file_gamma);

       void     png_set_gamma_fixed    (png_structp    png_ptr,    png_uint_32
       screen_gamma, png_uint_32 default_file_gamma);

       void png_set_gAMA  (png_structp  png_ptr,  png_infop  info_ptr,  double
       file_gamma);

       void   png_set_gAMA_fixed  (png_structp  png_ptr,  png_infop  info_ptr,
       png_uint_32 file_gamma);

       void png_set_gray_1_2_4_to_8 (png_structp png_ptr);

       void png_set_gray_to_rgb (png_structp png_ptr);

       void png_set_eXIf (png_structp png_ptr, png_infop  info_ptr,  png_bytep
       exif);

       void   png_set_eXIf_1   (png_structp   png_ptr,   png_infop   info_ptr,
       png_uint_32 num_exif, png_bytep exif);

       void   png_set_hIST   (png_structp   png_ptr,    png_infop    info_ptr,
       png_uint_16p hist);

       void    png_set_iCCP    (png_structp   png_ptr,   png_infop   info_ptr,
       png_const_charp name, int  compression_type,  png_const_bytep  profile,
       png_uint_32 proflen);

       int png_set_interlace_handling (png_structp png_ptr);

       void  png_set_invalid  (png_structp  png_ptr,  png_infop  info_ptr, int
       mask);

       void png_set_invert_alpha (png_structp png_ptr);

       void png_set_invert_mono (png_structp png_ptr);

       void png_set_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32
       width,  png_uint_32  height,  int bit_depth, int color_type, int inter-
       lace_type, int compression_type, int filter_type);

       void  png_set_keep_unknown_chunks  (png_structp  png_ptr,   int   keep,
       png_bytep chunk_list, int num_chunks);

       jmp_buf*   png_set_longjmp_fn   (png_structp  png_ptr,  png_longjmp_ptr
       longjmp_fn, size_t jmp_buf_size);

       void png_set_chunk_malloc_max  (png_structp  png_ptr,  png_alloc_size_t
       user_chunk_cache_max);

       void  png_set_compression_buffer_size (png_structp png_ptr, png_uint_32
       size);

       void png_set_mem_fn (png_structp png_ptr, png_voidp  mem_ptr,  png_mal-
       loc_ptr malloc_fn, png_free_ptr free_fn);

       void png_set_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32
       offset_x, png_uint_32 offset_y, int unit_type);

       int png_set_option(png_structrp png_ptr, int option, int onoff);

       void png_set_packing (png_structp png_ptr);

       void png_set_packswap (png_structp png_ptr);

       void png_set_palette_to_rgb (png_structp png_ptr);

       void png_set_pCAL (png_structp png_ptr, png_infop  info_ptr,  png_charp
       purpose, png_int_32 X0, png_int_32 X1, int type, int nparams, png_charp
       units, png_charpp params);

       void png_set_pHYs (png_structp png_ptr, png_infop info_ptr, png_uint_32
       res_x, png_uint_32 res_y, int unit_type);

       void  png_set_progressive_read_fn  (png_structp png_ptr, png_voidp pro-
       gressive_ptr, png_progressive_info_ptr info_fn, png_progressive_row_ptr
       row_fn, png_progressive_end_ptr end_fn);

       void  png_set_PLTE (png_structp png_ptr, png_infop info_ptr, png_colorp
       palette, int num_palette);

       void png_set_quantize (png_structp  png_ptr,  png_colorp  palette,  int
       num_palette, int maximum_colors, png_uint_16p histogram, int full_quan-
       tize);

       void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr
       read_data_fn);

       void  png_set_read_status_fn  (png_structp png_ptr, png_read_status_ptr
       read_row_fn);

       void   png_set_read_user_chunk_fn   (png_structp   png_ptr,   png_voidp
       user_chunk_ptr, png_user_chunk_ptr read_user_chunk_fn);

       void      png_set_read_user_transform_fn      (png_structp     png_ptr,
       png_user_transform_ptr read_user_transform_fn);

       void png_set_rgb_to_gray (png_structp png_ptr, int error_action, double
       red, double green);

       void  png_set_rgb_to_gray_fixed  (png_structp png_ptr, int error_action
       png_uint_32 red, png_uint_32 green);

       void png_set_rows (png_structp png_ptr, png_infop info_ptr,  png_bytepp
       row_pointers);

       void    png_set_sBIT    (png_structp   png_ptr,   png_infop   info_ptr,
       png_color_8p sig_bit);

       void png_set_sCAL (png_structp png_ptr, png_infop info_ptr,  int  unit,
       double width, double height);

       void  png_set_sCAL_fixed  (png_structp png_ptr, png_infop info_ptr, int
       unit, png_fixed_point width, png_fixed_point height);

       void png_set_sCAL_s (png_structp png_ptr, png_infop info_ptr, int unit,
       png_charp width, png_charp height);

       void png_set_scale_16 (png_structp png_ptr);

       void png_set_shift (png_structp png_ptr, png_color_8p true_bits);

       void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

       void    png_set_sPLT    (png_structp   png_ptr,   png_infop   info_ptr,
       png_spalette_p splt_ptr, int num_spalettes);

       void  png_set_sRGB  (png_structp  png_ptr,  png_infop   info_ptr,   int
       srgb_intent);

       void   png_set_sRGB_gAMA_and_cHRM   (png_structp   png_ptr,   png_infop
       info_ptr, int srgb_intent);

       void png_set_strip_16 (png_structp png_ptr);

       void png_set_strip_alpha (png_structp png_ptr);

       void  png_set_strip_error_numbers  (png_structp  png_ptr,   png_uint_32
       strip_mode);

       void png_set_swap (png_structp png_ptr);

       void png_set_swap_alpha (png_structp png_ptr);

       void  png_set_text  (png_structp png_ptr, png_infop info_ptr, png_textp
       text_ptr, int num_text);

       void png_set_text_compression_level (png_structp png_ptr, int level);

       void  png_set_text_compression_mem_level  (png_structp   png_ptr,   int
       mem_level);

       void png_set_text_compression_strategy (png_structp png_ptr, int strat-
       egy);

       void  png_set_text_compression_window_bits  (png_structp  png_ptr,  int
       window_bits);

       void png_set_text_compression_method (png_structp png_ptr, int method);

       void  png_set_tIME  (png_structp png_ptr, png_infop info_ptr, png_timep
       mod_time);

       void png_set_tRNS (png_structp png_ptr, png_infop  info_ptr,  png_bytep
       trans_alpha, int num_trans, png_color_16p trans_color);

       void png_set_tRNS_to_alpha (png_structp png_ptr);

       png_uint_32   png_set_unknown_chunks  (png_structp  png_ptr,  png_infop
       info_ptr, png_unknown_chunkp unknowns, int num, int location);

       void  png_set_unknown_chunk_location  (png_structp  png_ptr,  png_infop
       info_ptr, int chunk, int location);

       void     png_set_user_limits    (png_structp    png_ptr,    png_uint_32
       user_width_max, png_uint_32 user_height_max);

       void  png_set_user_transform_info   (png_structp   png_ptr,   png_voidp
       user_transform_ptr,  int user_transform_depth, int user_transform_chan-
       nels);

       void   png_set_write_fn   (png_structp   png_ptr,   png_voidp   io_ptr,
       png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);

       void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
       write_row_fn);

       void     png_set_write_user_transform_fn     (png_structp      png_ptr,
       png_user_transform_ptr write_user_transform_fn);

       int png_sig_cmp (png_bytep sig, size_t start, size_t num_to_check);

       void png_start_read_image (png_structp png_ptr);

       void png_warning (png_structp png_ptr, png_const_charp message);

       void   png_write_chunk   (png_structp  png_ptr,  png_bytep  chunk_name,
       png_bytep data, size_t length);

       void png_write_chunk_data (png_structp png_ptr, png_bytep data,  size_t
       length);

       void png_write_chunk_end (png_structp png_ptr);

       void  png_write_chunk_start (png_structp png_ptr, png_bytep chunk_name,
       png_uint_32 length);

       void png_write_end (png_structp png_ptr, png_infop info_ptr);

       void png_write_flush (png_structp png_ptr);

       void png_write_image (png_structp png_ptr, png_bytepp image);

       void png_write_info (png_structp png_ptr, png_infop info_ptr);

       void   png_write_info_before_PLTE   (png_structp   png_ptr,   png_infop
       info_ptr);

       void png_write_png (png_structp png_ptr, png_infop info_ptr, int trans-
       forms, png_voidp params);

       void png_write_row (png_structp png_ptr, png_bytep row);

       void png_write_rows (png_structp png_ptr, png_bytepp  row,  png_uint_32
       num_rows);

       void png_write_sig (png_structp png_ptr);

DESCRIPTION
       The  libpng  library supports encoding, decoding, and various manipula-
       tions of the Portable Network Graphics (PNG) format  image  files.   It
       uses  the  zlib(3)  compression  library.   Following  is a copy of the
       libpng-manual.txt file that accompanies libpng.

LIBPNG.TXT
       libpng-manual.txt - A description on how to use and modify libpng

        Copyright (c) 2018-2022 Cosmin Truta
        Copyright (c) 1998-2018 Glenn Randers-Pehrson

        This document is released under the libpng license.
        For conditions of distribution and use, see the disclaimer
        and license in png.h

        Based on:

        libpng version 1.6.36, December 2018, through 1.6.39 - November 2022
        Updated and distributed by Cosmin Truta
        Copyright (c) 2018-2022 Cosmin Truta

        libpng versions 0.97, January 1998, through 1.6.35 - July 2018
        Updated and distributed by Glenn Randers-Pehrson
        Copyright (c) 1998-2018 Glenn Randers-Pehrson

        libpng 1.0 beta 6 - version 0.96 - May 28, 1997
        Updated and distributed by Andreas Dilger
        Copyright (c) 1996, 1997 Andreas Dilger

        libpng 1.0 beta 2 - version 0.88 - January 26, 1996
        For conditions of distribution and use, see copyright
        notice in png.h. Copyright (c) 1995, 1996 Guy Eric
        Schalnat, Group 42, Inc.

        Updated/rewritten per request in the libpng FAQ
        Copyright (c) 1995, 1996 Frank J. T. Wojcik
        December 18, 1995 & January 20, 1996

        TABLE OF CONTENTS

           I. Introduction
          II. Structures
         III. Reading
          IV. Writing
           V. Simplified API
          VI. Modifying/Customizing libpng
         VII. MNG support
        VIII. Changes to Libpng from version 0.88
          IX. Changes to Libpng from version 1.0.x to 1.2.x
           X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
          XI. Changes to Libpng from version 1.4.x to 1.5.x
         XII. Changes to Libpng from version 1.5.x to 1.6.x
        XIII. Detecting libpng
         XIV. Source code repository
          XV. Coding style

I. Introduction
       This file describes how to use and modify  the  PNG  reference  library
       (known  as  libpng)  for your own use.  In addition to this file, exam-
       ple.c is a good starting point for using the library, as it is  heavily
       commented  and should include everything most people will need.  We as-
       sume that libpng is already installed; see the  INSTALL  file  for  in-
       structions on how to configure and install libpng.

       For  examples  of libpng usage, see the files "example.c", "pngtest.c",
       and the files in the "contrib" directory, all of which are included  in
       the libpng distribution.

       Libpng was written as a companion to the PNG specification, as a way of
       reducing the amount of time and effort it takes to support the PNG file
       format in application programs.

       The  PNG specification (second edition), November 2003, is available as
       a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at
       <https://www.w3.org/TR/2003/REC-PNG-20031110/>.   The W3C and ISO docu-
       ments have identical technical content.

       The PNG-1.2  specification  is  available  at  <https://png-mng.source-
       forge.io/pub/png/spec/1.2/>.   It  is technically equivalent to the PNG
       specification (second edition) but has some additional material.

       The PNG-1.0 specification is available as  RFC  2083  at  <https://png-
       mng.sourceforge.io/pub/png/spec/1.0/>  and  as  a W3C Recommendation at
       <https://www.w3.org/TR/REC-png-961001>.

       Some additional chunks are  described  in  the  special-purpose  public
       chunks documents at <http://www.libpng.org/pub/png/spec/register/>

       Other  information  about PNG, and the latest version of libpng, can be
       found at the PNG home page, <http://www.libpng.org/pub/png/>.

       Most users will not have to modify the library significantly;  advanced
       users may want to modify it more.  All attempts were made to make it as
       complete as possible, while keeping the code easy to understand.   Cur-
       rently,  this  library only supports C.  Support for other languages is
       being considered.

       Libpng has been designed to handle multiple sessions at one time, to be
       easily  modifiable,  to  be  portable  to the vast majority of machines
       (ANSI, K&R, 16-, 32-, and 64-bit) available, and to  be  easy  to  use.
       The  ultimate  goal  of  libpng is to promote the acceptance of the PNG
       file format in whatever way possible.  While there is still work to  be
       done (see the TODO file), libpng should cover the majority of the needs
       of its users.

       Libpng uses zlib for its compression and decompression  of  PNG  files.
       Further  information about zlib, and the latest version of zlib, can be
       found at the zlib home page, <https://zlib.net/>.  The zlib compression
       utility  is  a general purpose utility that is useful for more than PNG
       files, and can be used without libpng.  See the documentation delivered
       with  zlib for more details.  You can usually find the source files for
       the zlib utility wherever you find the libpng source files.

       Libpng is thread safe, provided the threads  are  using  different  in-
       stances  of the structures.  Each thread should have its own png_struct
       and png_info instances, and thus its own image.  Libpng does  not  pro-
       tect itself against two threads using the same instance of a structure.

II. Structures
       There  are two main structures that are important to libpng, png_struct
       and png_info.  Both are internal structures that are no longer  exposed
       in the libpng interface (as of libpng 1.5.0).

       The png_info structure is designed to provide information about the PNG
       file.  At one time, the fields of png_info were intended to be directly
       accessible  to  the  user.  However, this tended to cause problems with
       applications using dynamically loaded libraries, and as a result a  set
       of  interface  functions  for png_info (the png_get_*() and png_set_*()
       functions) was developed, and direct access to the png_info fields  was
       deprecated..

       The  png_struct structure is the object used by the library to decode a
       single image.  As of 1.5.0 this structure is also not exposed.

       Almost all libpng APIs require a pointer to a png_struct as  the  first
       argument.   Many  (in particular the png_set and png_get APIs) also re-
       quire a pointer to png_info as the second argument.   Some  application
       visible macros defined in png.h designed for basic data access (reading
       and writing integers in the PNG format) don't take a png_info  pointer,
       but  it's  almost  always safe to assume that a (png_struct*) has to be
       passed to call an API function.

       You can have more than one png_info structure associated with an image,
       as  illustrated  in  pngtest.c,  one for information valid prior to the
       IDAT chunks and another (called  "end_info"  below)  for  things  after
       them.

       The  png.h  header file is an invaluable reference for programming with
       libpng.  And while I'm on the topic, make sure you include  the  libpng
       header file:

       #include <png.h>

       and also (as of libpng-1.5.0) the zlib header file, if you need it:

       #include <zlib.h>

   Types
       The  png.h  header  file defines a number of integral types used by the
       APIs.  Most of these are fairly obvious; for example types  correspond-
       ing to integers of particular sizes and types for passing color values.

       One exception is how non-integral numbers are handled.  For application
       convenience most APIs that take such numbers have C (double) arguments;
       however, internally PNG, and libpng, use 32 bit signed integers and en-
       code the value by multiplying by 100,000.  As of libpng 1.5.0 a  conve-
       nience   macro   PNG_FP_1  is  defined  in  png.h  along  with  a  type
       (png_fixed_point) which is simply (png_int_32).

       All APIs that take (double) arguments also have  a  matching  API  that
       takes the corresponding fixed point integer arguments.  The fixed point
       API has the same name as the floating point one with "_fixed" appended.
       The  actual  range  of  values permitted in the APIs is frequently less
       than the full range of (png_fixed_point) (-21474 to +21474).  When APIs
       require  a  non-negative  argument  the type is recorded as png_uint_32
       above.  Consult the header file and the text below  for  more  informa-
       tion.

       Special  care  must  be take with sCAL chunk handling because the chunk
       itself uses non-integral values encoded as strings  containing  decimal
       floating point numbers.  See the comments in the header file.

   Configuration
       The  main header file function declarations are frequently protected by
       C preprocessing directives of the form:

           #ifdef PNG_feature_SUPPORTED
           declare-function
           #endif
           ...
           #ifdef PNG_feature_SUPPORTED
           use-function
           #endif

       The library can be built without support for  these  APIs,  although  a
       standard  build  will  have all implemented APIs.  Application programs
       should check the feature macros before using an API for maximum  porta-
       bility.   From  libpng 1.5.0 the feature macros set during the build of
       libpng are recorded in the header file "pnglibconf.h" and this file  is
       always included by png.h.

       If you don't need to change the library configuration from the default,
       skip to the next section ("Reading").

       Notice that some of the makefiles in the 'scripts'  directory  and  (in
       1.5.0)  all of the build project files in the 'projects' directory sim-
       ply copy scripts/pnglibconf.h.prebuilt  to  pnglibconf.h.   This  means
       that  these  build systems do not permit easy auto-configuration of the
       library - they only support the default configuration.

       The easiest way to make minor changes to the libpng configuration  when
       auto-configuration  is  supported  is to add definitions to the command
       line using (typically) CPPFLAGS.  For example:

       CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC

       will change the internal libpng math implementation for  gamma  correc-
       tion  and  other  arithmetic  calculations to fixed point, avoiding the
       need for fast floating point support.  The result can be  seen  in  the
       generated  pnglibconf.h  -  make  sure  it contains the changed feature
       macro setting.

       If you need to make more extensive configuration changes  -  more  than
       one  or two feature macro settings - you can either add -DPNG_USER_CON-
       FIG to the build command line and put a list of feature macro  settings
       in  pngusr.h  or  you  can set DFA_XTRA (a makefile variable) to a file
       containing the same information in the form of 'option' settings.

       A. Changing pnglibconf.h

       A variety of methods exist to build libpng.  Not all of  these  support
       reconfiguration  of  pnglibconf.h.  To reconfigure pnglibconf.h it must
       either be rebuilt from scripts/pnglibconf.dfa using awk or it  must  be
       edited by hand.

       Hand  editing  is  achieved by copying scripts/pnglibconf.h.prebuilt to
       pnglibconf.h and changing the lines defining  the  supported  features,
       paying   very   close   attention   to   the  'option'  information  in
       scripts/pnglibconf.dfa that describes those features and their require-
       ments.  This is easy to get wrong.

       B. Configuration using DFA_XTRA

       Rebuilding  from  pnglibconf.dfa  is  easy if a functioning 'awk', or a
       later variant such as 'nawk' or 'gawk', is  available.   The  configure
       build  will  automatically  find  an  appropriate awk and build pnglib-
       conf.h.  The scripts/pnglibconf.mak file contains a set of  make  rules
       for  doing  the  same  thing  if configure is not used, and many of the
       makefiles in the scripts directory use this approach.

       When rebuilding simply write a new file containing changed options  and
       set DFA_XTRA to the name of this file.  This causes the build to append
       the new file to the end of scripts/pnglibconf.dfa.  The pngusr.dfa file
       should contain lines of the following forms:

       everything = off

       This  turns  all optional features off.  Include it at the start of pn-
       gusr.dfa to make it easier to build a minimal configuration.  You  will
       need to turn at least some features on afterward to enable either read-
       ing or writing code, or both.

       option feature on option feature off

       Enable or disable a single feature.   This  will  automatically  enable
       other features required by a feature that is turned on or disable other
       features that require a feature which is turned off.  Conflicting  set-
       tings will cause an error message to be emitted by awk.

       setting feature default value

       Changes the default value of setting 'feature' to 'value'.  There are a
       small number of settings listed at the top of  pnglibconf.h,  they  are
       documented  in  the source code.  Most of these values have performance
       implications for the library but most of them have no visible effect on
       the API.  Some can also be overridden from the API.

       This  method  of  building  a customized pnglibconf.h is illustrated in
       contrib/pngminim/*.  See the "$(PNGCONF):" target in the  makefile  and
       pngusr.dfa in these directories.

       C. Configuration using PNG_USER_CONFIG

       If  -DPNG_USER_CONFIG  is  added  to  the CPPFLAGS when pnglibconf.h is
       built, the file pngusr.h will automatically be included before the  op-
       tions  in  scripts/pnglibconf.dfa  are  processed.   Your pngusr.h file
       should contain only macro definitions turning features  on  or  off  or
       setting settings.

       Apart from the global setting "everything = off" all the options listed
       above can be set using macros in pngusr.h:

       #define PNG_feature_SUPPORTED

       is equivalent to:

       option feature on

       #define PNG_NO_feature

       is equivalent to:

       option feature off

       #define PNG_feature value

       is equivalent to:

       setting feature default value

       Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
       pngusr file you supply override the contents of scripts/pnglibconf.dfa

       If confusing or incomprehensible behavior results it is possible to ex-
       amine the intermediate file pnglibconf.dfn to find the full set of  de-
       pendency  information  for  each setting and option.  Simply locate the
       feature in the file and read the C comments that precede it.

       This method is also illustrated in the contrib/pngminim/* makefiles and
       pngusr.h.

III. Reading
       We'll  now walk you through the possible functions to call when reading
       in a PNG file sequentially, briefly explaining the syntax  and  purpose
       of  each one.  See example.c and png.h for more detail.  While progres-
       sive reading is covered in the next section, you will still  need  some
       of the functions discussed in this section to read a PNG file.

   Setup
       You  will  want  to  do  the  I/O initialization(*) before you get into
       libpng, so if it doesn't work, you don't have much to undo.  Of course,
       you  will also want to insure that you are, in fact, dealing with a PNG
       file.  Libpng provides a simple check to see if a file is a  PNG  file.
       To  use  it, pass in the first 1 to 8 bytes of the file to the function
       png_sig_cmp(), and it will return 0 (false) if the bytes match the cor-
       responding bytes of the PNG signature, or nonzero (true) otherwise.  Of
       course, the more bytes you pass in, the greater  the  accuracy  of  the
       prediction.

       If  you  are intending to keep the file pointer open for use in libpng,
       you must ensure you don't read more than 8 bytes from the beginning  of
       the  file, and you also have to make a call to png_set_sig_bytes() with
       the number of bytes you read from the beginning.  Libpng will then only
       check the bytes (if any) that your program didn't read.

       (*):  If you are not using the standard I/O functions, you will need to
       replace them with custom functions.  See the discussion under Customiz-
       ing libpng.

           FILE *fp = fopen(file_name, "rb");
           if (!fp)
           {
              return ERROR;
           }

           if (fread(header, 1, number, fp) != number)
           {
              return ERROR;
           }

           is_png = !png_sig_cmp(header, 0, number);
           if (!is_png)
           {
              return NOT_PNG;
           }

       Next, png_struct and png_info need to be allocated and initialized.  In
       order to ensure that the size of these structures is correct even  with
       a  dynamically linked libpng, there are functions to initialize and al-
       locate the structures.  We also  pass  the  library  version,  optional
       pointers  to  error  handling functions, and a pointer to a data struct
       for use by the error functions, if necessary (the pointer and functions
       can  be  NULL  if  the default error handlers are to be used).  See the
       section on Changes to Libpng below  regarding  the  old  initialization
       functions.   The  structure allocation functions quietly return NULL if
       they fail to create the structure, so your application should check for
       that.

           png_structp png_ptr = png_create_read_struct
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn);

           if (!png_ptr)
              return ERROR;

           png_infop info_ptr = png_create_info_struct(png_ptr);

           if (!info_ptr)
           {
              png_destroy_read_struct(&png_ptr,
                  (png_infopp)NULL, (png_infopp)NULL);
              return ERROR;
           }

       If  you  want  to use your own memory allocation routines, use a libpng
       that was built with PNG_USER_MEM_SUPPORTED defined,  and  use  png_cre-
       ate_read_struct_2() instead of png_create_read_struct():

           png_structp png_ptr = png_create_read_struct_2
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn, (png_voidp)
               user_mem_ptr, user_malloc_fn, user_free_fn);

       The  error handling routines passed to png_create_read_struct() and the
       memory alloc/free routines passed  to  png_create_struct_2()  are  only
       necessary  if  you are not using the libpng supplied error handling and
       memory alloc/free functions.

       When libpng encounters an error, it expects to  longjmp  back  to  your
       routine.   Therefore,  you  will  need  to  call  setjmp  and pass your
       png_jmpbuf(png_ptr).  If you read the file from different routines, you
       will  need to update the longjmp buffer every time you enter a new rou-
       tine that will call a png_*() function.

       See your documentation of setjmp/longjmp for your compiler for more in-
       formation  on  setjmp/longjmp.  See the discussion on libpng error han-
       dling in the Customizing Libpng section below for more  information  on
       the  libpng  error  handling.  If an error occurs, and libpng longjmp's
       back to your setjmp, you will want to call png_destroy_read_struct() to
       free any memory.

           if (setjmp(png_jmpbuf(png_ptr)))
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                  &end_info);
              fclose(fp);
              return ERROR;
           }

       Pass  (png_infopp)NULL  instead  of  &end_info  if you didn't create an
       end_info structure.

       If you would rather avoid the complexity of setjmp/longjmp issues,  you
       can compile libpng with PNG_NO_SETJMP, in which case errors will result
       in a call to PNG_ABORT() which defaults to abort().

       You can #define PNG_ABORT() to a function that does something more use-
       ful than abort(), as long as your function does not return.

       Now  you  need  to set up the input code.  The default for libpng is to
       use the C function fread().  If you use this, you will need to  pass  a
       valid  FILE  * in the function png_init_io().  Be sure that the file is
       opened in binary mode.  If you wish to handle reading data  in  another
       way,  you  need  not call the png_init_io() function, but you must then
       implement the libpng I/O methods discussed in  the  Customizing  Libpng
       section below.

           png_init_io(png_ptr, fp);

       If  you  had  previously  opened the file and read any of the signature
       from the beginning in order to see if this was a PNG file, you need  to
       let libpng know that there are some bytes missing from the start of the
       file.

           png_set_sig_bytes(png_ptr, number);

       You can change the zlib compression buffer size to be used while  read-
       ing compressed data with

           png_set_compression_buffer_size(png_ptr, buffer_size);

       where  the  default  size  is 8192 bytes.  Note that the buffer size is
       changed immediately and the buffer is reallocated immediately,  instead
       of setting a flag to be acted upon later.

       If you want CRC errors to be handled in a different manner than the de-
       fault, use

           png_set_crc_action(png_ptr, crit_action, ancil_action);

       The values for png_set_crc_action() say how libpng is to handle CRC er-
       rors in ancillary and critical chunks, and whether to use the data con-
       tained therein. Starting with libpng-1.6.26, this also governs  how  an
       ADLER32  error is handled while reading the IDAT chunk. Note that it is
       impossible to "discard" data in a critical chunk.

       Choices for (int) crit_action are
          PNG_CRC_DEFAULT      0  error/quit
          PNG_CRC_ERROR_QUIT   1  error/quit
          PNG_CRC_WARN_USE     3  warn/use data
          PNG_CRC_QUIET_USE    4  quiet/use data
          PNG_CRC_NO_CHANGE    5  use the current value

       Choices for (int) ancil_action are
          PNG_CRC_DEFAULT      0  error/quit
          PNG_CRC_ERROR_QUIT   1  error/quit
          PNG_CRC_WARN_DISCARD 2  warn/discard data
          PNG_CRC_WARN_USE     3  warn/use data
          PNG_CRC_QUIET_USE    4  quiet/use data
          PNG_CRC_NO_CHANGE    5  use the current value

       When the setting for crit_action  is  PNG_CRC_QUIET_USE,  the  CRC  and
       ADLER32 checksums are not only ignored, but they are not evaluated.

   Setting up callback code
       You  can set up a callback function to handle any unknown chunks in the
       input stream. You must supply the function

           read_chunk_callback(png_structp png_ptr,
                png_unknown_chunkp chunk);
           {
              /* The unknown chunk structure contains your
                 chunk data, along with similar data for any other
                 unknown chunks: */

                  png_byte name[5];
                  png_byte *data;
                  size_t size;

              /* Note that libpng has already taken care of
                 the CRC handling */

              /* put your code here.  Search for your chunk in the
                 unknown chunk structure, process it, and return one
                 of the following: */

              return -n; /* chunk had an error */
              return 0; /* did not recognize */
              return n; /* success */
           }

       (You can give your function another  name  that  you  like  instead  of
       "read_chunk_callback")

       To inform libpng about your function, use

           png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
               read_chunk_callback);

       This names not only the callback function, but also a user pointer that
       you can retrieve with

           png_get_user_chunk_ptr(png_ptr);

       If you call the png_set_read_user_chunk_fn() function, then all unknown
       chunks which the callback does not handle will be saved when read.  You
       can cause them to be discarded by returning '1' ("handled") instead  of
       '0'.   This behavior will change in libpng 1.7 and the default handling
       set by the  png_set_keep_unknown_chunks()  function,  described  below,
       will be used when the callback returns 0.  If you want the existing be-
       havior you should set the global  default  to  PNG_HANDLE_CHUNK_IF_SAFE
       now;  this  is  compatible with all current versions of libpng and with
       1.7.  Libpng 1.6 issues a warning if you keep the default, or  PNG_HAN-
       DLE_CHUNK_NEVER, and the callback returns 0.

       At  this  point, you can set up a callback function that will be called
       after each row has been read, which you can use to control  a  progress
       meter  or the like.  It's demonstrated in pngtest.c.  You must supply a
       function

           void read_row_callback(png_structp png_ptr,
              png_uint_32 row, int pass);
           {
             /* put your code here */
           }

       (You can give it another name that you like instead of  "read_row_call-
       back")

       To inform libpng about your function, use

           png_set_read_status_fn(png_ptr, read_row_callback);

       When  this  function is called the row has already been completely pro-
       cessed and the 'row' and 'pass' refer to the next row  to  be  handled.
       For the non-interlaced case the row that was just handled is simply one
       less than the passed in row number, and pass will always be 0.  For the
       interlaced  case  the  same applies unless the row value is 0, in which
       case the row just handled was the last one from one  of  the  preceding
       passes.   Because  interlacing  may skip a pass you cannot be sure that
       the preceding pass is just 'pass-1'; if you really need  to  know  what
       the  last  pass is record (row,pass) from the callback and use the last
       recorded value each time.

       As with the user transform you  can  find  the  output  row  using  the
       PNG_ROW_FROM_PASS_ROW macro.

   Unknown-chunk handling
       Now  you get to set the way the library processes unknown chunks in the
       input PNG stream. Both known and unknown chunks will be  read.   Normal
       behavior  is that known chunks will be parsed into information in vari-
       ous info_ptr members while unknown chunks will be discarded.  This  be-
       havior  can  be  wasteful if your application will never use some known
       chunk types. To change this, you can call:

           png_set_keep_unknown_chunks(png_ptr, keep,
               chunk_list, num_chunks);

           keep       - 0: default unknown chunk handling
                        1: ignore; do not keep
                        2: keep only if safe-to-copy
                        3: keep even if unsafe-to-copy

                      You can use these definitions:
                        PNG_HANDLE_CHUNK_AS_DEFAULT   0
                        PNG_HANDLE_CHUNK_NEVER        1
                        PNG_HANDLE_CHUNK_IF_SAFE      2
                        PNG_HANDLE_CHUNK_ALWAYS       3

           chunk_list - list of chunks affected (a byte string,
                        five bytes per chunk, NULL or ' ' if
                        num_chunks is positive; ignored if
                        numchunks <= 0).

           num_chunks - number of chunks affected; if 0, all
                        unknown chunks are affected.  If positive,
                        only the chunks in the list are affected,
                        and if negative all unknown chunks and
                        all known chunks except for the IHDR,
                        PLTE, tRNS, IDAT, and IEND chunks are
                        affected.

       Unknown chunks declared in this way will be saved as raw  data  onto  a
       list  of  png_unknown_chunk  structures.   If  a chunk that is normally
       known to libpng is named in the list, it will be  handled  as  unknown,
       according  to  the "keep" directive.  If a chunk is named in successive
       instances of png_set_keep_unknown_chunks(),  the  final  instance  will
       take  precedence.   The  IHDR  and  IEND  chunks should not be named in
       chunk_list; if they are, libpng will process them normally anyway.   If
       you  know  that your application will never make use of some particular
       chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.

       Here is an example of the usage of png_set_keep_unknown_chunks(), where
       the  private "vpAg" chunk will later be processed by a user chunk call-
       back function:

           png_byte vpAg[5]={118, 112,  65, 103, (png_byte) ' '};

           #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
             png_byte unused_chunks[]=
             {
               104,  73,  83,  84, (png_byte) ' ',   /* hIST */
               105,  84,  88, 116, (png_byte) ' ',   /* iTXt */
               112,  67,  65,  76, (png_byte) ' ',   /* pCAL */
               115,  67,  65,  76, (png_byte) ' ',   /* sCAL */
               115,  80,  76,  84, (png_byte) ' ',   /* sPLT */
               116,  73,  77,  69, (png_byte) ' ',   /* tIME */
             };
           #endif

           ...

           #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
             /* ignore all unknown chunks
              * (use global setting "2" for libpng16 and earlier):
              */
             png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);

             /* except for vpAg: */
             png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);

             /* also ignore unused known chunks: */
             png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
                (int)(sizeof unused_chunks)/5);
           #endif

   User limits
       The PNG specification allows the width and height of an image to be  as
       large as 2^(31-1 (0x7fffffff), or about 2.147 billion rows and columns.
       For safety, libpng imposes a default limit of 1 million rows  and  col-
       umns.   Larger  images  will be rejected immediately with a png_error()
       call. If you wish to change these limits, you can use

          png_set_user_limits(png_ptr, width_max, height_max);

       to set your own limits (libpng may reject some very wide images  anyway
       because of potential buffer overflow conditions).

       You  should  put  this statement after you create the PNG structure and
       before calling png_read_info(), png_read_png(), or png_process_data().

       When writing a  PNG  datastream,  put  this  statement  before  calling
       png_write_info() or png_write_png().

       If you need to retrieve the limits that are being applied, use

          width_max = png_get_user_width_max(png_ptr);
          height_max = png_get_user_height_max(png_ptr);

       The  PNG  specification sets no limit on the number of ancillary chunks
       allowed in a PNG datastream.  By default, libpng imposes a limit  of  a
       total  of 1000 sPLT, tEXt, iTXt, zTXt, and unknown chunks to be stored.
       If you have set up both info_ptr and end_info_ptr,  the  limit  applies
       separately  to  each.   You can change the limit on the total number of
       such chunks that will be stored, with

          png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

       where 0x7fffffffL means unlimited.  You can retrieve this limit with

          chunk_cache_max = png_get_chunk_cache_max(png_ptr);

       Libpng imposes a limit of 8 Megabytes (8,000,000 bytes) on  the  amount
       of memory that any chunk other than IDAT can occupy, originally or when
       decompressed (prior to libpng-1.6.32 the limit was only applied to com-
       pressed chunks after decompression). You can change this limit with

          png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);

       and you can retrieve the limit with

          chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);

       Any  chunks that would cause either of these limits to be exceeded will
       be ignored.

   Information about your system
       If you intend to display the PNG or to incorporate it  in  other  image
       data  you need to tell libpng information about your display or drawing
       surface so that libpng can convert the values in the image to match the
       display.

       From  libpng-1.5.4  this  information can be set before reading the PNG
       file header.  In earlier versions png_set_gamma() existed  but  behaved
       incorrectly  if  called  before  the  PNG file header had been read and
       png_set_alpha_mode() did not exist.

       If you need to support versions prior to libpng-1.5.4 test the  version
       number  as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow
       the procedures described in the appropriate manual page.

       You give libpng the encoding expected by your  system  expressed  as  a
       'gamma'  value.   You  can  also specify a default encoding for the PNG
       file in case the required information is missing from the file.  By de-
       fault  libpng  assumes  that  the PNG data matches your system, to keep
       this default call:

          png_set_gamma(png_ptr, screen_gamma, output_gamma);

       or you can use the fixed point equivalent:

          png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
             PNG_FP_1*output_gamma);

       If you don't know the gamma for your system it is probably 2.2 - a good
       approximation  to  the IEC standard for display systems (sRGB).  If im-
       ages are too contrasty or washed out you got the value  wrong  -  check
       your system documentation!

       Many  systems  permit the system gamma to be changed via a lookup table
       in the display driver, a few systems, including older Macs, change  the
       response by default.  As of 1.5.4 three special values are available to
       handle common situations:

          PNG_DEFAULT_sRGB: Indicates that the system conforms to the
                            IEC 61966-2-1 standard.  This matches almost
                            all systems.
          PNG_GAMMA_MAC_18: Indicates that the system is an older
                            (pre Mac OS 10.6) Apple Macintosh system with
                            the default settings.
          PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
                            that the system expects data with no gamma
                            encoding.

       You would use the linear (unencoded) value if you need to  process  the
       pixel  values further because this avoids the need to decode and re-en-
       code each component value whenever arithmetic is performed.  A  lot  of
       graphics software uses linear values for this reason, often with higher
       precision component values to preserve overall accuracy.

       The output_gamma value expresses how to decode the output  values,  not
       how they are encoded.  The values used correspond to the normal numbers
       used to describe the overall gamma of a computer  display  system;  for
       example  2.2  for  an sRGB conformant system.  The values are scaled by
       100000 in the _fixed version of the API (so 220000 for sRGB.)

       The inverse of the value is always used to provide a  default  for  the
       PNG  file  encoding  if it has no gAMA chunk and if png_set_gamma() has
       not been called to override the PNG gamma information.

       When the ALPHA_OPTIMIZED mode is selected the output gamma is  used  to
       encode opaque pixels however pixels with lower alpha values are not en-
       coded, regardless of the output gamma setting.

       When the standard Porter Duff handling is requested  with  mode  1  the
       output  encoding is set to be linear and the output_gamma value is only
       relevant as a default for input data that  has  no  gamma  information.
       The  linear  output  encoding  will be overridden if png_set_gamma() is
       called - the results may be highly unexpected!

       The following numbers are derived from the sRGB standard  and  the  re-
       search  behind  it.   sRGB  is defined to be approximated by a PNG gAMA
       chunk value of 0.45455 (1/2.2) for PNG.  The value implicitly  includes
       any  viewing  correction required to take account of any differences in
       the color environment of the original scene and  the  intended  display
       environment; the value expresses how to *decode* the image for display,
       not how the original data was *encoded*.

       sRGB provides a peg for the PNG standard by defining a viewing environ-
       ment.   sRGB itself, and earlier TV standards, actually use a more com-
       plex transform (a linear portion then a gamma 2.4 power law)  than  PNG
       can express.  (PNG is limited to simple power laws.)  By saying that an
       image for direct display on an sRGB conformant system should be  stored
       with  a gAMA chunk value of 45455 (11.3.3.2 and 11.3.3.5 of the ISO PNG
       specification) the PNG specification makes it possible to derive values
       for other display systems and environments.

       The  Mac value is deduced from the sRGB based on an assumption that the
       actual extra viewing correction used in early Mac display  systems  was
       implemented as a power 1.45 lookup table.

       Any  system  where a programmable lookup table is used or where the be-
       havior of the final display device characteristics can be  changed  re-
       quires system specific code to obtain the current characteristic.  How-
       ever this can be difficult and most PNG gamma correction only  requires
       an approximate value.

       By  default, if png_set_alpha_mode() is not called, libpng assumes that
       all values are unencoded, linear, values and  that  the  output  device
       also  has  a linear characteristic.  This is only very rarely correct -
       it is invariably  better  to  call  png_set_alpha_mode()  with  PNG_DE-
       FAULT_sRGB  than  rely  on the default if you don't know what the right
       answer is!

       The special value PNG_GAMMA_MAC_18 indicates an older Mac  system  (pre
       Mac  OS  10.6)  which  used  a correction table to implement a somewhat
       lower gamma on an otherwise sRGB system.

       Both these values are reserved (not simple gamma values)  in  order  to
       allow more precise correction internally in the future.

       NOTE:  the  values  can be passed to either the fixed or floating point
       APIs, but the floating point API will also accept floating  point  val-
       ues.

       The  second  thing you may need to tell libpng about is how your system
       handles alpha channel information.  Some, but not all, PNG  files  con-
       tain  an  alpha  channel.  To display these files correctly you need to
       compose the data onto a suitable background, as described  in  the  PNG
       specification.

       Libpng only supports composing onto a single color (using png_set_back-
       ground; see below).  Otherwise you must  do  the  composition  yourself
       and, in this case, you may need to call png_set_alpha_mode:

          #if PNG_LIBPNG_VER >= 10504
             png_set_alpha_mode(png_ptr, mode, screen_gamma);
          #else
             png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
          #endif

       The  screen_gamma  value  is the same as the argument to png_set_gamma;
       however, how it affects the output depends on  the  mode.   png_set_al-
       pha_mode()  sets  the file gamma default to 1/screen_gamma, so normally
       you don't need to call png_set_gamma.  If you need  different  defaults
       call png_set_gamma() before png_set_alpha_mode() - if you call it after
       it will override the settings made by png_set_alpha_mode().

       The mode is as follows:

           PNG_ALPHA_PNG: The data is encoded according to the PNG  specifica-
       tion.  Red, green and blue, or gray, components are gamma encoded color
       values and are not premultiplied by the alpha value.  The  alpha  value
       is a linear measure of the contribution of the pixel to the correspond-
       ing final output pixel.

       You should normally use this format if you intend to perform color cor-
       rection on the color values; most, maybe all, color correction software
       has no handling for the alpha channel and, anyway, the math  to  handle
       pre-multiplied component values is unnecessarily complex.

       Before you do any arithmetic on the component values you need to remove
       the gamma encoding and multiply out the alpha  channel.   See  the  PNG
       specification  for  more  detail.  It is important to note that when an
       image with an alpha channel is scaled, linear  encoded,  pre-multiplied
       component values must be used!

       The  remaining modes assume you don't need to do any further color cor-
       rection or that if you do, your color  correction  software  knows  all
       about  alpha  (it  probably doesn't!).  They 'associate' the alpha with
       the color information by storing color channel values  that  have  been
       scaled  by  the alpha.  The advantage is that the color channels can be
       resampled (the image can be scaled) in this form.  The disadvantage  is
       that  normal  practice  is to store linear, not (gamma) encoded, values
       and this requires 16-bit channels for  still  images  rather  than  the
       8-bit  channels  that  are  just  about sufficient if gamma encoding is
       used.  In addition all non-transparent  pixel  values,  including  com-
       pletely  opaque ones, must be gamma encoded to produce the final image.
       These are the 'STANDARD', 'ASSOCIATED'  or  'PREMULTIPLIED'  modes  de-
       scribed below (the latter being the two common names for associated al-
       pha color channels). Note that PNG files always contain  non-associated
       color  channels;  png_set_alpha_mode() with one of the modes causes the
       decoder to convert the pixels to an associated  form  before  returning
       them to your application.

       Since  it is not necessary to perform arithmetic on opaque color values
       so long as they are not to be resampled and  are  in  the  final  color
       space  it  is possible to optimize the handling of alpha by storing the
       opaque pixels in the PNG format (adjusted for the output  color  space)
       while  storing partially opaque pixels in the standard, linear, format.
       The accuracy required for standard alpha composition is relatively low,
       because  the pixels are isolated, therefore typically the accuracy loss
       in storing 8-bit linear values is acceptable.  (This is not true if the
       alpha  channel  is used to simulate transparency over large areas - use
       16 bits or the PNG mode in this case!)  This is the  'OPTIMIZED'  mode.
       For  this  mode a pixel is treated as opaque only if the alpha value is
       equal to the maximum value.

           PNG_ALPHA_STANDARD:  The data libpng produces  is  encoded  in  the
       standard  way assumed by most correctly written graphics software.  The
       gamma encoding will be removed by libpng and the linear component  val-
       ues will be pre-multiplied by the alpha channel.

       With  this  format the final image must be re-encoded to match the dis-
       play gamma before the image is displayed.  If your  system  doesn't  do
       that,  yet  still seems to perform arithmetic on the pixels without de-
       coding them, it is broken - check out the modes below.

       With PNG_ALPHA_STANDARD libpng always produces linear component values,
       whatever  screen_gamma you supply.  The screen_gamma value is, however,
       used as a default for the file gamma if the PNG file has no  gamma  in-
       formation.

       If  you  call png_set_gamma() after png_set_alpha_mode() you will over-
       ride the linear encoding.  Instead the pre-multiplied pixel values will
       be  gamma encoded but the alpha channel will still be linear.  This may
       actually match the requirements of some broken software, but it is  un-
       likely.

       While linear 8-bit data is often used it has insufficient precision for
       any image with a reasonable dynamic range.  To avoid problems,  and  if
       your  software supports it, use png_set_expand_16() to force all compo-
       nents to 16 bits.

           PNG_ALPHA_OPTIMIZED: This mode is the  same  as  PNG_ALPHA_STANDARD
       except that completely opaque pixels are gamma encoded according to the
       screen_gamma value.  Pixels with alpha less than 1.0  will  still  have
       linear components.

       Use  this format if you have control over your compositing software and
       so don't do other arithmetic (such as scaling) on the data you get from
       libpng.  Your compositing software can simply copy opaque pixels to the
       output but still has linear values for the non-opaque pixels.

       In normal compositing, where the alpha channel  encodes  partial  pixel
       coverage  (as  opposed to broad area translucency), the inaccuracies of
       the 8-bit representation of non-opaque pixels are irrelevant.

       You can also try this format if your software is broken; it might  look
       better.

           PNG_ALPHA_BROKEN:  This  is PNG_ALPHA_STANDARD; however, all compo-
       nent values, including the alpha channel are gamma  encoded.   This  is
       broken  because,  in  practice, no implementation that uses this choice
       correctly undoes the encoding before handling alpha  composition.   Use
       this  choice  only  if other serious errors in the software or hardware
       you use mandate it.  In most cases of broken software or  hardware  the
       bug  in  the final display manifests as a subtle halo around composited
       parts of the image.  You may not even perceive this as a halo; the com-
       posited  part  of  the  image may simply appear separate from the back-
       ground, as though it had been cut out of paper and pasted on afterward.

       If you don't have to deal with bugs in software or hardware, or if  you
       can  fix  them,  there  are three recommended ways of using png_set_al-
       pha_mode():

          png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
              screen_gamma);

       You can do color correction on the result (libpng  does  not  currently
       support  color correction internally).  When you handle the alpha chan-
       nel you need to undo the gamma encoding and multiply out the alpha.

          png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
              screen_gamma);
          png_set_expand_16(png_ptr);

       If you are using the  high  level  interface,  don't  call  png_set_ex-
       pand_16(); instead pass PNG_TRANSFORM_EXPAND_16 to the interface.

       With  this  mode  you  can't do color correction, but you can do arith-
       metic, including composition and scaling, on the data  without  further
       processing.

          png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
              screen_gamma);

       You  can  avoid  the expansion to 16-bit components with this mode, but
       you lose the ability to scale the image or perform other linear  arith-
       metic.   All  you  can do is compose the result onto a matching output.
       Since this mode is libpng-specific you also need to write your own com-
       position software.

       The  following  are  examples of calls to png_set_alpha_mode to achieve
       the required overall gamma correction and, where necessary, alpha  pre-
       multiplication.

           png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);

       Choices for the alpha_mode are

           PNG_ALPHA_PNG           0 /* according to the PNG standard */
           PNG_ALPHA_STANDARD      1 /* according to Porter/Duff */
           PNG_ALPHA_ASSOCIATED     1 /* as above; this is the normal practice
       */
           PNG_ALPHA_PREMULTIPLIED 1 /* as above */
           PNG_ALPHA_OPTIMIZED     2 /* 'PNG' for opaque pixels,  else  'STAN-
       DARD' */
           PNG_ALPHA_BROKEN        3 /* the alpha channel is gamma encoded */

       PNG_ALPHA_PNG  is  the default libpng handling of the alpha channel. It
       is not pre-multiplied into the color components. In addition  the  call
       states  that  the  output is for a sRGB system and causes all PNG files
       without gAMA chunks to be assumed to be encoded using sRGB.

           png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);

       In this case the output is assumed to be something like an sRGB confor-
       mant  display preceded by a power-law lookup table of power 1.45.  This
       is how early Mac systems behaved.

           png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);

       This is the classic Jim Blinn approach and will work in academic  envi-
       ronments  where everything is done by the book.  It has the shortcoming
       of assuming that input PNG data with no gamma information is  linear  -
       this  is unlikely to be correct unless the PNG files were generated lo-
       cally.  Most of the time the output precision will be so low as to show
       significant banding in dark areas of the image.

           png_set_expand_16(pp);
           png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);

       This  is  a  somewhat  more realistic Jim Blinn inspired approach.  PNG
       files are assumed to have the sRGB encoding if not marked with a  gamma
       value and the output is always 16 bits per component.  This permits ac-
       curate scaling and processing of the data.  If you know that your input
       PNG  files  were  generated  locally  you might need to replace PNG_DE-
       FAULT_sRGB with the correct value for your system.

           png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);

       If you just need to composite the PNG image onto an existing background
       and if you control the code that does this you can use the optimization
       setting.  In this case you just copy completely opaque  pixels  to  the
       output.   For pixels that are not completely transparent (you just skip
       those) you do the composition math using png_composite  or  png_compos-
       ite_16  below then encode the resultant 8-bit or 16-bit values to match
       the output encoding.

           Other cases

       If neither the PNG nor the standard linear encoding work  for  you  be-
       cause  of the software or hardware you use then you have a big problem.
       The PNG case will probably result in halos around the image.  The  lin-
       ear  encoding  will  probably result in a washed out, too bright, image
       (it's actually too contrasty.)  Try the ALPHA_OPTIMIZED  mode  above  -
       this will probably substantially reduce the halos.  Alternatively try:

           png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);

       This  option  will also reduce the halos, but there will be slight dark
       halos round the opaque parts of  the  image  where  the  background  is
       light.   In  the OPTIMIZED mode the halos will be light halos where the
       background is dark.  Take your pick - the halos are unavoidable  unless
       you  can  get your hardware/software fixed!  (The OPTIMIZED approach is
       slightly faster.)

       When the default gamma of PNG files doesn't match the output gamma.  If
       you  have PNG files with no gamma information png_set_alpha_mode allows
       you to provide a default gamma, but it also sets the  output  gamma  to
       the  matching  value.   If  you  know  your PNG files have a gamma that
       doesn't match the output you  can  take  advantage  of  the  fact  that
       png_set_alpha_mode  always  sets the output gamma but only sets the PNG
       default if it is not already set:

           png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
           png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);

       The first call sets both the default and the output gamma  values,  the
       second  call  overrides  the output gamma without changing the default.
       This is easier than achieving the same effect with png_set_gamma.   You
       must  use  PNG_ALPHA_PNG  for  the  first  call  - internal checking in
       png_set_alpha will fire if more than one call to png_set_alpha_mode and
       png_set_background is made in the same read operation, however multiple
       calls with PNG_ALPHA_PNG are ignored.

       If you don't need, or can't handle, the  alpha  channel  you  can  call
       png_set_background() to remove it by compositing against a fixed color.
       Don't call png_set_strip_alpha() to do this - it  will  leave  spurious
       pixel values in transparent parts of this image.

          png_set_background(png_ptr, &background_color,
              PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);

       The background_color is an RGB or grayscale value according to the data
       format libpng will produce for you.  Because you  don't  yet  know  the
       format  of  the  PNG file, if you call png_set_background at this point
       you must arrange for the format produced by libpng to always have 8-bit
       or  16-bit  components  and  then store the color as an 8-bit or 16-bit
       color as appropriate.  The color contains separate gray and RGB  compo-
       nent values, so you can let libpng produce gray or RGB output according
       to the input format, but low bit depth grayscale images must always  be
       converted  to  at  least  8-bit  format.   (Even  though  low bit depth
       grayscale images can't have an alpha channel they can have a  transpar-
       ent color!)

       You  set  the  transforms  you  need later, either as flags to the high
       level interface or libpng API calls for the low level  interface.   For
       reference the settings and API calls required are:

       8-bit values:
          PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
          png_set_expand(png_ptr); png_set_scale_16(png_ptr);

          If you must get exactly the same inaccurate results
          produced by default in versions prior to libpng-1.5.4,
          use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
          instead.

       16-bit values:
          PNG_TRANSFORM_EXPAND_16
          png_set_expand_16(png_ptr);

       In either case palette image data will be expanded to RGB.  If you just
       want   color   data   you   can   add   PNG_TRANSFORM_GRAY_TO_RGB    or
       png_set_gray_to_rgb(png_ptr) to the list.

       Calling  png_set_background before the PNG file header is read will not
       work prior to libpng-1.5.4.  Because the failure may  result  in  unex-
       pected   warnings  or  errors  it  is  therefore  much  safer  to  call
       png_set_background after the head has been  read.   Unfortunately  this
       means  that prior to libpng-1.5.4 it cannot be used with the high level
       interface.

   The high-level read interface
       At this point there are two ways to  proceed;  through  the  high-level
       read  interface,  or  through  a sequence of low-level read operations.
       You can use the high-level interface if (a) you are willing to read the
       entire image into memory, and (b) the input transformations you want to
       do are limited to the following set:

           PNG_TRANSFORM_IDENTITY      No transformation
           PNG_TRANSFORM_SCALE_16      Strip 16-bit samples to
                                       8-bit accurately
           PNG_TRANSFORM_STRIP_16      Chop 16-bit samples to
                                       8-bit less accurately
           PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel
           PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit
                                       samples to bytes
           PNG_TRANSFORM_PACKSWAP      Change order of packed
                                       pixels to LSB first
           PNG_TRANSFORM_EXPAND        Perform set_expand()
           PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
           PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                       sBIT depth
           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
           PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                       to transparency
           PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
           PNG_TRANSFORM_GRAY_TO_RGB   Expand grayscale samples
                                       to RGB (or GA to RGBA)
           PNG_TRANSFORM_EXPAND_16     Expand samples to 16 bits

       (This excludes setting a background color, doing gamma  transformation,
       quantizing, and setting filler.)  If this is the case, simply do this:

           png_read_png(png_ptr, info_ptr, png_transforms, NULL)

       where  png_transforms  is  an integer containing the bitwise OR of some
       set  of   transformation   flags.    This   call   is   equivalent   to
       png_read_info(),  followed  the set of transformations indicated by the
       transform mask, then png_read_image(), and finally png_read_end().

       (The final parameter of this call is not yet used.   Someday  it  might
       point to transformation parameters required by some future input trans-
       form.)

       You must use png_transforms and not call any png_set_transform()  func-
       tions when you use png_read_png().

       After  you  have called png_read_png(), you can retrieve the image data
       with

          row_pointers = png_get_rows(png_ptr, info_ptr);

       where row_pointers is an array of pointers to the pixel data  for  each
       row:

          png_bytep row_pointers[height];

       If you know your image size and pixel size ahead of time, you can allo-
       cate row_pointers prior to calling png_read_png() with

          if (height > PNG_UINT_32_MAX/(sizeof (png_byte)))
             png_error (png_ptr,
                 "Image is too tall to process in memory");

          if (width > PNG_UINT_32_MAX/pixel_size)
             png_error (png_ptr,
                 "Image is too wide to process in memory");

          row_pointers = png_malloc(png_ptr,
              height*(sizeof (png_bytep)));

          for (int i=0; i<height, i++)
             row_pointers[i]=NULL;  /* security precaution */

          for (int i=0; i<height, i++)
             row_pointers[i]=png_malloc(png_ptr,
                 width*pixel_size);

          png_set_rows(png_ptr, info_ptr, &row_pointers);

       Alternatively you could allocate your image in one big block and define
       row_pointers[i]  to  point  into  the  proper places in your block, but
       first be sure that your platform is able to allocate such a large  buf-
       fer:

          /* Guard against integer overflow */
          if (height > PNG_SIZE_MAX/(width*pixel_size)) {
               png_error(png_ptr,"image_data buffer would be too large");
          }

          png_bytep buffer=png_malloc(png_ptr,height*width*pixel_size);

          for (int i=0; i<height, i++)
             row_pointers[i]=buffer+i*width*pixel_size;

          png_set_rows(png_ptr, info_ptr, &row_pointers);

       If  you  use png_set_rows(), the application is responsible for freeing
       row_pointers (and row_pointers[i], if they were separately allocated).

       If you don't allocate row_pointers ahead of time,  png_read_png()  will
       do it, and it'll be free'ed by libpng when you call png_destroy_*().

   The low-level read interface
       If you are going the low-level route, you are now ready to read all the
       file information up to the actual image data.  You do this with a  call
       to png_read_info().

           png_read_info(png_ptr, info_ptr);

       This will process all chunks up to but not including the image data.

       This  also  copies  some  of the data from the PNG file into the decode
       structure for use  in  later  transformations.   Important  information
       copied in is:

       1) The PNG file gamma from the gAMA chunk.  This overwrites the default
       value provided by an  earlier  call  to  png_set_gamma  or  png_set_al-
       pha_mode.

       2)  Prior to libpng-1.5.4 the background color from a bKGd chunk.  This
       damages the information provided by an earlier  call  to  png_set_back-
       ground  resulting  in unexpected behavior.  Libpng-1.5.4 no longer does
       this.

       3) The number of significant bits in each component value.  Libpng uses
       this  to  optimize gamma handling by reducing the internal lookup table
       sizes.

       4) The transparent color information from a tRNS chunk.   This  can  be
       modified by a later call to png_set_tRNS.

   Querying the info structure
       Functions are used to get the information from the info_ptr once it has
       been read.  Note that these fields may not be completely filled in  un-
       til png_read_end() has read the chunk data following the image.

           png_get_IHDR(png_ptr, info_ptr, &width, &height,
              &bit_depth, &color_type, &interlace_type,
              &compression_type, &filter_method);

           width          - holds the width of the image
                            in pixels (up to 2^31).

           height         - holds the height of the image
                            in pixels (up to 2^31).

           bit_depth      - holds the bit depth of one of the
                            image channels.  (valid values are
                            1, 2, 4, 8, 16 and depend also on
                            the color_type.  See also
                            significant bits (sBIT) below).

           color_type     - describes which color/alpha channels
                                are present.
                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - (PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7)

           compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                            for PNG 1.0)

           filter_method  - (must be PNG_FILTER_TYPE_BASE
                            for PNG 1.0, and can also be
                            PNG_INTRAPIXEL_DIFFERENCING if
                            the PNG datastream is embedded in
                            a MNG-1.0 datastream)

           Any of width, height, color_type, bit_depth,
           interlace_type, compression_type, or filter_method can
           be NULL if you are not interested in their values.

           Note that png_get_IHDR() returns 32-bit data into
           the application's width and height variables.
           This is an unsafe situation if these are not png_uint_32
           variables.  In such situations, the
           png_get_image_width() and png_get_image_height()
           functions described below are safer.

           width            = png_get_image_width(png_ptr,
                                info_ptr);

           height           = png_get_image_height(png_ptr,
                                info_ptr);

           bit_depth        = png_get_bit_depth(png_ptr,
                                info_ptr);

           color_type       = png_get_color_type(png_ptr,
                                info_ptr);

           interlace_type   = png_get_interlace_type(png_ptr,
                                info_ptr);

           compression_type = png_get_compression_type(png_ptr,
                                info_ptr);

           filter_method    = png_get_filter_type(png_ptr,
                                info_ptr);

           channels = png_get_channels(png_ptr, info_ptr);

           channels       - number of channels of info for the
                            color type (valid values are 1 (GRAY,
                            PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
                            4 (RGB_ALPHA or RGB + filler byte))

           rowbytes = png_get_rowbytes(png_ptr, info_ptr);

           rowbytes       - number of bytes needed to hold a row
                            This value, the bit_depth, color_type,
                            and the number of channels can change
                            if you use transforms such as
                            png_set_expand(). See
                            png_read_update_info(), below.

           signature = png_get_signature(png_ptr, info_ptr);

           signature      - holds the signature read from the
                            file (if any).  The data is kept in
                            the same offset it would be if the
                            whole signature were read (i.e. if an
                            application had already read in 4
                            bytes of signature before starting
                            libpng, the remaining 4 bytes would
                            be in signature[4] through signature[7]
                            (see png_set_sig_bytes())).

       These  are  also  important,  but their validity depends on whether the
       chunk   has   been   read.    The   png_get_valid(png_ptr,    info_ptr,
       PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr, info_ptr, ...) functions
       return non-zero if the data has been read, or zero if  it  is  missing.
       The parameters to the png_get_<chunk> are set directly if they are sim-
       ple data types, or a pointer into the info_ptr is returned for any com-
       plex types.

       The  colorspace  data  from  gAMA, cHRM, sRGB, iCCP, and sBIT chunks is
       simply returned to give the application information about how the image
       was  encoded.   Libpng  itself only does transformations using the file
       gamma when combining semitransparent pixels with the background  color,
       and,  since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit
       linear pixels within the simplified API.  Libpng  also  uses  the  file
       gamma  when converting RGB to gray, beginning with libpng-1.0.5, if the
       application calls png_set_rgb_to_gray()).

           png_get_PLTE(png_ptr, info_ptr, &palette,
                            &num_palette);

           palette        - the palette for the file
                            (array of png_color)

           num_palette    - number of entries in the palette

           png_get_gAMA(png_ptr, info_ptr, &file_gamma);
           png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);

           file_gamma     - the gamma at which the file is
                            written (PNG_INFO_gAMA)

           int_file_gamma - 100,000 times the gamma at which the
                            file is written

           png_get_cHRM(png_ptr, info_ptr,  &white_x, &white_y, &red_x,
                            &red_y, &green_x, &green_y, &blue_x, &blue_y)
           png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z,
                            &green_X, &green_Y, &green_Z, &blue_X, &blue_Y,
                            &blue_Z)
           png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x,
                            &int_white_y, &int_red_x, &int_red_y,
                            &int_green_x, &int_green_y, &int_blue_x,
                            &int_blue_y)
           png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
                            &int_red_Z, &int_green_X, &int_green_Y,
                            &int_green_Z, &int_blue_X, &int_blue_Y,
                            &int_blue_Z)

           {white,red,green,blue}_{x,y}
                            A color space encoding specified using the
                            chromaticities of the end points and the
                            white point. (PNG_INFO_cHRM)

           {red,green,blue}_{X,Y,Z}
                            A color space encoding specified using the
                            encoding end points - the CIE tristimulus
                            specification of the intended color of the red,
                            green and blue channels in the PNG RGB data.
                            The white point is simply the sum of the three
                            end points. (PNG_INFO_cHRM)

           png_get_sRGB(png_ptr, info_ptr, &srgb_intent);

           srgb_intent -    the rendering intent (PNG_INFO_sRGB)
                            The presence of the sRGB chunk
                            means that the pixel data is in the
                            sRGB color space.  This chunk also
                            implies specific values of gAMA and
                            cHRM.

           png_get_iCCP(png_ptr, info_ptr, &name,
              &compression_type, &profile, &proflen);

           name             - The profile name.

           compression_type - The compression type; always
                              PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                              You may give NULL to this argument to
                              ignore it.

           profile          - International Color Consortium color
                              profile data. May contain NULs.

           proflen          - length of profile data in bytes.

           png_get_sBIT(png_ptr, info_ptr, &sig_bit);

           sig_bit        - the number of significant bits for
                            (PNG_INFO_sBIT) each of the gray,
                            red, green, and blue channels,
                            whichever are appropriate for the
                            given color type (png_color_16)

           png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
                            &num_trans, &trans_color);

           trans_alpha    - array of alpha (transparency)
                            entries for palette (PNG_INFO_tRNS)

           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)

           trans_color    - graylevel or color sample values of
                            the single transparent color for
                            non-paletted images (PNG_INFO_tRNS)

           png_get_eXIf_1(png_ptr, info_ptr, &num_exif, &exif);
                            (PNG_INFO_eXIf)

           exif           - Exif profile (array of png_byte)

           png_get_hIST(png_ptr, info_ptr, &hist);
                            (PNG_INFO_hIST)

           hist           - histogram of palette (array of
                            png_uint_16)

           png_get_tIME(png_ptr, info_ptr, &mod_time);

           mod_time       - time image was last modified
                           (PNG_VALID_tIME)

           png_get_bKGD(png_ptr, info_ptr, &background);

           background     - background color (of type
                            png_color_16p) (PNG_VALID_bKGD)
                            valid 16-bit red, green and blue
                            values, regardless of color_type

           num_comments   = png_get_text(png_ptr, info_ptr,
                            &text_ptr, &num_text);

           num_comments   - number of comments

           text_ptr       - array of png_text holding image
                            comments

           text_ptr[i].compression - type of compression used
                        on "text" PNG_TEXT_COMPRESSION_NONE
                                  PNG_TEXT_COMPRESSION_zTXt
                                  PNG_ITXT_COMPRESSION_NONE
                                  PNG_ITXT_COMPRESSION_zTXt

           text_ptr[i].key   - keyword for comment.  Must contain
                                1-79 characters.

           text_ptr[i].text  - text comments for current
                                keyword.  Can be empty.

           text_ptr[i].text_length - length of text string,
                        after decompression, 0 for iTXt

           text_ptr[i].itxt_length - length of itxt string,
                        after decompression, 0 for tEXt/zTXt

           text_ptr[i].lang  - language of comment (empty
                                string for unknown).

           text_ptr[i].lang_key  - keyword in UTF-8
                                (empty string for unknown).

           Note that the itxt_length, lang, and lang_key
           members of the text_ptr structure only exist when the
           library is built with iTXt chunk support.  Prior to
           libpng-1.4.0 the library was built by default without
           iTXt support. Also note that when iTXt is supported,
           they contain NULL pointers when the "compression"
           field contains PNG_TEXT_COMPRESSION_NONE or
           PNG_TEXT_COMPRESSION_zTXt.

           num_text       - number of comments (same as
                            num_comments; you can put NULL here
                            to avoid the duplication)

           Note while png_set_text() will accept text, language,
           and translated keywords that can be NULL pointers, the
           structure returned by png_get_text will always contain
           regular zero-terminated C strings.  They might be
           empty strings but they will never be NULL pointers.

           num_spalettes = png_get_sPLT(png_ptr, info_ptr,
              &palette_ptr);

           num_spalettes  - number of sPLT chunks read.

           palette_ptr    - array of palette structures holding
                            contents of one or more sPLT chunks
                            read.

           png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
              &unit_type);

           offset_x       - positive offset from the left edge
                            of the screen (can be negative)

           offset_y       - positive offset from the top edge
                            of the screen (can be negative)

           unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

           png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
              &unit_type);

           res_x          - pixels/unit physical resolution in
                            x direction

           res_y          - pixels/unit physical resolution in
                            x direction

           unit_type      - PNG_RESOLUTION_UNKNOWN,
                            PNG_RESOLUTION_METER

           png_get_sCAL(png_ptr, info_ptr, &unit, &width,
              &height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units

           height      - height of a pixel in physical scale units
                        (width and height are doubles)

           png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
              &height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units
                         (expressed as a string)

           height      - height of a pixel in physical scale units
                        (width and height are strings like "2.54")

           num_unknown_chunks = png_get_unknown_chunks(png_ptr,
              info_ptr, &unknowns)

           unknowns          - array of png_unknown_chunk
                               structures holding unknown chunks

           unknowns[i].name  - name of unknown chunk

           unknowns[i].data  - data of unknown chunk

           unknowns[i].size  - size of unknown chunk's data

           unknowns[i].location - position of chunk in file

           The value of "i" corresponds to the order in which the
           chunks were read from the PNG file or inserted with the
           png_set_unknown_chunks() function.

           The value of "location" is a bitwise "or" of

                PNG_HAVE_IHDR  (0x01)
                PNG_HAVE_PLTE  (0x02)
                PNG_AFTER_IDAT (0x08)

       The data from the pHYs chunk can be  retrieved  in  several  convenient
       forms:

           res_x = png_get_x_pixels_per_meter(png_ptr,
              info_ptr)

           res_y = png_get_y_pixels_per_meter(png_ptr,
              info_ptr)

           res_x_and_y = png_get_pixels_per_meter(png_ptr,
              info_ptr)

           res_x = png_get_x_pixels_per_inch(png_ptr,
              info_ptr)

           res_y = png_get_y_pixels_per_inch(png_ptr,
              info_ptr)

           res_x_and_y = png_get_pixels_per_inch(png_ptr,
              info_ptr)

           aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
              info_ptr)

           Each of these returns 0 [signifying "unknown"] if
              the data is not present or if res_x is 0;
              res_x_and_y is 0 if res_x != res_y

           Note that because of the way the resolutions are
              stored internally, the inch conversions won't
              come out to exactly even number.  For example,
              72 dpi is stored as 0.28346 pixels/meter, and
              when this is retrieved it is 71.9988 dpi, so
              be sure to round the returned value appropriately
              if you want to display a reasonable-looking result.

       The  data  from  the  oFFs chunk can be retrieved in several convenient
       forms:

           x_offset = png_get_x_offset_microns(png_ptr, info_ptr);

           y_offset = png_get_y_offset_microns(png_ptr, info_ptr);

           x_offset = png_get_x_offset_inches(png_ptr, info_ptr);

           y_offset = png_get_y_offset_inches(png_ptr, info_ptr);

           Each of these returns 0 [signifying "unknown" if both
              x and y are 0] if the data is not present or if the
              chunk is present but the unit is the pixel.  The
              remark about inexact inch conversions applies here
              as well, because a value in inches can't always be
              converted to microns and back without some loss
              of precision.

       For more information, see the PNG specification for chunk contents.  Be
       careful  with  trusting  rowbytes, as some of the transformations could
       increase the space needed to hold a row (expand,  filler,  gray_to_rgb,
       etc.).  See png_read_update_info(), below.

       A  quick word about text_ptr and num_text.  PNG stores comments in key-
       word/text pairs, one pair per chunk, with no limit  on  the  number  of
       text chunks, and a 2^31 byte limit on their size.  While there are sug-
       gested keywords, there is no requirement to restrict the use  to  these
       strings.   It  is strongly suggested that keywords and text be sensible
       to humans (that's the point), so don't use abbreviations.  Non-printing
       symbols  are  not allowed.  See the PNG specification for more details.
       There is also no requirement to have text after the keyword.

       Keywords should be limited to 79 Latin-1 characters without leading  or
       trailing spaces, but non-consecutive spaces are allowed within the key-
       word.  It is possible to have the same keyword  any  number  of  times.
       The text_ptr is an array of png_text structures, each holding a pointer
       to a language string, a pointer to a keyword and a pointer  to  a  text
       string.   The text string, language code, and translated keyword may be
       empty or NULL pointers.  The keyword/text pairs are put into the  array
       in  the order that they are received.  However, some or all of the text
       chunks may be after the image, so, to make sure you have read  all  the
       text chunks, don't mess with these until after you read the stuff after
       the image.  This will be mentioned again below in the  discussion  that
       goes with png_read_end().

   Input transformations
       After you've read the header information, you can set up the library to
       handle any special transformations of the image data.  The various ways
       to  transform  the data will be described in the order that they should
       occur.  This is important, as some  of  these  change  the  color  type
       and/or  bit  depth  of  the  data, and some others only work on certain
       color types and bit depths.

       Transformations you request are ignored if they don't have any  meaning
       for  a  particular input data format.  However some transformations can
       have an effect as a result of a previous transformation.  If you  spec-
       ify a contradictory set of transformations, for example both adding and
       removing the alpha channel, you cannot predict the final result.

       The color used for the transparency values should be  supplied  in  the
       same  format/depth as the current image data.  It is stored in the same
       format/depth as the image data in a tRNS chunk, so this is what  libpng
       expects for this data.

       The  color used for the background value depends on the need_expand ar-
       gument as described below.

       Data will be decoded into the supplied row buffers  packed  into  bytes
       unless  the  library has been told to transform it into another format.
       For example, 4 bit/pixel paletted or grayscale data will be returned  2
       pixels/byte with the leftmost pixel in the high-order bits of the byte,
       unless png_set_packing() is called.  8-bit RGB data will be  stored  in
       RGB  RGB  RGB  format unless png_set_filler() or png_set_add_alpha() is
       called to insert filler bytes, either before or after each RGB triplet.

       16-bit RGB data will be returned RRGGBB RRGGBB, with the most  signifi-
       cant byte of the color value first, unless png_set_scale_16() is called
       to transform it to regular RGB RGB  triplets,  or  png_set_filler()  or
       png_set_add alpha() is called to insert two filler bytes, either before
       or after each RRGGBB triplet.  Similarly,  8-bit  or  16-bit  grayscale
       data   can  be  modified  with  png_set_filler(),  png_set_add_alpha(),
       png_set_strip_16(), or png_set_scale_16().

       The following code transforms grayscale images of  less  than  8  to  8
       bits,  changes paletted images to RGB, and adds a full alpha channel if
       there is transparency information in a tRNS chunk.  This is most useful
       on  grayscale  images with bit depths of 2 or 4 or if there is a multi-
       ple-image viewing application that wishes to treat all  images  in  the
       same way.

           if (color_type == PNG_COLOR_TYPE_PALETTE)
               png_set_palette_to_rgb(png_ptr);

           if (png_get_valid(png_ptr, info_ptr,
               PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);

           if (color_type == PNG_COLOR_TYPE_GRAY &&
               bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);

       The  first  two  functions  are  actually aliases for png_set_expand(),
       added in libpng version 1.0.4, with the function names expanded to  im-
       prove  code  readability.   In some future version they may actually do
       different things.

       As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was added.
       It expands the sample depth without changing tRNS to alpha.

       As  of libpng version 1.5.2, png_set_expand_16() was added.  It behaves
       as png_set_expand(); however,  the  resultant  channels  have  16  bits
       rather  than  8.   Use  this when the output color or gray channels are
       made linear to avoid fairly severe accuracy loss.

          if (bit_depth < 16)
             png_set_expand_16(png_ptr);

       PNG can have files with 16 bits per channel.  If you only can handle  8
       bits per channel, this will strip the pixels down to 8-bit.

           if (bit_depth == 16) #if PNG_LIBPNG_VER >= 10504
              png_set_scale_16(png_ptr); #else
              png_set_strip_16(png_ptr); #endif

       (The  more accurate "png_set_scale_16()" API became available in libpng
       version 1.5.4).

       If you need to process the alpha channel on the image  separately  from
       the  image  data (for example if you convert it to a bitmap mask) it is
       possible to have libpng strip the channel  leaving  just  RGB  or  gray
       data:

           if (color_type & PNG_COLOR_MASK_ALPHA)
              png_set_strip_alpha(png_ptr);

       If you strip the alpha channel you need to find some other way of deal-
       ing with the information.  If, instead, you want to convert  the  image
       to  an opaque version with no alpha channel use png_set_background; see
       below.

       As of libpng version 1.5.2, almost all useful expansions are supported,
       the  major  omissions  are  conversion  of  grayscale to indexed images
       (which can be done trivially in the application) and conversion of  in-
       dexed  to grayscale (which can be done by a trivial manipulation of the
       palette.)

       In the following table, the 01 means grayscale with depth<8,  31  means
       indexed  with  depth<8,  other  numerals  represent the color type, "T"
       means the tRNS chunk is present, A means an alpha channel  is  present,
       and  O  means  tRNS or alpha is present but all pixels in the image are
       opaque.

         FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O  6A  6O
          TO
          01    -  [G]  -   -   -   -   -   -   -   -   -   -   -   -   -
          31   [Q]  Q  [Q] [Q] [Q]  Q   Q   Q   Q   Q   Q  [Q] [Q]  Q   Q
           0    1   G   +   .   .   G   G   G   G   G   G   B   B  GB  GB
          0T    lt  Gt  t   +   .   Gt  G   G   Gt  G   G   Bt  Bt GBt GBt
          0O    lt  Gt  t   .   +   Gt  Gt  G   Gt  Gt  G   Bt  Bt GBt GBt
           2    C   P   C   C   C   +   .   .   C   -   -  CB  CB   B   B
          2T    Ct  -   Ct  C   C   t   +   t   -   -   -  CBt CBt  Bt  Bt
          2O    Ct  -   Ct  C   C   t   t   +   -   -   -  CBt CBt  Bt  Bt
           3   [Q]  p  [Q] [Q] [Q]  Q   Q   Q   +   .   .  [Q] [Q]  Q   Q
          3T   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   +   t  [Qt][Qt] Qt  Qt
          3O   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   t   +  [Qt][Qt] Qt  Qt
          4A    lA  G   A   T   T   GA  GT  GT  GA  GT  GT  +   BA  G  GBA
          4O    lA GBA  A   T   T   GA  GT  GT  GA  GT  GT  BA  +  GBA  G
          6A    CA  PA  CA  C   C   A   T  tT   PA  P   P   C  CBA  +   BA
          6O    CA PBA  CA  C   C   A  tT   T   PA  P   P  CBA  C   BA  +

       Within the matrix,
            "+" identifies entries where 'from' and 'to' are the same.
            "-" means the transformation is not supported.
            "." means nothing is necessary (a tRNS chunk can just be ignored).
            "t" means the transformation is obtained by png_set_tRNS.
            "A" means the transformation is obtained by png_set_add_alpha().
            "X" means the transformation is obtained by png_set_expand().
            "1" means the transformation is obtained by
                png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
                if there is no transparency in the original or the final
                format).
            "C" means the transformation is obtained by png_set_gray_to_rgb().
            "G" means the transformation is obtained by png_set_rgb_to_gray().
            "P" means the transformation is obtained by
                png_set_expand_palette_to_rgb().
            "p" means the transformation is obtained by png_set_packing().
            "Q" means the transformation is obtained by png_set_quantize().
            "T" means the transformation is obtained by
                png_set_tRNS_to_alpha().
            "B" means the transformation is obtained by
                png_set_background(), or png_strip_alpha().

       When an entry has multiple transforms listed all are required to  cause
       the right overall transformation.  When two transforms are separated by
       a comma either will do the job.  When transforms are enclosed in [] the
       transform  should  do  the  job but this is currently unimplemented - a
       different format will result if the suggested transformations are used.

       In PNG files, the alpha channel in an image is the  level  of  opacity.
       If  you need the alpha channel in an image to be the level of transpar-
       ency instead of opacity, you can invert the alpha channel (or the  tRNS
       chunk  data)  after  it's  read,  so that 0 is fully opaque and 255 (in
       8-bit or paletted images) or 65535 (in 16-bit images) is  fully  trans-
       parent, with

           png_set_invert_alpha(png_ptr);

       PNG  files pack pixels of bit depths 1, 2, and 4 into bytes as small as
       they can, resulting in, for example, 8 pixels per byte for 1 bit files.
       This  code  expands  to 1 pixel per byte without changing the values of
       the pixels:

           if (bit_depth < 8)
              png_set_packing(png_ptr);

       PNG files have possible bit depths of 1, 2, 4, 8, and 16.   All  pixels
       stored  in  a  PNG image have been "scaled" or "shifted" up to the next
       higher possible bit depth (e.g. from 5 bits/sample in the range  [0,31]
       to  8 bits/sample in the range [0, 255]).  However, it is also possible
       to convert the PNG pixel data back to the original bit depth of the im-
       age.  This call reduces the pixels back down to the original bit depth:

           png_color_8p sig_bit;

           if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
              png_set_shift(png_ptr, sig_bit);

       PNG  files  store  3-color pixels in red, green, blue order.  This code
       changes the storage of the pixels to blue, green, red:

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_RGB_ALPHA)
              png_set_bgr(png_ptr);

       PNG files store RGB pixels packed into 3 or 6 bytes. This code  expands
       them  into  4  or  8 bytes for windowing systems that need them in this
       format:

           if (color_type == PNG_COLOR_TYPE_RGB)
              png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);

       where "filler" is the 8-bit or 16-bit number to fill with, and the  lo-
       cation  is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon
       whether you want the filler before the RGB or after.  When  filling  an
       8-bit  pixel, the least significant 8 bits of the number are used, if a
       16-bit number is supplied.  This transformation does not affect  images
       that already have full alpha channels.  To add an opaque alpha channel,
       use filler=0xffff and PNG_FILLER_AFTER which will generate RGBA pixels.

       Note that png_set_filler() does not change the color type.  If you want
       to do that, you can add a true alpha channel with

           if (color_type == PNG_COLOR_TYPE_RGB ||
              color_type == PNG_COLOR_TYPE_GRAY)
              png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);

       where  "filler"  contains the alpha value to assign to each pixel.  The
       png_set_add_alpha() function was added in libpng-1.2.7.

       If you are reading an image with an alpha channel,  and  you  need  the
       data as ARGB instead of the normal PNG format RGBA:

           if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
              png_set_swap_alpha(png_ptr);

       For some uses, you may want a grayscale image to be represented as RGB.
       This code will do that conversion:

           if (color_type == PNG_COLOR_TYPE_GRAY ||
               color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
              png_set_gray_to_rgb(png_ptr);

       Conversely, you can convert an  RGB  or  RGBA  image  to  grayscale  or
       grayscale with alpha.

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_RGB_ALPHA)
              png_set_rgb_to_gray(png_ptr, error_action,
                 double red_weight, double green_weight);

           error_action = 1: silently do the conversion

           error_action = 2: issue a warning if the original
                             image has any pixel where
                             red != green or red != blue

           error_action = 3: issue an error and abort the
                             conversion if the original
                             image has any pixel where
                             red != green or red != blue

           red_weight:       weight of red component

           green_weight:     weight of green component
                             If either weight is negative, default
                             weights are used.

       In  the  corresponding  fixed point API the red_weight and green_weight
       values are simply scaled by 100,000:

           png_set_rgb_to_gray(png_ptr, error_action,
              png_fixed_point red_weight,
              png_fixed_point green_weight);

       If you have set error_action = 1 or 2, you can later check whether  the
       image  really  was  gray,  after  processing  the  image rows, with the
       png_get_rgb_to_gray_status(png_ptr)  function.   It   will   return   a
       png_byte that is zero if the image was gray or 1 if there were any non-
       gray pixels.  Background and sBIT data will be  silently  converted  to
       grayscale, using the green channel data for sBIT, regardless of the er-
       ror_action setting.

       The default values come from the PNG file cHRM chunk if present; other-
       wise, the defaults correspond to the ITU-R recommendation 709, and also
       the sRGB color space, as recommended in the  Charles  Poynton's  Colour
       FAQ, Copyright (c) 2006-11-28 Charles Poynton, in section 9:

       <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>

           Y = 0.2126 * R + 0.7152 * G + 0.0722 * B

       Previous  versions  of  this document, 1998 through 2002, recommended a
       slightly different formula:

           Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

       Libpng uses an integer approximation:

           Y = (6968 * R + 23434 * G + 2366 * B)/32768

       The calculation is done in a linear colorspace, if the image gamma  can
       be determined.

       The  png_set_background() function has been described already; it tells
       libpng to composite images with alpha or  simple  transparency  against
       the  supplied  background  color.   For  compatibility with versions of
       libpng earlier than libpng-1.5.4 it is recommended that  you  call  the
       function  after  reading the file header, even if you don't want to use
       the color in a bKGD chunk, if one exists.

       If the PNG file contains a bKGD chunk (PNG_INFO_bKGD  valid),  you  may
       use  this  color, or supply another color more suitable for the current
       display (e.g., the background color from a web page).  You need to tell
       libpng  how  the color is represented, both the format of the component
       values in the color (the number of bits) and the gamma encoding of  the
       color.   The  function  takes  two arguments, background_gamma_mode and
       need_expand to convey this information; however, only two  combinations
       are likely to be useful:

           png_color_16 my_background;
           png_color_16p image_background;

           if (png_get_bKGD(png_ptr, info_ptr, &image_background))
              png_set_background(png_ptr, image_background,
                  PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
           else
              png_set_background(png_ptr, &my_background,
                  PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);

       The second call was described above - my_background is in the format of
       the final, display, output produced by libpng.  Because  you  now  know
       the format of the PNG it is possible to avoid the need to choose either
       8-bit or 16-bit output and to retain palette images (the palette colors
       will  be  modified appropriately and the tRNS chunk removed.)  However,
       if you are doing this, take great care not to ask  for  transformations
       without checking first that they apply!

       In  the  first call the background color has the original bit depth and
       color type of the PNG file.  So, for palette images the color  is  sup-
       plied  as a palette index and for low bit greyscale images the color is
       a reduced bit value in image_background->gray.

       If you didn't call png_set_gamma() before reading the file header,  for
       example  if you need your code to remain compatible with older versions
       of libpng prior to libpng-1.5.4, this is the place to call it.

       Do not call it if you called png_set_alpha_mode(); doing so will damage
       the  settings  put  in  place by png_set_alpha_mode().  (If png_set_al-
       pha_mode() is supported then you can certainly do  png_set_gamma()  be-
       fore reading the PNG header.)

       This  API  unconditionally sets the screen and file gamma values, so it
       will override the value in the PNG file unless it is called before  the
       PNG  file reading starts.  For this reason you must always call it with
       the PNG file value when you call it in this position:

          if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
             png_set_gamma(png_ptr, screen_gamma, file_gamma);

          else
             png_set_gamma(png_ptr, screen_gamma, 0.45455);

       If you need to reduce an RGB file to a paletted file, or if a  paletted
       file  has more entries than will fit on your screen, png_set_quantize()
       will do that.  Note that this  is  a  simple  match  quantization  that
       merely finds the closest color available.  This should work fairly well
       with optimized palettes, but fairly badly with linear color cubes.   If
       you  pass  a  palette that is larger than maximum_colors, the file will
       reduce the number of colors in the palette so it will  fit  into  maxi-
       mum_colors.   If  there is a histogram, libpng will use it to make more
       intelligent choices when reducing the palette.  If  there  is  no  his-
       togram, it may not do as good a job.

          if (color_type & PNG_COLOR_MASK_COLOR)
          {
             if (png_get_valid(png_ptr, info_ptr,
                 PNG_INFO_PLTE))
             {
                png_uint_16p histogram = NULL;

                png_get_hIST(png_ptr, info_ptr,
                    &histogram);
                png_set_quantize(png_ptr, palette, num_palette,
                   max_screen_colors, histogram, 1);
             }

             else
             {
                png_color std_color_cube[MAX_SCREEN_COLORS] =
                   { ... colors ... };

                png_set_quantize(png_ptr, std_color_cube,
                   MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
                   NULL,0);
             }
          }

       PNG  files describe monochrome as black being zero and white being one.
       The following code will reverse this (make black be one  and  white  be
       zero):

          if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
             png_set_invert_mono(png_ptr);

       This  function  can also be used to invert grayscale and gray-alpha im-
       ages:

          if (color_type == PNG_COLOR_TYPE_GRAY ||
              color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
             png_set_invert_mono(png_ptr);

       PNG files store 16-bit pixels in network byte  order  (big-endian,  ie.
       most  significant  bits  first).   This code changes the storage to the
       other way (little-endian, i.e. least significant bits  first,  the  way
       PCs store them):

           if (bit_depth == 16)
              png_set_swap(png_ptr);

       If  you  are using packed-pixel images (1, 2, or 4 bits/pixel), and you
       need to change the order the pixels are packed into bytes, you can use:

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

       Finally, you can write your own transformation function if none of  the
       existing  ones  meets  your  needs.  This is done by setting a callback
       with

           png_set_read_user_transform_fn(png_ptr,
               read_transform_fn);

       You must supply the function

           void read_transform_fn(png_structp png_ptr, png_row_infop
               row_info, png_bytep data)

       See pngtest.c for a working example.  Your function will be called  af-
       ter  all  of  the other transformations have been processed.  Take care
       with interlaced images if you do the interlace yourself - the width  of
       the row is the width in 'row_info', not the overall image width.

       If supported, libpng provides two information routines that you can use
       to find where you are in processing the image:

          png_get_current_pass_number(png_structp png_ptr);
          png_get_current_row_number(png_structp png_ptr);

       Don't try using these outside a transform callback - firstly  they  are
       only supported if user transforms are supported, secondly they may well
       return unexpected results unless the row is actually being processed at
       the moment they are called.

       With  interlaced images the value returned is the row in the input sub-
       image    image.     Use    PNG_ROW_FROM_PASS_ROW(row,     pass)     and
       PNG_COL_FROM_PASS_COL(col,  pass)  to find the output pixel (x,y) given
       an interlaced sub-image pixel (row,col,pass).

       The discussion of interlace handling above contains more information on
       how to use these values.

       You can also set up a pointer to a user structure for use by your call-
       back function, and you can inform libpng that your  transform  function
       will change the number of channels or bit depth with the function

           png_set_user_transform_info(png_ptr, user_ptr,
               user_depth, user_channels);

       The  user's  application, not libpng, is responsible for allocating and
       freeing any memory required for the user structure.

       You can retrieve  the  pointer  via  the  function  png_get_user_trans-
       form_ptr().  For example:

           voidp read_user_transform_ptr =
               png_get_user_transform_ptr(png_ptr);

       The  last thing to handle is interlacing; this is covered in detail be-
       low, but you must call the function here if you want libpng  to  handle
       expansion of the interlaced image.

           number_of_passes = png_set_interlace_handling(png_ptr);

       After  setting  the  transformations,  libpng  can update your png_info
       structure to reflect any transformations  you've  requested  with  this
       call.

           png_read_update_info(png_ptr, info_ptr);

       This  is  most  useful to update the info structure's rowbytes field so
       you can use it to allocate your image memory.  This function will  also
       update  your  palette  with  the correct screen_gamma and background if
       these have been  given  with  the  calls  above.   You  may  only  call
       png_read_update_info() once with a particular info_ptr.

       After  you call png_read_update_info(), you can allocate any memory you
       need to hold the image.  The row data is simply raw byte data  for  all
       forms  of  images.  As the actual allocation varies among applications,
       no example will be given.  If you are allocating one large  chunk,  you
       will  need  to  build  an  array of pointers to each row, as it will be
       needed for some of the functions below.

       Be sure that your platform can allocate the buffer  that  you'll  need.
       libpng internally checks for oversize width, but you'll need to do your
       own check for number_of_rows*width*pixel_size if you are using a multi-
       ple-row buffer:

          /* Guard against integer overflow */
          if (number_of_rows > PNG_SIZE_MAX/(width*pixel_size)) {
               png_error(png_ptr,"image_data buffer would be too large");
          }

       Remember: Before you call png_read_update_info(), the png_get_*() func-
       tions return the values corresponding to the original PNG image.  After
       you call png_read_update_info the values refer to the image that libpng
       will output.  Consequently you must call all the png_set_ functions be-
       fore  you  call png_read_update_info().  This is particularly important
       for  png_set_interlace_handling()  -  if  you   are   going   to   call
       png_read_update_info()  you  must call png_set_interlace_handling() be-
       fore it unless you want to receive interlaced output.

   Reading image data
       After you've allocated memory, you can read the image data.   The  sim-
       plest  way  to  do this is in one function call.  If you are allocating
       enough memory to hold the whole image, you can just  call  png_read_im-
       age() and libpng will read in all the image data and put it in the mem-
       ory area supplied.  You will need to pass in an array  of  pointers  to
       each row.

       This  function  automatically handles interlacing, so you don't need to
       call  png_set_interlace_handling()  (unless   you   call   png_read_up-
       date_info()) or call this function multiple times, or any of that other
       stuff necessary with png_read_rows().

          png_read_image(png_ptr, row_pointers);

       where row_pointers is:

          png_bytep row_pointers[height];

       You can point to void or char or whatever you use for pixels.

       If you don't want to read in the whole  image  at  once,  you  can  use
       png_read_rows()  instead.   If  there  is  no interlacing (check inter-
       lace_type == PNG_INTERLACE_NONE), this is simple:

           png_read_rows(png_ptr, row_pointers, NULL,
               number_of_rows);

       where row_pointers is the same as in the png_read_image() call.

       If you are doing this just one row at a time, you can do  this  with  a
       single row_pointer instead of an array of row_pointers:

           png_bytep row_pointer = row;
           png_read_row(png_ptr, row_pointer, NULL);

       If  the  file  is  interlaced  (interlace_type != 0 in the IHDR chunk),
       things get somewhat harder.  The only current (PNG  Specification  ver-
       sion  1.2)  interlacing  type  for PNG is (interlace_type == PNG_INTER-
       LACE_ADAM7); a somewhat  complicated  2D  interlace  scheme,  known  as
       Adam7,  that  breaks down an image into seven smaller images of varying
       size, based on an 8x8 grid.  This number is defined (from  libpng  1.5)
       as PNG_INTERLACE_ADAM7_PASSES in png.h

       libpng  can  fill  out those images or it can give them to you "as is".
       It is almost always better to have libpng handle  the  interlacing  for
       you.  If you want the images filled out, there are two ways to do that.
       The one mentioned in the PNG specification is to expand each  pixel  to
       cover  those  pixels  that  have  not  been  read  yet (the "rectangle"
       method).  This results in a blocky image  for  the  first  pass,  which
       gradually smooths out as more pixels are read.  The other method is the
       "sparkle" method, where pixels are drawn only in their final locations,
       with the rest of the image remaining whatever colors they were initial-
       ized to before the start of the read.  The first method  usually  looks
       better,  but tends to be slower, as there are more pixels to put in the
       rows.

       If, as is likely, you want libpng to expand the images, call  this  be-
       fore calling png_start_read_image() or png_read_update_info():

           if (interlace_type == PNG_INTERLACE_ADAM7)
              number_of_passes
                  = png_set_interlace_handling(png_ptr);

       This  will  return  the  number  of  passes needed.  Currently, this is
       seven, but may change if another interlace type is added.   This  func-
       tion  can  be  called even if the file is not interlaced, where it will
       return one  pass.   You  then  need  to  read  the  whole  image  'num-
       ber_of_passes'  times.   Each  time will distribute the pixels from the
       current pass to the correct place in the output image, so you  need  to
       supply the same rows to png_read_rows in each pass.

       If  you are not going to display the image after each pass, but are go-
       ing to wait until the entire image is read in, use the sparkle  effect.
       This  effect  is  faster and the end result of either method is exactly
       the same.  If you are planning on displaying the image after each pass,
       the "rectangle" effect is generally considered the better looking one.

       If  you  only  want  the  "sparkle" effect, just call png_read_row() or
       png_read_rows() as normal, with the third parameter  NULL.   Make  sure
       you  make  pass  over  the  image number_of_passes times, and you don't
       change the data in the rows between calls.  You can  change  the  loca-
       tions of the data, just not the data.  Each pass only writes the pixels
       appropriate for that pass, and assumes the data from previous passes is
       still valid.

           png_read_rows(png_ptr, row_pointers, NULL,
               number_of_rows);
           or
           png_read_row(png_ptr, row_pointers, NULL);

       If  you only want the first effect (the rectangles), do the same as be-
       fore except pass the row buffer in the third parameter, and  leave  the
       second parameter NULL.

           png_read_rows(png_ptr, NULL, row_pointers,
               number_of_rows);
           or
           png_read_row(png_ptr, NULL, row_pointers);

       If  you  don't want libpng to handle the interlacing details, just call
       png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the im-
       ages.  Each of the images is a valid image by itself; however, you will
       almost certainly need to distribute the pixels from each  sub-image  to
       the correct place.  This is where everything gets very tricky.

       If  you  want to retrieve the separate images you must pass the correct
       number of rows to each successive call of png_read_rows().  The  calcu-
       lation  gets pretty complicated for small images, where some sub-images
       may not even exist because either their width or height ends  up  zero.
       libpng provides two macros to help you in 1.5 and later versions:

          png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
          png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);

       Respectively  these tell you the width and height of the sub-image cor-
       responding to the numbered pass.  'pass' is in in the range 0  to  6  -
       this  can  be  confusing  because  the specification refers to the same
       passes as 1 to 7!  Be careful, you must check both the width and height
       before  calling png_read_rows() and not call it for that pass if either
       is zero.

       You can, of course, read each sub-image row by row.   If  you  want  to
       produce  optimal code to make a pixel-by-pixel transformation of an in-
       terlaced image this is the best approach; read each row of  each  pass,
       transform it, and write it out to a new interlaced image.

       If  you want to de-interlace the image yourself libpng provides further
       macros to help that tell you where to place the pixels  in  the  output
       image.   Because the interlacing scheme is rectangular - sub-image pix-
       els are always arranged on a rectangular grid - all you  need  to  know
       for each pass is the starting column and row in the output image of the
       first pixel plus the spacing between each  pixel.   As  of  libpng  1.5
       there are four macros to retrieve this information:

          png_uint_32 x = PNG_PASS_START_COL(pass);
          png_uint_32 y = PNG_PASS_START_ROW(pass);
          png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
          png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);

       These allow you to write the obvious loop:

          png_uint_32 input_y = 0;
          png_uint_32 output_y = PNG_PASS_START_ROW(pass);

          while (output_y < output_image_height)
          {
             png_uint_32 input_x = 0;
             png_uint_32 output_x = PNG_PASS_START_COL(pass);

             while (output_x < output_image_width)
             {
                image[output_y][output_x] =
                    subimage[pass][input_y][input_x++];

                output_x += xStep;
             }

             ++input_y;
             output_y += yStep;
          }

       Notice  that  the  steps between successive output rows and columns are
       returned as shifts.  This is possible because the pixels in the  subim-
       ages are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the orig-
       inal image.  In practice you may need to directly calculate the  output
       coordinate  given  an  input  coordinate.   libpng provides two further
       macros for this purpose:

          png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
          png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);

       Finally a pair of macros are provided to tell you if a particular image
       row or column appears in a given pass:

          int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
          int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);

       Bear  in  mind  that you will probably also need to check the width and
       height of the pass in addition to the above to be sure  the  pass  even
       exists!

       With  any  luck you are convinced by now that you don't want to do your
       own interlace handling.  In reality normally the only good  reason  for
       doing this is if you are processing PNG files on a pixel-by-pixel basis
       and don't want to load the whole file into memory  when  it  is  inter-
       laced.

       libpng  includes a test program, pngvalid, that illustrates reading and
       writing of interlaced images.  If you can't get interlacing to work  in
       your  code  and  don't  want to leave it to libpng (the recommended ap-
       proach), see how pngvalid.c does it.

   Finishing a sequential read
       After you are finished reading the image through the  low-level  inter-
       face, you can finish reading the file.

       If  you  want  to use a different crc action for handling CRC errors in
       chunks after the image data, you can call png_set_crc_action() again at
       this point.

       If  you  are interested in comments or time, which may be stored either
       before or after the image data, you should pass the  separate  png_info
       struct if you want to keep the comments from before and after the image
       separate.

           png_infop end_info = png_create_info_struct(png_ptr);

           if (!end_info)
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                  (png_infopp)NULL);
              return ERROR;
           }

          png_read_end(png_ptr, end_info);

       If you are not interested, you should still call png_read_end() but you
       can  pass  NULL, avoiding the need to create an end_info structure.  If
       you do this, libpng will not process any chunks after IDAT  other  than
       skipping  over  them  and perhaps (depending on whether you have called
       png_set_crc_action) checking their CRCs  while  looking  for  the  IEND
       chunk.

          png_read_end(png_ptr, (png_infop)NULL);

       If  you  don't call png_read_end(), then your file pointer will be left
       pointing to the first chunk after the last IDAT, which is probably  not
       what you want if you expect to read something beyond the end of the PNG
       datastream.

       When you are done, you can free all memory  allocated  by  libpng  like
       this:

          png_destroy_read_struct(&png_ptr, &info_ptr,
              &end_info);

       or, if you didn't create an end_info structure,

          png_destroy_read_struct(&png_ptr, &info_ptr,
              (png_infopp)NULL);

       It  is  also  possible  to  individually free the info_ptr members that
       point to libpng-allocated storage with the following function:

           png_free_data(png_ptr, info_ptr, mask, seq)

           mask - identifies data to be freed, a mask
                  containing the bitwise OR of one or
                  more of
                    PNG_FREE_PLTE, PNG_FREE_TRNS,
                    PNG_FREE_HIST, PNG_FREE_ICCP,
                    PNG_FREE_PCAL, PNG_FREE_ROWS,
                    PNG_FREE_SCAL, PNG_FREE_SPLT,
                    PNG_FREE_TEXT, PNG_FREE_UNKN,
                  or simply PNG_FREE_ALL

           seq  - sequence number of item to be freed
                  (-1 for all items)

       This function may be safely called when the relevant  storage  has  al-
       ready  been  freed,  or has not yet been allocated, or was allocated by
       the user and not by libpng,  and will in those cases do  nothing.   The
       "seq"  parameter is ignored if only one item of the selected data type,
       such as PLTE, is allowed.  If "seq" is not -1, and multiple  items  are
       allowed for the data type identified in the mask, such as text or sPLT,
       only the n'th item in the structure is freed, where n is "seq".

       The default behavior is only to free data that was allocated internally
       by libpng.  This can be changed, so that libpng will not free the data,
       or so that it will free data  that  was  allocated  by  the  user  with
       png_malloc()  or png_calloc() and passed in via a png_set_*() function,
       with

           png_data_freer(png_ptr, info_ptr, freer, mask)

           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

           mask   - which data elements are affected
                    same choices as in png_free_data()

       This function only affects data that has already been  allocated.   You
       can  call  this  function after reading the PNG data but before calling
       any  png_set_*()  functions,  to  control  whether  the  user  or   the
       png_set_*()  function is responsible for freeing any existing data that
       might be present, and again after the png_set_*() functions to  control
       whether the user or png_destroy_*() is supposed to free the data.  When
       the user assumes responsibility for libpng-allocated data, the applica-
       tion  must  use  png_free() to free it, and when the user transfers re-
       sponsibility to libpng for data that the user has allocated,  the  user
       must have used png_malloc() or png_calloc() to allocate it.

       If  you  allocated  your  row_pointers  in a single block, as suggested
       above in the description of the high level read interface, you must not
       transfer   responsibility   for  freeing  it  to  the  png_set_rows  or
       png_read_destroy function, because they would also try to free the  in-
       dividual row_pointers[i].

       If  you  allocated  text_ptr.text,  text_ptr.lang,  and text_ptr.trans-
       lated_keyword separately, do not transfer  responsibility  for  freeing
       text_ptr  to  libpng, because when libpng fills a png_text structure it
       combines these members with the key member,  and  png_free_data()  will
       free  only text_ptr.key.  Similarly, if you transfer responsibility for
       free'ing text_ptr from libpng to  your  application,  your  application
       must not separately free those members.

       The  png_free_data()  function  will turn off the "valid" flag for any-
       thing it frees.  If you need to turn the flag off for a chunk that  was
       freed by your application instead of by libpng, you can use

           png_set_invalid(png_ptr, info_ptr, mask);

           mask - identifies the chunks to be made invalid,
                  containing the bitwise OR of one or
                  more of
                    PNG_INFO_gAMA, PNG_INFO_sBIT,
                    PNG_INFO_cHRM, PNG_INFO_PLTE,
                    PNG_INFO_tRNS, PNG_INFO_bKGD,
                    PNG_INFO_eXIf,
                    PNG_INFO_hIST, PNG_INFO_pHYs,
                    PNG_INFO_oFFs, PNG_INFO_tIME,
                    PNG_INFO_pCAL, PNG_INFO_sRGB,
                    PNG_INFO_iCCP, PNG_INFO_sPLT,
                    PNG_INFO_sCAL, PNG_INFO_IDAT

       For  a  more compact example of reading a PNG image, see the file exam-
       ple.c.

   Reading PNG files progressively
       The progressive reader is slightly different from  the  non-progressive
       reader.   Instead  of  calling  png_read_info(),  png_read_rows(),  and
       png_read_end(), you make one call to  png_process_data(),  which  calls
       callbacks  when  it  has the info, a row, or the end of the image.  You
       set up these callbacks with png_set_progressive_read_fn().   You  don't
       have  to  worry  about the input/output functions of libpng, as you are
       giving the library the data directly in png_process_data().  I will as-
       sume  that  you  have read the section on reading PNG files above, so I
       will only highlight the differences (although I will show  all  of  the
       code).

       png_structp png_ptr; png_infop info_ptr;

        /*  An example code fragment of how you would
            initialize the progressive reader in your
            application. */
        int
        initialize_png_reader()
        {
           png_ptr = png_create_read_struct
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
                user_error_fn, user_warning_fn);

           if (!png_ptr)
               return ERROR;

           info_ptr = png_create_info_struct(png_ptr);

           if (!info_ptr)
           {
              png_destroy_read_struct(&png_ptr,
                 (png_infopp)NULL, (png_infopp)NULL);
              return ERROR;
           }

           if (setjmp(png_jmpbuf(png_ptr)))
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                 (png_infopp)NULL);
              return ERROR;
           }

           /* This one's new.  You can provide functions
              to be called when the header info is valid,
              when each row is completed, and when the image
              is finished.  If you aren't using all functions,
              you can specify NULL parameters.  Even when all
              three functions are NULL, you need to call
              png_set_progressive_read_fn().  You can use
              any struct as the user_ptr (cast to a void pointer
              for the function call), and retrieve the pointer
              from inside the callbacks using the function

                 png_get_progressive_ptr(png_ptr);

              which will return a void pointer, which you have
              to cast appropriately.
            */
           png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
               info_callback, row_callback, end_callback);

           return 0;
        }

        /* A code fragment that you call as you receive blocks
          of data */
        int
        process_data(png_bytep buffer, png_uint_32 length)
        {
           if (setjmp(png_jmpbuf(png_ptr)))
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                  (png_infopp)NULL);
              return ERROR;
           }

           /* This one's new also.  Simply give it a chunk
              of data from the file stream (in order, of
              course).  On machines with segmented memory
              models machines, don't give it any more than
              64K.  The library seems to run fine with sizes
              of 4K. Although you can give it much less if
              necessary (I assume you can give it chunks of
              1 byte, I haven't tried less than 256 bytes
              yet).  When this function returns, you may
              want to display any rows that were generated
              in the row callback if you don't already do
              so there.
            */
           png_process_data(png_ptr, info_ptr, buffer, length);

           /* At this point you can call png_process_data_skip if
              you want to handle data the library will skip yourself;
              it simply returns the number of bytes to skip (and stops
              libpng skipping that number of bytes on the next
              png_process_data call).
           return 0;
        }

        /* This function is called (as set by
           png_set_progressive_read_fn() above) when enough data
           has been supplied so all of the header has been
           read.
        */
        void
        info_callback(png_structp png_ptr, png_infop info)
        {
           /* Do any setup here, including setting any of
              the transformations mentioned in the Reading
              PNG files section.  For now, you _must_ call
              either png_start_read_image() or
              png_read_update_info() after all the
              transformations are set (even if you don't set
              any).  You may start getting rows before
              png_process_data() returns, so this is your
              last chance to prepare for that.

              This is where you turn on interlace handling,
              assuming you don't want to do it yourself.

              If you need to you can stop the processing of
              your original input data at this point by calling
              png_process_data_pause.  This returns the number
              of unprocessed bytes from the last png_process_data
              call - it is up to you to ensure that the next call
              sees these bytes again.  If you don't want to bother
              with this you can get libpng to cache the unread
              bytes by setting the 'save' parameter (see png.h) but
              then libpng will have to copy the data internally.
            */
        }

        /* This function is called when each row of image
           data is complete */
        void
        row_callback(png_structp png_ptr, png_bytep new_row,
           png_uint_32 row_num, int pass)
        {
           /* If the image is interlaced, and you turned
              on the interlace handler, this function will
              be called for every row in every pass.  Some
              of these rows will not be changed from the
              previous pass.  When the row is not changed,
              the new_row variable will be NULL.  The rows
              and passes are called in order, so you don't
              really need the row_num and pass, but I'm
              supplying them because it may make your life
              easier.

              If you did not turn on interlace handling then
              the callback is called for each row of each
              sub-image when the image is interlaced.  In this
              case 'row_num' is the row in the sub-image, not
              the row in the output image as it is in all other
              cases.

              For the non-NULL rows of interlaced images when
              you have switched on libpng interlace handling,
              you must call png_progressive_combine_row()
              passing in the row and the old row.  You can
              call this function for NULL rows (it will just
              return) and for non-interlaced images (it just
              does the memcpy for you) if it will make the
              code easier.  Thus, you can just do this for
              all cases if you switch on interlace handling;
            */

               png_progressive_combine_row(png_ptr, old_row,
                 new_row);

           /* where old_row is what was displayed
              previously for the row.  Note that the first
              pass (pass == 0, really) will completely cover
              the old row, so the rows do not have to be
              initialized.  After the first pass (and only
              for interlaced images), you will have to pass
              the current row, and the function will combine
              the old row and the new row.

              You can also call png_process_data_pause in this
              callback - see above.
           */
        }

        void
        end_callback(png_structp png_ptr, png_infop info)
        {
           /* This function is called after the whole image
              has been read, including any chunks after the
              image (up to and including the IEND).  You
              will usually have the same info chunk as you
              had in the header, although some data may have
              been added to the comments and time fields.

              Most people won't do much here, perhaps setting
              a flag that marks the image as finished.
            */
        }

IV. Writing
       Much of this is very similar to reading.  However, everything of impor-
       tance is repeated here, so you won't have to constantly look back up in
       the reading section to understand writing.

   Setup
       You  will want to do the I/O initialization before you get into libpng,
       so if it doesn't work, you don't have anything to undo. If you are  not
       using  the  standard  I/O functions, you will need to replace them with
       custom writing functions.  See the discussion under Customizing libpng.

           FILE *fp = fopen(file_name, "wb");

           if (!fp)
              return ERROR;

       Next, png_struct and png_info need to be allocated and initialized.  As
       these  can be both relatively large, you may not want to store these on
       the stack, unless you have stack space to spare.  Of course,  you  will
       want  to check if they return NULL.  If you are also reading, you won't
       want to  name  your  read  structure  and  your  write  structure  both
       "png_ptr";  you can call them anything you like, such as "read_ptr" and
       "write_ptr".  Look at pngtest.c, for example.

           png_structp png_ptr = png_create_write_struct
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn);

           if (!png_ptr)
              return ERROR;

           png_infop info_ptr = png_create_info_struct(png_ptr);
           if (!info_ptr)
           {
              png_destroy_write_struct(&png_ptr,
                  (png_infopp)NULL);
              return ERROR;
           }

       If you  want  to  use  your  own  memory  allocation  routines,  define
       PNG_USER_MEM_SUPPORTED  and  use png_create_write_struct_2() instead of
       png_create_write_struct():

           png_structp png_ptr = png_create_write_struct_2
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn, (png_voidp)
               user_mem_ptr, user_malloc_fn, user_free_fn);

       After you have these structures, you will need to set up the error han-
       dling.   When  libpng encounters an error, it expects to longjmp() back
       to your routine.  Therefore, you will need to call  setjmp()  and  pass
       the  png_jmpbuf(png_ptr).   If  you  write the file from different rou-
       tines, you will need to update the png_jmpbuf(png_ptr) every  time  you
       enter  a new routine that will call a png_*() function.  See your docu-
       mentation of setjmp/longjmp for your compiler for more  information  on
       setjmp/longjmp.   See  the  discussion  on libpng error handling in the
       Customizing Libpng section below for more information on the libpng er-
       ror handling.

           if (setjmp(png_jmpbuf(png_ptr)))
           {
           png_destroy_write_struct(&png_ptr, &info_ptr);
              fclose(fp);
              return ERROR;
           }
           ...
           return;

       If  you would rather avoid the complexity of setjmp/longjmp issues, you
       can compile libpng with PNG_NO_SETJMP, in which case errors will result
       in a call to PNG_ABORT() which defaults to abort().

       You can #define PNG_ABORT() to a function that does something more use-
       ful than abort(), as long as your function does not return.

       Checking for invalid palette index on write was added at libpng 1.5.10.
       If a pixel contains an invalid (out-of-range) index libpng issues a be-
       nign error.  This is enabled by default because this  condition  is  an
       error  according to the PNG specification, Clause 11.3.2, but the error
       can be ignored in each png_ptr with

          png_set_check_for_invalid_index(png_ptr, 0);

       If the error is ignored, or if png_benign_error() treats it as a  warn-
       ing,  any invalid pixels are written as-is by the encoder, resulting in
       an invalid PNG datastream as output.  In this case the  application  is
       responsible  for  ensuring  that the pixel indexes are in range when it
       writes a PLTE chunk with fewer entries than the bit depth would allow.

       Now you need to set up the output code.  The default for libpng  is  to
       use  the C function fwrite().  If you use this, you will need to pass a
       valid FILE * in the function png_init_io().  Be sure that the  file  is
       opened  in  binary  mode.  Again, if you wish to handle writing data in
       another way, see the discussion on libpng I/O handling in the Customiz-
       ing Libpng section below.

           png_init_io(png_ptr, fp);

       If  you are embedding your PNG into a datastream such as MNG, and don't
       want libpng to write the 8-byte signature, or if you have already writ-
       ten the signature in your application, use

           png_set_sig_bytes(png_ptr, 8);

       to inform libpng that it should not write a signature.

   Write callbacks
       At  this  point, you can set up a callback function that will be called
       after each row has been  written,  which  you  can  use  to  control  a
       progress  meter or the like.  It's demonstrated in pngtest.c.  You must
       supply a function

           void write_row_callback(png_structp png_ptr, png_uint_32 row,
              int pass);
           {
             /* put your code here */
           }

       (You can give it another name that you like instead of "write_row_call-
       back")

       To inform libpng about your function, use

           png_set_write_status_fn(png_ptr, write_row_callback);

       When  this  function is called the row has already been completely pro-
       cessed and it has also been written out.  The 'row' and 'pass' refer to
       the  next  row to be handled.  For the non-interlaced case the row that
       was just handled is simply one less than the passed in row number,  and
       pass will always be 0.  For the interlaced case the same applies unless
       the row value is 0, in which case the row just handled was the last one
       from  one of the preceding passes.  Because interlacing may skip a pass
       you cannot be sure that the preceding pass is just 'pass-1', if you re-
       ally  need  to  know  what  the last pass is record (row,pass) from the
       callback and use the last recorded value each time.

       As with the user transform you  can  find  the  output  row  using  the
       PNG_ROW_FROM_PASS_ROW macro.

       You  now  have the option of modifying how the compression library will
       run.  The following functions are mainly for testing, but may be useful
       in  some  cases, like if you need to write PNG files extremely fast and
       are willing to give up some compression, or if you want to get the max-
       imum  possible  compression  at  the expense of slower writing.  If you
       have no special needs in this area, let the library do what it wants by
       not  calling  this  function  at all, as it has been tuned to deliver a
       good speed/compression ratio. The second parameter to  png_set_filter()
       is  the filter method, for which the only valid values are 0 (as of the
       July 1999 PNG specification, version 1.2) or 64 (if you are  writing  a
       PNG  datastream that is to be embedded in a MNG datastream).  The third
       parameter is a flag that indicates  which  filter  type(s)  are  to  be
       tested for each scanline.  See the PNG specification for details on the
       specific filter types.

           /* turn on or off filtering, and/or choose
              specific filters.  You can use either a single
              PNG_FILTER_VALUE_NAME or the bitwise OR of one
              or more PNG_FILTER_NAME masks.
            */
           png_set_filter(png_ptr, 0,
              PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
              PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
              PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
              PNG_FILTER_AVG   | PNG_FILTER_VALUE_AVG  |
              PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
              PNG_ALL_FILTERS  | PNG_FAST_FILTERS);

       If an application wants to start and stop using particular filters dur-
       ing compression, it should start out with all of the filters (to ensure
       that the previous row of pixels will be  stored  in  case  it's  needed
       later), and then add and remove them after the start of compression.

       If  you  are  writing  a PNG datastream that is to be embedded in a MNG
       datastream, the second parameter can be either 0 or 64.

       The png_set_compression_*() functions interface to the zlib compression
       library,  and  should mostly be ignored unless you really know what you
       are  doing.   The  only  generally  useful  call  is   png_set_compres-
       sion_level()  which changes how much time zlib spends on trying to com-
       press the image data.  See the Compression Library  (zlib.h  and  algo-
       rithm.txt,  distributed  with zlib) for details on the compression lev-
       els.

           #include zlib.h

           /* Set the zlib compression level */
           png_set_compression_level(png_ptr,
               Z_BEST_COMPRESSION);

           /* Set other zlib parameters for compressing IDAT */
           png_set_compression_mem_level(png_ptr, 8);
           png_set_compression_strategy(png_ptr,
               Z_DEFAULT_STRATEGY);
           png_set_compression_window_bits(png_ptr, 15);
           png_set_compression_method(png_ptr, 8);
           png_set_compression_buffer_size(png_ptr, 8192)

           /* Set zlib parameters for text compression
            * If you don't call these, the parameters
            * fall back on those defined for IDAT chunks
            */
           png_set_text_compression_mem_level(png_ptr, 8);
           png_set_text_compression_strategy(png_ptr,
               Z_DEFAULT_STRATEGY);
           png_set_text_compression_window_bits(png_ptr, 15);
           png_set_text_compression_method(png_ptr, 8);

   Setting the contents of info for output
       You now need to fill in the png_info structure with all  the  data  you
       wish  to  write  before the actual image.  Note that the only thing you
       are allowed to write after the image is the text chunks  and  the  time
       chunk  (as  of PNG Specification 1.2, anyway).  See png_write_end() and
       the latest PNG specification for more information on that.  If you wish
       to write them before the image, fill them in now, and flag that data as
       being valid.  If you want to wait until after the data, don't fill them
       until  png_write_end().   For all the fields in png_info and their data
       types, see png.h.  For explanations of what the fields contain, see the
       PNG specification.

       Some of the more important parts of the png_info are:

           png_set_IHDR(png_ptr, info_ptr, width, height,
              bit_depth, color_type, interlace_type,
              compression_type, filter_method)

           width          - holds the width of the image
                            in pixels (up to 2^31).

           height         - holds the height of the image
                            in pixels (up to 2^31).

           bit_depth      - holds the bit depth of one of the
                            image channels.
                            (valid values are 1, 2, 4, 8, 16
                            and depend also on the
                            color_type.  See also significant
                            bits (sBIT) below).

           color_type     - describes which color/alpha
                            channels are present.
                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7

           compression_type - (must be
                            PNG_COMPRESSION_TYPE_DEFAULT)

           filter_method  - (must be PNG_FILTER_TYPE_DEFAULT
                            or, if you are writing a PNG to
                            be embedded in a MNG datastream,
                            can also be
                            PNG_INTRAPIXEL_DIFFERENCING)

       If  you  call  png_set_IHDR(),  the  call must appear before any of the
       other png_set_*() functions, because they might require access to  some
       of  the  IHDR  settings.   The  remaining  png_set_*() functions can be
       called in any order.

       If you wish, you can reset  the  compression_type,  interlace_type,  or
       filter_method  later  by  calling png_set_IHDR() again; if you do this,
       the width, height, bit_depth, and color_type must be the same  in  each
       call.

           png_set_PLTE(png_ptr, info_ptr, palette,
              num_palette);

           palette        - the palette for the file
                            (array of png_color)
           num_palette    - number of entries in the palette

           png_set_gAMA(png_ptr, info_ptr, file_gamma);
           png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);

           file_gamma     - the gamma at which the image was
                            created (PNG_INFO_gAMA)

           int_file_gamma - 100,000 times the gamma at which
                            the image was created

           png_set_cHRM(png_ptr, info_ptr,  white_x, white_y, red_x, red_y,
                            green_x, green_y, blue_x, blue_y)
           png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
                            green_Y, green_Z, blue_X, blue_Y, blue_Z)
           png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
                            int_red_x, int_red_y, int_green_x, int_green_y,
                            int_blue_x, int_blue_y)
           png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
                            int_red_Z, int_green_X, int_green_Y, int_green_Z,
                            int_blue_X, int_blue_Y, int_blue_Z)

           {white,red,green,blue}_{x,y}
                            A  color  space encoding specified using the chro-
       maticities
                            of the end points and the white point.

           {red,green,blue}_{X,Y,Z}
                            A color space encoding specified using the  encod-
       ing end
                            points  - the CIE tristimulus specification of the
       intended
                            color of the red, green and blue channels  in  the
       PNG RGB
                            data.   The  white  point is simply the sum of the
       three end
                            points.

           png_set_sRGB(png_ptr, info_ptr, srgb_intent);

           srgb_intent    - the rendering intent
                            (PNG_INFO_sRGB) The presence of
                            the sRGB chunk means that the pixel
                            data is in the sRGB color space.
                            This chunk also implies specific
                            values of gAMA and cHRM.  Rendering
                            intent is the CSS-1 property that
                            has been defined by the International
                            Color Consortium
                            (http://www.color.org).
                            It can be one of
                            PNG_sRGB_INTENT_SATURATION,
                            PNG_sRGB_INTENT_PERCEPTUAL,
                            PNG_sRGB_INTENT_ABSOLUTE, or
                            PNG_sRGB_INTENT_RELATIVE.

           png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
              srgb_intent);

           srgb_intent    - the rendering intent
                            (PNG_INFO_sRGB) The presence of the
                            sRGB chunk means that the pixel
                            data is in the sRGB color space.
                            This function also causes gAMA and
                            cHRM chunks with the specific values
                            that are consistent with sRGB to be
                            written.

           png_set_iCCP(png_ptr, info_ptr, name, compression_type,
                              profile, proflen);

           name             - The profile name.

           compression_type - The compression type; always
                              PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                              You may give NULL to this argument to
                              ignore it.

           profile          - International Color Consortium color
                              profile data. May contain NULs.

           proflen          - length of profile data in bytes.

           png_set_sBIT(png_ptr, info_ptr, sig_bit);

           sig_bit        - the number of significant bits for
                            (PNG_INFO_sBIT) each of the gray, red,
                            green, and blue channels, whichever are
                            appropriate for the given color type
                            (png_color_16)

           png_set_tRNS(png_ptr, info_ptr, trans_alpha,
              num_trans, trans_color);

           trans_alpha    - array of alpha (transparency)
                            entries for palette (PNG_INFO_tRNS)

           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)

           trans_color    - graylevel or color sample values
                            (in order red, green, blue) of the
                            single transparent color for
                            non-paletted images (PNG_INFO_tRNS)

           png_set_eXIf_1(png_ptr, info_ptr, num_exif, exif);

           exif           - Exif profile (array of
                            png_byte) (PNG_INFO_eXIf)

           png_set_hIST(png_ptr, info_ptr, hist);

           hist           - histogram of palette (array of
                            png_uint_16) (PNG_INFO_hIST)

           png_set_tIME(png_ptr, info_ptr, mod_time);

           mod_time       - time image was last modified
                            (PNG_VALID_tIME)

           png_set_bKGD(png_ptr, info_ptr, background);

           background     - background color (of type
                            png_color_16p) (PNG_VALID_bKGD)

           png_set_text(png_ptr, info_ptr, text_ptr, num_text);

           text_ptr       - array of png_text holding image
                            comments

           text_ptr[i].compression - type of compression used
                        on "text" PNG_TEXT_COMPRESSION_NONE
                                  PNG_TEXT_COMPRESSION_zTXt
                                  PNG_ITXT_COMPRESSION_NONE
                                  PNG_ITXT_COMPRESSION_zTXt
           text_ptr[i].key   - keyword for comment.  Must contain
                        1-79 characters.
           text_ptr[i].text  - text comments for current
                                keyword.  Can be NULL or empty.
           text_ptr[i].text_length - length of text string,
                        after decompression, 0 for iTXt
           text_ptr[i].itxt_length - length of itxt string,
                        after decompression, 0 for tEXt/zTXt
           text_ptr[i].lang  - language of comment (NULL or
                                empty for unknown).
           text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
                                or empty for unknown).

           Note that the itxt_length, lang, and lang_key
           members of the text_ptr structure only exist when the
           library is built with iTXt chunk support.  Prior to
           libpng-1.4.0 the library was built by default without
           iTXt support. Also note that when iTXt is supported,
           they contain NULL pointers when the "compression"
           field contains PNG_TEXT_COMPRESSION_NONE or
           PNG_TEXT_COMPRESSION_zTXt.

           num_text       - number of comments

           png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
              num_spalettes);

           palette_ptr    - array of png_sPLT_struct structures
                            to be added to the list of palettes
                            in the info structure.
           num_spalettes  - number of palette structures to be
                            added.

           png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
               unit_type);

           offset_x  - positive offset from the left
                            edge of the screen

           offset_y  - positive offset from the top
                            edge of the screen

           unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

           png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
               unit_type);

           res_x       - pixels/unit physical resolution
                         in x direction

           res_y       - pixels/unit physical resolution
                         in y direction

           unit_type   - PNG_RESOLUTION_UNKNOWN,
                         PNG_RESOLUTION_METER

           png_set_sCAL(png_ptr, info_ptr, unit, width, height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units

           height      - height of a pixel in physical scale units
                         (width and height are doubles)

           png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units
                         expressed as a string

           height      - height of a pixel in physical scale units
                        (width and height are strings like "2.54")

           png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
              num_unknowns)

           unknowns          - array of png_unknown_chunk
                               structures holding unknown chunks
           unknowns[i].name  - name of unknown chunk
           unknowns[i].data  - data of unknown chunk
           unknowns[i].size  - size of unknown chunk's data
           unknowns[i].location - position to write chunk in file
                                  0: do not write chunk
                                  PNG_HAVE_IHDR: before PLTE
                                  PNG_HAVE_PLTE: before IDAT
                                  PNG_AFTER_IDAT: after IDAT

       The "location" member is set automatically according to  what  part  of
       the output file has already been written.  You can change its value af-
       ter calling  png_set_unknown_chunks()  as  demonstrated  in  pngtest.c.
       Within  each  of the "locations", the chunks are sequenced according to
       their position in the structure (that is, the value of  "i",  which  is
       the order in which the chunk was either read from the input file or de-
       fined with png_set_unknown_chunks).

       A quick word about text and num_text.  text is  an  array  of  png_text
       structures.   num_text  is the number of valid structures in the array.
       Each png_text structure holds a language code, a keyword, a text value,
       and a compression type.

       The  compression  types  have the same valid numbers as the compression
       types of the image data.  Currently, the only  valid  number  is  zero.
       However,  you  can store text either compressed or uncompressed, unlike
       images, which always have to be compressed.  So if you don't  want  the
       text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
       Because tEXt and zTXt chunks don't have a language field, if you  spec-
       ify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt any language
       code or translated keyword will not be written out.

       Until text gets around a few hundred bytes, it is not worth compressing
       it.   After  the text has been written out to the file, the compression
       type  is  set  to  PNG_TEXT_COMPRESSION_NONE_WR  or   PNG_TEXT_COMPRES-
       SION_zTXt_WR,  so  that  it isn't written out again at the end (in case
       you are calling png_write_end() with the same struct).

       The keywords that are given in the PNG Specification are:

           Title            Short (one line) title or
                            caption for image

           Author           Name of image's creator

           Description      Description of image (possibly long)

           Copyright        Copyright notice

           Creation Time    Time of original image creation
                            (usually RFC 1123 format, see below)

           Software         Software used to create the image

           Disclaimer       Legal disclaimer

           Warning          Warning of nature of content

           Source           Device used to create the image

           Comment          Miscellaneous comment; conversion
                            from other image format

       The keyword-text pairs work like this.  Keywords should be short simple
       descriptions  of  what the comment is about.  Some typical keywords are
       found in the PNG specification, as is some recommendations on keywords.
       You can repeat keywords in a file.  You can even write some text before
       the image and some after.  For example, you may want to put a  descrip-
       tion  of the image before the image, but leave the disclaimer until af-
       ter, so viewers working over modem connections don't have to  wait  for
       the disclaimer to go over the modem before they start seeing the image.
       Finally, keywords should be full words,  not  abbreviations.   Keywords
       and  text  are in the ISO 8859-1 (Latin-1) character set (a superset of
       regular ASCII) and can not contain NUL characters, and should not  con-
       tain  control  or  other  unprintable characters.  To make the comments
       widely readable, stick with basic ASCII,  and  avoid  machine  specific
       character  set  extensions  like the IBM-PC character set.  The keyword
       must be present, but you can leave off  the  text  string  on  non-com-
       pressed  pairs.   Compressed pairs must have a text string, as only the
       text string is compressed anyway, so the compression would be  meaning-
       less.

       PNG supports modification time via the png_time structure.  Two conver-
       sion routines are provided, png_convert_from_time_t()  for  time_t  and
       png_convert_from_struct_tm()  for  struct  tm.  The time_t routine uses
       gmtime().  You don't have to use either of these, but if  you  wish  to
       fill in the png_time structure directly, you should provide the time in
       universal time (GMT) if possible instead of your local time.  Note that
       the  year  number  is the full year (e.g. 1998, rather than 98 - PNG is
       year 2000 compliant!), and that months start with 1.

       If you want to store the time  of  the  original  image  creation,  you
       should  use  a plain tEXt chunk with the "Creation Time" keyword.  This
       is necessary because the "creation time" of a  PNG  image  is  somewhat
       vague,  depending  on whether you mean the PNG file, the time the image
       was created in a non-PNG format, a still photo from which the image was
       scanned, or possibly the subject matter itself.  In order to facilitate
       machine-readable dates, it is recommended that the "Creation Time" tEXt
       chunk  use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"), al-
       though this isn't a requirement.  Unlike the tIME chunk, the  "Creation
       Time"  tEXt  chunk  is  not expected to be automatically changed by the
       software.  To facilitate the use of RFC 1123 dates, a function png_con-
       vert_to_rfc1123_buffer(buffer,  png_timep)  is provided to convert from
       PNG time to an RFC 1123 format  string.   The  caller  must  provide  a
       writeable buffer of at least 29 bytes.

   Writing unknown chunks
       You  can  use  the  png_set_unknown_chunks function to queue up private
       chunks for writing.  You give it a chunk name, location, raw data,  and
       a size.  You also must use png_set_keep_unknown_chunks() to ensure that
       libpng will handle them.  That's all there is to it.  The  chunks  will
       be   written   by   the   next   following  png_write_info_before_PLTE,
       png_write_info, or png_write_end function, depending upon the specified
       location.   Any  chunks  previously  read into the info structure's un-
       known-chunk list will also be written out in a sequence that  satisfies
       the PNG specification's ordering rules.

       Here is an example of writing two private chunks, prVt and miNE:

           #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
           /* Set unknown chunk data */
           png_unknown_chunk unk_chunk[2];
           strcpy((char *) unk_chunk[0].name, "prVt";
           unk_chunk[0].data = (unsigned char *) "PRIVATE DATA";
           unk_chunk[0].size = strlen(unk_chunk[0].data)+1;
           unk_chunk[0].location = PNG_HAVE_IHDR;
           strcpy((char *) unk_chunk[1].name, "miNE";
           unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA";
           unk_chunk[1].size = strlen(unk_chunk[0].data)+1;
           unk_chunk[1].location = PNG_AFTER_IDAT;
           png_set_unknown_chunks(write_ptr, write_info_ptr,
               unk_chunk, 2);
           /* Needed because miNE is not safe-to-copy */
           png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS,
              (png_bytep) "miNE", 1);
           # if PNG_LIBPNG_VER < 10600
             /* Deal with unknown chunk location bug in 1.5.x and earlier */
             png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR);
             png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT);
           # endif
           # if PNG_LIBPNG_VER < 10500
             /*  PNG_AFTER_IDAT  writes  two  copies  of  the  chunk  prior to
       libpng-1.5.0,
              * one before IDAT and another after IDAT, so don't use it;  only
       use
              *  PNG_HAVE_IHDR location.  This call resets the location previ-
       ously
              * set by  assignment  and  png_set_unknown_chunk_location()  for
       chunk 1.
              */
             png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR);
           # endif
           #endif

   The high-level write interface
       At  this  point  there  are two ways to proceed; through the high-level
       write interface, or through a sequence of low-level  write  operations.
       You  can  use the high-level interface if your image data is present in
       the info structure.  All defined output transformations are  permitted,
       enabled by the following masks.

           PNG_TRANSFORM_IDENTITY      No transformation
           PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit samples
           PNG_TRANSFORM_PACKSWAP      Change order of packed
                                       pixels to LSB first
           PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
           PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                       sBIT depth
           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
           PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                       to transparency
           PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
           PNG_TRANSFORM_STRIP_FILLER        Strip out filler
                                             bytes (deprecated).
           PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
                                             filler bytes
           PNG_TRANSFORM_STRIP_FILLER_AFTER  Strip out trailing
                                             filler bytes

       If  you  have  valid  image  data  in  the  info structure (you can use
       png_set_rows() to put image data in  the  info  structure),  simply  do
       this:

           png_write_png(png_ptr, info_ptr, png_transforms, NULL)

       where  png_transforms  is  an integer containing the bitwise OR of some
       set  of   transformation   flags.    This   call   is   equivalent   to
       png_write_info(),  followed the set of transformations indicated by the
       transform mask, then png_write_image(), and finally png_write_end().

       (The final parameter of this call is not yet used.   Someday  it  might
       point  to  transformation  parameters  required  by  some future output
       transform.)

       You must use png_transforms and not call any png_set_transform()  func-
       tions when you use png_write_png().

   The low-level write interface
       If  you  are  going  the  low-level route instead, you are now ready to
       write all the file information up to the actual  image  data.   You  do
       this with a call to png_write_info().

           png_write_info(png_ptr, info_ptr);

       Note  that  there  is  one  transformation  you  may  need to do before
       png_write_info().  In PNG files, the alpha channel in an image  is  the
       level of opacity.  If your data is supplied as a level of transparency,
       you can invert the alpha channel before you write  it,  so  that  0  is
       fully  transparent  and  255 (in 8-bit or paletted images) or 65535 (in
       16-bit images) is fully opaque, with

           png_set_invert_alpha(png_ptr);

       This must appear before png_write_info()  instead  of  later  with  the
       other  transformations  because in the case of paletted images the tRNS
       chunk data has to be inverted before the tRNS  chunk  is  written.   If
       your  image is not a paletted image, the tRNS data (which in such cases
       represents a single color to be rendered as transparent) won't need  to
       be  changed,  and  you  can  safely  do  this transformation after your
       png_write_info() call.

       If you need to write a private chunk that you want to appear before the
       PLTE  chunk  when  PLTE  is  present, you can write the PNG info in two
       steps, and insert code to write your own chunk between them:

           png_write_info_before_PLTE(png_ptr, info_ptr);
           png_set_unknown_chunks(png_ptr, info_ptr, ...);
           png_write_info(png_ptr, info_ptr);

       After you've written the file information, you can set up  the  library
       to  handle  any special transformations of the image data.  The various
       ways to transform the data will be described in  the  order  that  they
       should  occur.   This  is  important, as some of these change the color
       type and/or bit depth of the data, and some others only work on certain
       color  types and bit depths.  Even though each transformation checks to
       see if it has data that it can do something with, you should make  sure
       to  only enable a transformation if it will be valid for the data.  For
       example, don't swap red and blue on grayscale data.

       PNG files store RGB pixels packed into 3 or 6 bytes.  This  code  tells
       the library to strip input data that has 4 or 8 bytes per pixel down to
       3 or 6 bytes (or strip 2 or 4-byte grayscale+filler  data  to  1  or  2
       bytes per pixel).

           png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

       where  the 0 is unused, and the location is either PNG_FILLER_BEFORE or
       PNG_FILLER_AFTER, depending upon whether the filler byte in  the  pixel
       is stored XRGB or RGBX.

       PNG  files pack pixels of bit depths 1, 2, and 4 into bytes as small as
       they can, resulting in, for example, 8 pixels per byte for 1 bit files.
       If  the data is supplied at 1 pixel per byte, use this code, which will
       correctly pack the pixels into a single byte:

           png_set_packing(png_ptr);

       PNG files reduce possible bit depths to 1, 2, 4, 8, and  16.   If  your
       data is of another bit depth, you can write an sBIT chunk into the file
       so that decoders can recover the original data if desired.

           /* Set the true bit depth of the image data */
           if (color_type & PNG_COLOR_MASK_COLOR)
           {
              sig_bit.red = true_bit_depth;
              sig_bit.green = true_bit_depth;
              sig_bit.blue = true_bit_depth;
           }

           else
           {
              sig_bit.gray = true_bit_depth;
           }

           if (color_type & PNG_COLOR_MASK_ALPHA)
           {
              sig_bit.alpha = true_bit_depth;
           }

           png_set_sBIT(png_ptr, info_ptr, &sig_bit);

       If the data is stored in the row buffer in a bit depth other  than  one
       supported  by  PNG  (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
       this will scale the values to appear to be the correct bit depth as  is
       required by PNG.

           png_set_shift(png_ptr, &sig_bit);

       PNG  files  store  16-bit pixels in network byte order (big-endian, ie.
       most significant bits first).  This code would be used if they are sup-
       plied  the other way (little-endian, i.e. least significant bits first,
       the way PCs store them):

           if (bit_depth > 8)
              png_set_swap(png_ptr);

       If you are using packed-pixel images (1, 2, or 4 bits/pixel),  and  you
       need to change the order the pixels are packed into bytes, you can use:

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

       PNG  files  store  3 color pixels in red, green, blue order.  This code
       would be used if they are supplied as blue, green, red:

           png_set_bgr(png_ptr);

       PNG files describe monochrome as black being zero and white being  one.
       This  code  would be used if the pixels are supplied with this reversed
       (black being one and white being zero):

           png_set_invert_mono(png_ptr);

       Finally, you can write your own transformation function if none of  the
       existing  ones  meets  your  needs.  This is done by setting a callback
       with

           png_set_write_user_transform_fn(png_ptr,
              write_transform_fn);

       You must supply the function

           void write_transform_fn(png_structp png_ptr, png_row_infop
              row_info, png_bytep data)

       See pngtest.c for a working example.  Your function will be called  be-
       fore  any  of  the  other  transformations are processed.  If supported
       libpng also supplies an information routine that  may  be  called  from
       your callback:

          png_get_current_row_number(png_ptr);
          png_get_current_pass_number(png_ptr);

       This  returns the current row passed to the transform.  With interlaced
       images the value returned is the row in the input sub-image image.  Use
       PNG_ROW_FROM_PASS_ROW(row,  pass)  and PNG_COL_FROM_PASS_COL(col, pass)
       to find the output pixel (x,y)  given  an  interlaced  sub-image  pixel
       (row,col,pass).

       The discussion of interlace handling above contains more information on
       how to use these values.

       You can also set up a pointer to a user structure for use by your call-
       back function.

           png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

       The  user_channels  and  user_depth parameters of this function are ig-
       nored when writing; you can set them to zero as shown.

       You can retrieve  the  pointer  via  the  function  png_get_user_trans-
       form_ptr().  For example:

           voidp write_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

       It  is  possible  to have libpng flush any pending output, either manu-
       ally, or automatically after a certain number of lines have been  writ-
       ten.  To flush the output stream a single time call:

           png_write_flush(png_ptr);

       and to have libpng flush the output stream periodically after a certain
       number of scanlines have been written, call:

           png_set_flush(png_ptr, nrows);

       Note  that  the  distance  between  rows  is   from   the   last   time
       png_write_flush()  was  called, or the first row of the image if it has
       never been called.  So if you write 50 lines,  and  then  png_set_flush
       25,  it  will flush the output on the next scanline, and every 25 lines
       thereafter, unless png_write_flush() is called  before  25  more  lines
       have been written.  If nrows is too small (less than about 10 lines for
       a 640 pixel wide RGB image) the image compression may decrease  notice-
       ably (although this may be acceptable for real-time applications).  In-
       frequent flushing will only degrade the compression  performance  by  a
       few percent over images that do not use flushing.

   Writing the image data
       That's  it  for the transformations.  Now you can write the image data.
       The simplest way to do this is in one function call.  If you  have  the
       whole  image  in memory, you can just call png_write_image() and libpng
       will write the image.  You will need to pass in an array of pointers to
       each  row.   This  function  automatically  handles interlacing, so you
       don't need to call png_set_interlace_handling() or call  this  function
       multiple   times,   or   any   of   that  other  stuff  necessary  with
       png_write_rows().

           png_write_image(png_ptr, row_pointers);

       where row_pointers is:

           png_byte *row_pointers[height];

       You can point to void or char or whatever you use for pixels.

       If you don't want to write  the  whole  image  at  once,  you  can  use
       png_write_rows()  instead.  If the file is not interlaced, this is sim-
       ple:

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

       row_pointers is the same as in the png_write_image() call.

       If you are just writing one row at a time, you can do this with a  sin-
       gle row_pointer instead of an array of row_pointers:

           png_bytep row_pointer = row;

           png_write_row(png_ptr, row_pointer);

       When  the  file  is interlaced, things can get a good deal more compli-
       cated.  The only currently (as of the PNG  Specification  version  1.2,
       dated  July  1999)  defined  interlacing  scheme  for  PNG files is the
       "Adam7" interlace scheme, that breaks down an image into seven  smaller
       images of varying size.  libpng will build these images for you, or you
       can do them yourself.  If you want to build them yourself, see the  PNG
       specification for details of which pixels to write when.

       If  you  don't  want libpng to handle the interlacing details, just use
       png_set_interlace_handling() and call png_write_rows() the correct num-
       ber  of times to write all the sub-images (png_set_interlace_handling()
       returns the number of sub-images.)

       If you want libpng to build the sub-images, call this before you  start
       writing any rows:

           number_of_passes = png_set_interlace_handling(png_ptr);

       This  will  return  the  number  of  passes needed.  Currently, this is
       seven, but may change if another interlace type is added.

       Then write the complete image number_of_passes times.

           png_write_rows(png_ptr, row_pointers, number_of_rows);

       Think carefully before you write an interlaced image.   Typically  code
       that  reads  such  images  reads all the image data into memory, uncom-
       pressed, before doing any processing.  Only code that  can  display  an
       image  on  the  fly can take advantage of the interlacing and even then
       the image has to be exactly the correct size for the output device, be-
       cause  scaling  an  image  requires  adjacent  pixels and these are not
       available until all the passes have been read.

       If you do write an interlaced image you will hardly ever need to handle
       the  interlacing  yourself.   Call png_set_interlace_handling() and use
       the approach described above.

       The only time it is conceivable that you will really need to  write  an
       interlaced  image  pass-by-pass  is when you have read one pass by pass
       and made some pixel-by-pixel transformation to it, as described in  the
       read  code above.  In this case use the PNG_PASS_ROWS and PNG_PASS_COLS
       macros to determine the size of each sub-image in turn and simply write
       the rows you obtained from the read code.

   Finishing a sequential write
       After you are finished writing the image, you should finish writing the
       file.  If you are interested in writing comments or  time,  you  should
       pass  an  appropriately filled png_info pointer.  If you are not inter-
       ested, you can pass NULL.

           png_write_end(png_ptr, info_ptr);

       When you are done, you can free all memory used by libpng like this:

           png_destroy_write_struct(&png_ptr, &info_ptr);

       It is also possible to individually  free  the  info_ptr  members  that
       point to libpng-allocated storage with the following function:

           png_free_data(png_ptr, info_ptr, mask, seq)

           mask  - identifies data to be freed, a mask
                   containing the bitwise OR of one or
                   more of
                     PNG_FREE_PLTE, PNG_FREE_TRNS,
                     PNG_FREE_HIST, PNG_FREE_ICCP,
                     PNG_FREE_PCAL, PNG_FREE_ROWS,
                     PNG_FREE_SCAL, PNG_FREE_SPLT,
                     PNG_FREE_TEXT, PNG_FREE_UNKN,
                   or simply PNG_FREE_ALL

           seq   - sequence number of item to be freed
                   (-1 for all items)

       This  function  may  be safely called when the relevant storage has al-
       ready been freed, or has not yet been allocated, or  was  allocated  by
       the  user  and not by libpng,  and will in those cases do nothing.  The
       "seq" parameter is ignored if only one item of the selected data  type,
       such  as  PLTE, is allowed.  If "seq" is not -1, and multiple items are
       allowed for the data type identified in the mask, such as text or sPLT,
       only the n'th item in the structure is freed, where n is "seq".

       If  you  allocated  data such as a palette that you passed in to libpng
       with png_set_*, you must not free it until  just  before  the  call  to
       png_destroy_write_struct().

       The default behavior is only to free data that was allocated internally
       by libpng.  This can be changed, so that libpng will not free the data,
       or  so  that  it  will  free  data  that was allocated by the user with
       png_malloc() or png_calloc() and passed in via a png_set_*()  function,
       with

           png_data_freer(png_ptr, info_ptr, freer, mask)

           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

           mask   - which data elements are affected
                    same choices as in png_free_data()

       For  example,  to  transfer  responsibility  for  some data from a read
       structure to a write structure, you could use

           png_data_freer(read_ptr, read_info_ptr,
              PNG_USER_WILL_FREE_DATA,
              PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

           png_data_freer(write_ptr, write_info_ptr,
              PNG_DESTROY_WILL_FREE_DATA,
              PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

       thereby briefly reassigning responsibility for freeing to the user  but
       immediately  afterwards  reassigning  it once more to the write_destroy
       function.  Having done this, it would then be safe to destroy the  read
       structure  and  continue  to  use  the PLTE, tRNS, and hIST data in the
       write structure.

       This function only affects data that has already been  allocated.   You
       can  call  this function before calling after the png_set_*() functions
       to control whether the user or png_destroy_*() is supposed to free  the
       data.   When the user assumes responsibility for libpng-allocated data,
       the application must use png_free() to  free  it,  and  when  the  user
       transfers  responsibility  to  libpng  for data that the user has allo-
       cated, the user must have used png_malloc() or png_calloc() to allocate
       it.

       If  you  allocated  text_ptr.text,  text_ptr.lang,  and text_ptr.trans-
       lated_keyword separately, do not transfer  responsibility  for  freeing
       text_ptr  to  libpng, because when libpng fills a png_text structure it
       combines these members with the key member,  and  png_free_data()  will
       free  only text_ptr.key.  Similarly, if you transfer responsibility for
       free'ing text_ptr from libpng to  your  application,  your  application
       must  not separately free those members.  For a more compact example of
       writing a PNG image, see the file example.c.

V. Simplified API
       The simplified API, which became available in libpng-1.6.0,  hides  the
       details  of  both libpng and the PNG file format itself.  It allows PNG
       files to be read into a very limited number of in-memory bitmap formats
       or to be written from the same formats.  If these formats do not accom-
       modate your needs then you can, and should, use the more  sophisticated
       APIs  above  -  these support a wide variety of in-memory formats and a
       wide variety of sophisticated transformations to those formats as  well
       as a wide variety of APIs to manipulate ancillary information.

       To read a PNG file using the simplified API:

         1) Declare a 'png_image' structure (see below) on the stack, set the
            version  field  to  PNG_IMAGE_VERSION  and the 'opaque' pointer to
       NULL
            (this is REQUIRED, your program may crash if you don't do it.)

         2) Call the appropriate png_image_begin_read... function.

         3) Set the png_image 'format' member to the required sample format.

         4) Allocate a buffer for the image and, if required, the color-map.

         5) Call png_image_finish_read to read the image and, if required, the
            color-map into your buffers.

       There are no restrictions on the format of the PNG  input  itself;  all
       valid  color  types,  bit depths, and interlace methods are acceptable,
       and the input image is transformed as necessary to  the  requested  in-
       memory format during the png_image_finish_read() step.  The only caveat
       is that if you request a color-mapped image from a PNG  that  is  full-
       color  or  makes  complex use of an alpha channel the transformation is
       extremely lossy and the result may look terrible.

       To write a PNG file using the simplified API:

         1) Declare a 'png_image' structure on the stack and memset()
            it to all zero.

         2) Initialize the members of the structure that describe the
            image, setting the 'format' member to the format of the
            image samples.

         3) Call the appropriate png_image_write... function with a
            pointer to the image and, if necessary, the color-map to write
            the PNG data.

       png_image is a structure that describes the in-memory format of an  im-
       age  when  it is being read or defines the in-memory format of an image
       that you need to write.  The "png_image" structure contains the follow-
       ing members:

          png_controlp opaque  Initialize to NULL, free with png_image_free
          png_uint_32  version Set to PNG_IMAGE_VERSION
          png_uint_32  width   Image width in pixels (columns)
          png_uint_32  height  Image height in pixels (rows)
          png_uint_32  format  Image format as defined below
          png_uint_32  flags   A bit mask containing informational flags
          png_uint_32  colormap_entries; Number of entries in the color-map
          png_uint_32  warning_or_error;
          char         message[64];

       In  the  event of an error or warning the "warning_or_error" field will
       be set to a non-zero value and the 'message' field will contain  a  ' '
       terminated  string  with  the libpng error or warning message.  If both
       warnings and an error were encountered, only the error is recorded.  If
       there are multiple warnings, only the first one is recorded.

       The upper 30 bits of the "warning_or_error" value are reserved; the low
       two bits contain a two bit code such that a value more than 1 indicates
       a failure in the API just called:

          0 - no warning or error
          1 - warning
          2 - error
          3 - error preceded by warning

       The  pixels (samples) of the image have one to four channels whose com-
       ponents have original values in the range 0 to 1.0:

         1: A single gray or luminance channel (G).
         2: A gray/luminance channel and an alpha channel (GA).
         3: Three red, green, blue color channels (RGB).
         4: Three color channels and an alpha channel (RGBA).

       The channels are encoded in one of two ways:

         a) As a small integer, value 0..255, contained in a single byte.  For
       the  alpha  channel  the  original  value is simply value/255.  For the
       color or luminance channels the value is encoded according to the  sRGB
       specification  and matches the 8-bit format expected by typical display
       devices.

       The color/gray channels are not scaled (pre-multiplied)  by  the  alpha
       channel and are suitable for passing to color management software.

         b)  As  a value in the range 0..65535, contained in a 2-byte integer,
       in the native byte order of the platform on which  the  application  is
       running.  All channels can be converted to the original value by divid-
       ing by 65535; all channels are linear.  Color channels use the RGB  en-
       coding  (RGB  end-points)  of the sRGB specification.  This encoding is
       identified by the PNG_FORMAT_FLAG_LINEAR flag below.

       When the simplified API needs to convert between sRGB and  linear  col-
       orspaces, the actual sRGB transfer curve defined in the sRGB specifica-
       tion (see the article at https://en.wikipedia.org/wiki/SRGB)  is  used,
       not the gamma=1/2.2 approximation used elsewhere in libpng.

       When  an alpha channel is present it is expected to denote pixel cover-
       age of the color or luminance channels and is returned as an associated
       alpha  channel:  the color/gray channels are scaled (pre-multiplied) by
       the alpha value.

       The samples are either contained directly in the image data, between  1
       and  8  bytes  per  pixel  according  to the encoding, or are held in a
       color-map indexed by bytes in the image data.  In the case of a  color-
       map the color-map entries are individual samples, encoded as above, and
       the image data has one byte per pixel to  select  the  relevant  sample
       from the color-map.

       PNG_FORMAT_*

       The  #defines to be used in png_image::format.  Each #define identifies
       a particular layout of channel data  and,  if  present,  alpha  values.
       There are separate defines for each of the two component encodings.

       A  format  is  built up using single bit flag values.  All combinations
       are valid.  Formats can be built up from the flag values or you can use
       one  of  the  predefined values below.  When testing formats always use
       the FORMAT_FLAG macros to test for individual features  -  future  ver-
       sions of the library may add new flags.

       When reading or writing color-mapped images the format should be set to
       the  format  of   the   entries   in   the   color-map   then   png_im-
       age_{read,write}_colormap called to read or write the color-map and set
       the format correctly for the image  data.   Do  not  set  the  PNG_FOR-
       MAT_FLAG_COLORMAP bit directly!

       NOTE: libpng can be built with particular features disabled. If you see
       compiler errors because the definition of one of  the  following  flags
       has  been  compiled out it is because libpng does not have the required
       support.  It is possible, however, for the libpng configuration to  en-
       able the format on just read or just write; in that case you may see an
       error at run time.  You can guard against this by checking for the def-
       inition of the appropriate "_SUPPORTED" macro, one of:

          PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED

          PNG_FORMAT_FLAG_ALPHA    format with an alpha channel
          PNG_FORMAT_FLAG_COLOR    color format: otherwise grayscale
          PNG_FORMAT_FLAG_LINEAR   2-byte channels else 1-byte
          PNG_FORMAT_FLAG_COLORMAP image data is color-mapped
          PNG_FORMAT_FLAG_BGR      BGR colors, else order is RGB
          PNG_FORMAT_FLAG_AFIRST   alpha channel comes first

       Supported  formats  are as follows.  Future versions of libpng may sup-
       port more formats; for compatibility with older versions  simply  check
       if  the  format  macro is defined using #ifdef.  These defines describe
       the in-memory layout of the components of the pixels of the image.

       First the single byte (sRGB) formats:

          PNG_FORMAT_GRAY
          PNG_FORMAT_GA
          PNG_FORMAT_AG
          PNG_FORMAT_RGB
          PNG_FORMAT_BGR
          PNG_FORMAT_RGBA
          PNG_FORMAT_ARGB
          PNG_FORMAT_BGRA
          PNG_FORMAT_ABGR

       Then the linear 2-byte formats.  When naming these "Y" is used to indi-
       cate  a luminance (gray) channel.  The component order within the pixel
       is always the same - there is no provision for swapping  the  order  of
       the  components  in the linear format.  The components are 16-bit inte-
       gers in the native byte order for your platform, and there is no provi-
       sion for swapping the bytes to a different endian condition.

          PNG_FORMAT_LINEAR_Y
          PNG_FORMAT_LINEAR_Y_ALPHA
          PNG_FORMAT_LINEAR_RGB
          PNG_FORMAT_LINEAR_RGB_ALPHA

       With  color-mapped  formats  the image data is one byte for each pixel.
       The byte is an index into the color-map which is  formatted  as  above.
       To  obtain  a  color-mapped  format  it  is  sufficient just to add the
       PNG_FOMAT_FLAG_COLORMAP to one of the above definitions, or you can use
       one of the definitions below.

          PNG_FORMAT_RGB_COLORMAP
          PNG_FORMAT_BGR_COLORMAP
          PNG_FORMAT_RGBA_COLORMAP
          PNG_FORMAT_ARGB_COLORMAP
          PNG_FORMAT_BGRA_COLORMAP
          PNG_FORMAT_ABGR_COLORMAP

       PNG_IMAGE macros

       These  are  convenience  macros  to derive information from a png_image
       structure.  The PNG_IMAGE_SAMPLE_ macros return values  appropriate  to
       the actual image sample values - either the entries in the color-map or
       the pixels in the image.  The  PNG_IMAGE_PIXEL_  macros  return  corre-
       sponding  values  for  the  pixels  and will always return 1 for color-
       mapped formats.  The remaining macros return information about the rows
       in the image and the complete image.

       NOTE:  All  the macros that take a png_image::format parameter are com-
       pile time constants if the format parameter  is,  itself,  a  constant.
       Therefore  these  macros can be used in array declarations and case la-
       bels where required.  Similarly the macros are also pre-processor  con-
       stants (sizeof is not used) so they can be used in #if tests.

         PNG_IMAGE_SAMPLE_CHANNELS(fmt)
           Returns the total number of channels in a given format: 1..4

         PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt)
           Returns  the  size  in  bytes  of  a single component of a pixel or
       color-map
           entry (as appropriate) in the image: 1 or 2.

         PNG_IMAGE_SAMPLE_SIZE(fmt)
           This is the size of the sample data for one sample.  If  the  image
       is
           color-mapped  it is the size of one color-map entry (and image pix-
       els are
           one byte in size), otherwise it is the size of one image pixel.

         PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt)
           The maximum size of the color-map required by the format  expressed
       in a
           count of components.  This can be used to compile-time allocate a
           color-map:

           png_uint_16     colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(lin-
       ear_fmt)];

           png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)];

           Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro  below  to  use
       the
           information  from one of the png_image_begin_read_ APIs and dynami-
       cally
           allocate the required memory.

         PNG_IMAGE_COLORMAP_SIZE(fmt)
          The size of the color-map required by the format; this is  the  size
       of the
          color-map buffer passed to the png_image_{read,write}_colormap APIs.
       It is
          a fixed number determined by the format so can easily  be  allocated
       on the
          stack if necessary.

       Corresponding information about the pixels

         PNG_IMAGE_PIXEL_CHANNELS(fmt)
          The number of separate channels (components) in a pixel; 1 for a
          color-mapped image.

         PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)    The  size,  in  bytes, of each
       component in a pixel; 1 for a color-mapped
          image.

         PNG_IMAGE_PIXEL_SIZE(fmt)
          The size, in bytes, of a complete pixel; 1 for a color-mapped image.

       Information about the whole row, or whole image

         PNG_IMAGE_ROW_STRIDE(image)
          Returns the total number of components in a single row of the image;
       this
          is the minimum 'row stride', the minimum count of components between
       each
          row.  For a color-mapped image this is the minimum number  of  bytes
       in a
          row.

          If you need the stride measured in bytes, row_stride_bytes is
          PNG_IMAGE_ROW_STRIDE(image) * PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)
          plus any padding bytes that your application might need, for example
          to start the next row on a 4-byte boundary.

         PNG_IMAGE_BUFFER_SIZE(image, row_stride)
          Return  the size, in bytes, of an image buffer given a png_image and
       a row
          stride - the number of components to leave space for in each row.

         PNG_IMAGE_SIZE(image)
          Return the size, in bytes, of the  image  in  memory  given  just  a
       png_image;
          the row stride is the minimum stride required for the image.

         PNG_IMAGE_COLORMAP_SIZE(image)
          Return  the  size, in bytes, of the color-map of this image.  If the
       image
          format is not a color-map format this will return a size  sufficient
       for
          256 entries in the given format; check PNG_FORMAT_FLAG_COLORMAP if
          you don't want to allocate a color-map in this case.

       PNG_IMAGE_FLAG_*

       Flags containing additional information about the image are held in the
       'flags' field of png_image.

         PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01
           This indicates that the RGB values of the in-memory bitmap do not
           correspond to the red, green and blue end-points defined by sRGB.

         PNG_IMAGE_FLAG_FAST == 0x02
          On write emphasise speed over compression; the  resultant  PNG  file
       will be
          larger  but  will  be  produced significantly faster, particular for
       large
          images.  Do not use this option for images which  will  be  distrib-
       uted, only
          used it when producing intermediate files that will be read back in
          repeatedly.   For  a typical 24-bit image the option will double the
       read
          speed at the cost of increasing the image size by 25%,  however  for
       many
          more  compressible  images  the PNG file can be 10 times larger with
       only a
          slight speed gain.

         PNG_IMAGE_FLAG_16BIT_sRGB == 0x04
           On read if the image is a 16-bit per component image and  there  is
       no gAMA
           or  sRGB chunk assume that the components are sRGB encoded.  Notice
       that
           images output by the simplified API always have gamma  information;
       setting
           this flag only affects the interpretation of 16-bit images from an
           external  source.   It  is  recommended that the application expose
       this flag
           to the user; the user can normally easily recognize the  difference
       between
           linear  and  sRGB encoding.  This flag has no effect on write - the
       data
           passed to the write APIs must have the correct encoding (as defined
           above.)

           If the flag is not set (the default)  input  16-bit  per  component
       data is
           assumed to be linear.

           NOTE:  the  flag  can  only  be set after the png_image_begin_read_
       call,
           because that call initializes the 'flags' field.

       READ APIs

          The png_image passed to the read APIs must have been initialized  by
       setting
          the  png_controlp  field  'opaque'  to  NULL (or, better, memset the
       whole thing.)

          int png_image_begin_read_from_file( png_imagep image,
            const char *file_name)

            The named file is opened for read and the image header
            is filled in from the PNG header in the file.

          int png_image_begin_read_from_stdio (png_imagep image,
            FILE* file)

             The PNG header is read from the stdio FILE object.

          int png_image_begin_read_from_memory(png_imagep image,
             png_const_voidp memory, size_t size)

             The PNG header is read from the given memory buffer.

          int png_image_finish_read(png_imagep image,
             png_colorp background, void *buffer,
             png_int_32 row_stride, void *colormap));

             Finish reading the image into the supplied buffer and
             clean up the png_image structure.

             row_stride is the step, in png_byte or png_uint_16 units
             as appropriate, between adjacent rows.  A positive stride
             indicates that the top-most row is first in the buffer -
             the normal top-down arrangement.  A negative stride
             indicates that the bottom-most row is first in the buffer.

             background need only be supplied if an alpha channel must
             be removed from a png_byte format and the removal is to be
             done by compositing on a solid color; otherwise it may be
             NULL and any composition will be done directly onto the
             buffer.  The value is an sRGB color to use for the
             background, for grayscale output the green channel is used.

             For linear output removing the alpha channel is always done
             by compositing on black.

          void png_image_free(png_imagep image)

             Free any data allocated by libpng in image->opaque,
             setting the pointer to NULL.  May be called at any time
             after the structure is initialized.

       When the simplified API needs to convert between sRGB and  linear  col-
       orspaces, the actual sRGB transfer curve defined in the sRGB specifica-
       tion (see the article at https://en.wikipedia.org/wiki/SRGB)  is  used,
       not the gamma=1/2.2 approximation used elsewhere in libpng.

       WRITE APIS

       For write you must initialize a png_image structure to describe the im-
       age to be written:

          version: must be set to PNG_IMAGE_VERSION
          opaque: must be initialized to NULL
          width: image width in pixels
          height: image height in rows
          format: the format of the data you wish to write
          flags: set to 0 unless one of the defined flags applies; set
             PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images
             where the RGB values do not correspond to the colors in sRGB.
          colormap_entries: set to the number of entries in the  color-map  (0
       to 256)

          int png_image_write_to_file, (png_imagep image,
             const char *file, int convert_to_8bit, const void *buffer,
             png_int_32 row_stride, const void *colormap));

             Write the image to the named file.

          int png_image_write_to_memory (png_imagep image, void *memory,
             png_alloc_size_t * PNG_RESTRICT memory_bytes,
             int convert_to_8_bit, const void *buffer, ptrdiff_t row_stride,
             const void *colormap));

             Write the image to memory.

          int png_image_write_to_stdio(png_imagep image, FILE *file,
             int convert_to_8_bit, const void *buffer,
             png_int_32 row_stride, const void *colormap)

             Write the image to the given (FILE*).

       With  all  write  APIs  if  image  is in one of the linear formats with
       (png_uint_16) data then setting convert_to_8_bit will cause the  output
       to  be  a (png_byte) PNG gamma encoded according to the sRGB specifica-
       tion, otherwise a 16-bit linear encoded PNG file is written.

       With all APIs row_stride is handled as in the read APIs  -  it  is  the
       spacing  from  one row to the next in component sized units (float) and
       if negative indicates a bottom-up row layout in  the  buffer.   If  you
       pass  zero, libpng will calculate the row_stride for you from the width
       and number of channels.

       Note that the write API does not support interlacing, sub-8-bit pixels,
       indexed (paletted) images, or most ancillary chunks.

VI. Modifying/Customizing libpng
       There are two issues here.  The first is changing how libpng does stan-
       dard things like memory allocation, input/output, and  error  handling.
       The  second  deals with more complicated things like adding new chunks,
       adding new transformations, and generally changing  how  libpng  works.
       Both  of those are compile-time issues; that is, they are generally de-
       termined at the time the code is written, and there is rarely a need to
       provide the user with a means of changing them.

       Memory allocation, input/output, and error handling

       All  of  the  memory  allocation,  input/output,  and error handling in
       libpng goes through callbacks that are user-settable.  The default rou-
       tines  are  in  pngmem.c,  pngrio.c,  pngwio.c, and pngerror.c, respec-
       tively.  To change these functions, call the appropriate png_set_*_fn()
       function.

       Memory  allocation is done through the functions png_malloc(), png_cal-
       loc(), and png_free().  The png_malloc() and png_free() functions  cur-
       rently  just  call  the  standard  C  functions  and png_calloc() calls
       png_malloc() and then clears the newly allocated memory to  zero;  note
       that  png_calloc(png_ptr,  size)  is not the same as the calloc(number,
       size) function provided by stdlib.h.  There is limited support for cer-
       tain  systems  with  segmented  memory  architectures  and the types of
       pointers declared by png.h match this; you will have to use appropriate
       pointers  in your application.  If you prefer to use a different method
       of allocating and freeing data, you can use  png_create_read_struct_2()
       or  png_create_write_struct_2()  to  register your own functions as de-
       scribed above.  These functions also provide a void pointer that can be
       retrieved via

           mem_ptr=png_get_mem_ptr(png_ptr);

       Your replacement memory functions must have prototypes as follows:

           png_voidp malloc_fn(png_structp png_ptr,
              png_alloc_size_t size);

           void free_fn(png_structp png_ptr, png_voidp ptr);

       Your malloc_fn() must return NULL in case of failure.  The png_malloc()
       function will normally call png_error() if it receives a NULL from  the
       system memory allocator or from your replacement malloc_fn().

       Your  free_fn()  will  never  be called with a NULL ptr, since libpng's
       png_free() checks for NULL before calling free_fn().

       Input/Output in libpng is  done  through  png_read()  and  png_write(),
       which  currently  just call fread() and fwrite().  The FILE * is stored
       in png_struct and is initialized via png_init_io().   If  you  wish  to
       change  the  method of I/O, the library supplies callbacks that you can
       set through the function png_set_read_fn()  and  png_set_write_fn()  at
       run  time,  instead of calling the png_init_io() function.  These func-
       tions also provide a void pointer that can be retrieved via  the  func-
       tion png_get_io_ptr().  For example:

           png_set_read_fn(png_structp read_ptr,
               voidp read_io_ptr, png_rw_ptr read_data_fn)

           png_set_write_fn(png_structp write_ptr,
               voidp write_io_ptr, png_rw_ptr write_data_fn,
               png_flush_ptr output_flush_fn);

           voidp read_io_ptr = png_get_io_ptr(read_ptr);
           voidp write_io_ptr = png_get_io_ptr(write_ptr);

       The replacement I/O functions must have prototypes as follows:

           void user_read_data(png_structp png_ptr,
               png_bytep data, size_t length);

           void user_write_data(png_structp png_ptr,
               png_bytep data, size_t length);

           void user_flush_data(png_structp png_ptr);

       The user_read_data() function is responsible for detecting and handling
       end-of-data errors.

       Supplying NULL for the read, write, or flush functions sets  them  back
       to  using  the  default  C stream functions, which expect the io_ptr to
       point to a standard *FILE structure.  It is probably a mistake  to  use
       NULL for one of write_data_fn and output_flush_fn but not both of them,
       unless you have built libpng with PNG_NO_WRITE_FLUSH defined.  It is an
       error to read from a write stream, and vice versa.

       Error handling in libpng is done through png_error() and png_warning().
       Errors handled through png_error() are fatal, meaning that  png_error()
       should  never  return  to  its  caller.  Currently, this is handled via
       setjmp()  and  longjmp()  (unless  you  have   compiled   libpng   with
       PNG_NO_SETJMP,  in  which  case it is handled via PNG_ABORT()), but you
       could change this to do things like exit() if you should wish, as  long
       as your function does not return.

       On  non-fatal  errors,  png_warning() is called to print a warning mes-
       sage, and then control returns to the calling code.  By default png_er-
       ror()  and png_warning() print a message on stderr via fprintf() unless
       the library is compiled with  PNG_NO_CONSOLE_IO  defined  (because  you
       don't  want  the  messages)  or PNG_NO_STDIO defined (because fprintf()
       isn't available).  If you wish to change  the  behavior  of  the  error
       functions,  you  will need to set up your own message callbacks.  These
       functions are normally supplied at the time that the png_struct is cre-
       ated.   It is also possible to redirect errors and warnings to your own
       replacement functions after png_create_*_struct() has  been  called  by
       calling:

           png_set_error_fn(png_structp png_ptr,
               png_voidp error_ptr, png_error_ptr error_fn,
               png_error_ptr warning_fn);

       If  NULL is supplied for either error_fn or warning_fn, then the libpng
       default function will be used, calling fprintf() and/or longjmp() if  a
       problem  is  encountered.   The replacement error functions should have
       parameters as follows:

           void user_error_fn(png_structp png_ptr,
               png_const_charp error_msg);

           void user_warning_fn(png_structp png_ptr,
               png_const_charp warning_msg);

       Then, within your user_error_fn or user_warning_fn,  you  can  retrieve
       the error_ptr if you need it, by calling

           png_voidp error_ptr = png_get_error_ptr(png_ptr);

       The motivation behind using setjmp() and longjmp() is the C++ throw and
       catch exception handling methods.  This makes the code much  easier  to
       write, as there is no need to check every return code of every function
       call.  However, there are some uncertainties about the status of  local
       variables after a longjmp, so the user may want to be careful about do-
       ing anything after setjmp returns non-zero  besides  returning  itself.
       Consult  your compiler documentation for more details.  For an alterna-
       tive approach,  you  may  wish  to  use  the  "cexcept"  facility  (see
       https://cexcept.sourceforge.io/),  which  is  illustrated in pngvalid.c
       and in contrib/visupng.

       Beginning  in  libpng-1.4.0,  the  png_set_benign_errors()  API  became
       available.  You can use this to handle certain errors (normally handled
       as errors) as warnings.

           png_set_benign_errors (png_ptr, int allowed);

           allowed: 0: treat png_benign_error() as an error.
                    1: treat png_benign_error() as a warning.

       As of libpng-1.6.0, the default condition is to treat benign errors  as
       warnings while reading and as errors while writing.

   Custom chunks
       If  you need to read or write custom chunks, you may need to get deeper
       into the libpng code.  The library now has mechanisms for  storing  and
       writing chunks of unknown type; you can even declare callbacks for cus-
       tom chunks.  However, this may not be good enough if the  library  code
       itself needs to know about interactions between your chunk and existing
       `intrinsic' chunks.

       If you need to write a new intrinsic chunk, first read the PNG specifi-
       cation.  Acquire  a  first level of understanding of how it works.  Pay
       particular attention to the sections that  describe  chunk  names,  and
       look at how other chunks were designed, so you can do things similarly.
       Second, check out the sections of libpng that read  and  write  chunks.
       Try  to find a chunk that is similar to yours and use it as a template.
       More details can be found in the comments inside the code.  It is  best
       to  handle  private or unknown chunks in a generic method, via callback
       functions, instead of by modifying libpng  functions.  This  is  illus-
       trated in pngtest.c, which uses a callback function to handle a private
       "vpAg" chunk and the new  "sTER"  chunk,  which  are  both  unknown  to
       libpng.

       If you wish to write your own transformation for the data, look through
       the part of the code that does the transformations, and check out  some
       of  the  simpler  ones  to get an idea of how they work.  Try to find a
       similar transformation to the one you want to add and copy off  of  it.
       More details can be found in the comments inside the code itself.

   Configuring for gui/windowing platforms:
       You will need to write new error and warning functions that use the GUI
       interface, as described previously, and set them to be  the  error  and
       warning  functions at the time that png_create_*_struct() is called, in
       order to have them available during the structure initialization.  They
       can  be  changed  later via png_set_error_fn().  On some compilers, you
       may also have to change the memory allocators (png_malloc, etc.).

   Configuring zlib:
       There are special functions to configure the compression.  Perhaps  the
       most useful one changes the compression level, which currently uses in-
       put compression values in the range 0 - 9.  The library  normally  uses
       the  default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests have
       shown that for a large majority of images, compression  values  in  the
       range  3-6  compress  nearly  as  well as higher levels, and do so much
       faster.  For online applications it may be desirable  to  have  maximum
       speed  (Z_BEST_SPEED  = 1).  With versions of zlib after v0.99, you can
       also specify no compression (Z_NO_COMPRESSION = 0), but this would cre-
       ate files larger than just storing the raw bitmap.  You can specify the
       compression level by calling:

           #include zlib.h
           png_set_compression_level(png_ptr, level);

       Another useful one is to reduce the memory level used by  the  library.
       The  memory level defaults to 8, but it can be lowered if you are short
       on memory (running DOS, for example, where you only have  640K).   Note
       that  the  memory level does have an effect on compression; among other
       things, lower levels will result in sections of incompressible data be-
       ing  emitted  in  smaller  stored blocks, with a correspondingly larger
       relative overhead of up to 15% in the worst case.

           #include zlib.h
           png_set_compression_mem_level(png_ptr, level);

       The other functions are for configuring zlib.  They are not recommended
       for  normal  use  and  may  result in writing an invalid PNG file.  See
       zlib.h for more information on what these mean.

           #include zlib.h
           png_set_compression_strategy(png_ptr,
               strategy);

           png_set_compression_window_bits(png_ptr,
               window_bits);

           png_set_compression_method(png_ptr, method);

       This controls the size of the IDAT chunks (default 8192):

           png_set_compression_buffer_size(png_ptr, size);

       As of libpng version 1.5.4, additional APIs  became  available  to  set
       these separately for non-IDAT compressed chunks such as zTXt, iTXt, and
       iCCP:

           #include zlib.h
           #if PNG_LIBPNG_VER >= 10504
           png_set_text_compression_level(png_ptr, level);

           png_set_text_compression_mem_level(png_ptr, level);

           png_set_text_compression_strategy(png_ptr,
               strategy);

           png_set_text_compression_window_bits(png_ptr,
               window_bits);

           png_set_text_compression_method(png_ptr, method);
           #endif

   Controlling row filtering
       If you want to control whether libpng uses filtering or not, which fil-
       ters  are used, and how it goes about picking row filters, you can call
       one of these functions.  The selection and configuration of row filters
       can  have  a  significant  impact  on the size and encoding speed and a
       somewhat lesser impact on the decoding speed of an image.  Filtering is
       enabled  by  default for RGB and grayscale images (with and without al-
       pha), but not for paletted images nor for any images  with  bit  depths
       less than 8 bits/pixel.

       The  'method'  parameter  sets the main filtering method, which is cur-
       rently only '0' in the PNG 1.2 specification.  The 'filters'  parameter
       sets which filter(s), if any, should be used for each scanline.  Possi-
       ble values are PNG_ALL_FILTERS, PNG_NO_FILTERS, or PNG_FAST_FILTERS  to
       turn  filtering on and off, or to turn on just the fast-decoding subset
       of filters, respectively.

       Individual filter types are PNG_FILTER_NONE,  PNG_FILTER_SUB,  PNG_FIL-
       TER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise ORed to-
       gether with '|' to specify one or more filters to use.   These  filters
       are  described  in more detail in the PNG specification.  If you intend
       to change the filter type during the course of writing the  image,  you
       should start with flags set for all of the filters you intend to use so
       that libpng can initialize its internal  structures  appropriately  for
       all  of the filter types.  (Note that this means the first row must al-
       ways be adaptively filtered, because libpng currently does not allocate
       the filter buffers until png_write_row() is called for the first time.)

           filters = PNG_NO_FILTERS;
           filters = PNG_ALL_FILTERS;
           filters = PNG_FAST_FILTERS;

           or

           filters = PNG_FILTER_NONE | PNG_FILTER_SUB |
                     PNG_FILTER_UP | PNG_FILTER_AVG |
                     PNG_FILTER_PAETH;

           png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
              filters);

                     The second parameter can also be
                     PNG_INTRAPIXEL_DIFFERENCING if you are
                     writing a PNG to be embedded in a MNG
                     datastream.  This parameter must be the
                     same as the value of filter_method used
                     in png_set_IHDR().

   Requesting debug printout
       The  macro definition PNG_DEBUG can be used to request debugging print-
       out.  Set it to an integer value in the range 0 to 3.   Higher  numbers
       result in increasing amounts of debugging information.  The information
       is printed to the "stderr" file, unless another file name is  specified
       in the PNG_DEBUG_FILE macro definition.

       When PNG_DEBUG > 0, the following functions (macros) become available:

          png_debug(level, message)
          png_debug1(level, message, p1)
          png_debug2(level, message, p1, p2)

       in  which  "level"  is compared to PNG_DEBUG to decide whether to print
       the message, "message" is the formatted string to be  printed,  and  p1
       and  p2  are parameters that are to be embedded in the string according
       to printf-style formatting directives.  For example,

          png_debug1(2, "foo=%d", foo);

       is expanded to

          if (PNG_DEBUG > 2)
             fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);

       When PNG_DEBUG is defined but is zero, the macros aren't  defined,  but
       you can still use PNG_DEBUG to control your own debugging:

          #ifdef PNG_DEBUG
              fprintf(stderr, ...
          #endif

       When  PNG_DEBUG  = 1, the macros are defined, but only png_debug state-
       ments having level = 0 will be printed.  There aren't any  such  state-
       ments  in  this  version of libpng, but if you insert some they will be
       printed.

VII. MNG support
       The MNG specification (available at http://www.libpng.org/pub/mng)  al-
       lows  certain extensions to PNG for PNG images that are embedded in MNG
       datastreams.  Libpng can support some of these extensions.   To  enable
       them, use the png_permit_mng_features() function:

          feature_set = png_permit_mng_features(png_ptr, mask)

          mask is a png_uint_32 containing the bitwise OR of the
               features you want to enable.  These include
               PNG_FLAG_MNG_EMPTY_PLTE
               PNG_FLAG_MNG_FILTER_64
               PNG_ALL_MNG_FEATURES

          feature_set is a png_uint_32 that is the bitwise AND of
             your mask with the set of MNG features that is
             supported by the version of libpng that you are using.

       It  is  an  error to use this function when reading or writing a stand-
       alone PNG file with the PNG 8-byte signature.  The PNG datastream  must
       be  wrapped  in  a  MNG datastream.  As a minimum, it must have the MNG
       8-byte signature and the MHDR and MEND chunks.  Libpng does not provide
       support  for  these or any other MNG chunks; your application must pro-
       vide its own support for them.  You may wish to consider  using  libmng
       (available at https://www.libmng.com/) instead.

VIII. Changes to Libpng from version 0.88
       It should be noted that versions of libpng later than 0.96 are not dis-
       tributed by the original libpng author, Guy Schalnat,  nor  by  Andreas
       Dilger,  who had taken over from Guy during 1996 and 1997, and distrib-
       uted versions 0.89 through 0.96, but rather by another  member  of  the
       original  PNG  Group, Glenn Randers-Pehrson.  Guy and Andreas are still
       alive and well, but they have moved on to other things.

       The   old   libpng   functions    png_read_init(),    png_write_init(),
       png_info_init(),  png_read_destroy(), and png_write_destroy() have been
       moved to PNG_INTERNAL in version 0.95 to discourage their  use.   These
       functions will be removed from libpng version 1.4.0.

       The preferred method of creating and initializing the libpng structures
       is via  the  png_create_read_struct(),  png_create_write_struct(),  and
       png_create_info_struct()  because  they  isolate the size of the struc-
       tures from the application, allow version error checking, and also  al-
       low  the  use  of custom error handling routines during the initializa-
       tion, which the old functions do not.  The functions png_read_destroy()
       and png_write_destroy() do not actually free the memory that libpng al-
       located for these structs, but just reset the data structures, so  they
       can   be   used   instead   of  png_destroy_read_struct()  and  png_de-
       stroy_write_struct() if you feel there is too much system overhead  al-
       locating and freeing the png_struct for each image read.

       Setting   the   error   callbacks   via   png_set_message_fn()   before
       png_read_init() as was suggested in libpng-0.88 is no longer  supported
       because this caused applications that do not use custom error functions
       to fail if the png_ptr was not initialized to zero.  It is still possi-
       ble to set the error callbacks AFTER png_read_init(), or to change them
       with png_set_error_fn(), which is essentially the  same  function,  but
       with  a new name to force compilation errors with applications that try
       to use the old method.

       Support for the  sCAL,  iCCP,  iTXt,  and  sPLT  chunks  was  added  at
       libpng-1.0.6; however, iTXt support was not enabled by default.

       Starting  with version 1.0.7, you can find out which version of the li-
       brary you are using at run-time:

          png_uint_32 libpng_vn = png_access_version_number();

       The number libpng_vn is constructed from the major version, minor  ver-
       sion  with  leading  zero, and release number with leading zero, (e.g.,
       libpng_vn for version 1.0.7 is 10007).

       Note that this function does not take a png_ptr, so you can call it be-
       fore you've created one.

       You  can also check which version of png.h you used when compiling your
       application:

          png_uint_32 application_vn = PNG_LIBPNG_VER;

IX. Changes to Libpng from version 1.0.x to 1.2.x
       Support for user memory management was enabled by default.   To  accom-
       plish   this,   the   functions   png_create_read_struct_2(),  png_cre-
       ate_write_struct_2(),  png_set_mem_fn(),  png_get_mem_ptr(),   png_mal-
       loc_default(), and png_free_default() were added.

       Support  for  the  iTXt chunk has been enabled by default as of version
       1.2.41.

       Support for certain MNG features was enabled.

       Support for numbered error messages was added.  However, we  never  got
       around   to  actually  numbering  the  error  messages.   The  function
       png_set_strip_error_numbers() was added (Note: the prototype  for  this
       function  was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
       builds of libpng-1.2.15.  It was restored in libpng-1.2.36).

       The png_malloc_warn() function was added at libpng-1.2.3.  This  issues
       a png_warning and returns NULL instead of aborting when it fails to ac-
       quire the requested memory allocation.

       Support for setting user limits on image width and height  was  enabled
       by       default.        The      functions      png_set_user_limits(),
       png_get_user_width_max(), and png_get_user_height_max() were  added  at
       libpng-1.2.6.

       The png_set_add_alpha() function was added at libpng-1.2.7.

       The    function    png_set_expand_gray_1_2_4_to_8()    was   added   at
       libpng-1.2.9.  Unlike png_set_gray_1_2_4_to_8(), the new function  does
       not expand the tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() func-
       tion is deprecated.

       A number of macro definitions in support of runtime selection of assem-
       bler  code  features  (especially Intel MMX code support) were added at
       libpng-1.2.0:

           PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
           PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
           PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
           PNG_ASM_FLAG_MMX_READ_INTERLACE
           PNG_ASM_FLAG_MMX_READ_FILTER_SUB
           PNG_ASM_FLAG_MMX_READ_FILTER_UP
           PNG_ASM_FLAG_MMX_READ_FILTER_AVG
           PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
           PNG_ASM_FLAGS_INITIALIZED
           PNG_MMX_READ_FLAGS
           PNG_MMX_FLAGS
           PNG_MMX_WRITE_FLAGS
           PNG_MMX_FLAGS

       We added the following functions in support of runtime selection of as-
       sembler code features:

           png_get_mmx_flagmask()
           png_set_mmx_thresholds()
           png_get_asm_flags()
           png_get_mmx_bitdepth_threshold()
           png_get_mmx_rowbytes_threshold()
           png_set_asm_flags()

       We  replaced all of these functions with simple stubs in libpng-1.2.20,
       when the Intel assembler code was removed due to a licensing issue.

       These macros are deprecated:

           PNG_READ_TRANSFORMS_NOT_SUPPORTED
           PNG_PROGRESSIVE_READ_NOT_SUPPORTED
           PNG_NO_SEQUENTIAL_READ_SUPPORTED
           PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
           PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
           PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED

       They have been replaced, respectively, by:

           PNG_NO_READ_TRANSFORMS
           PNG_NO_PROGRESSIVE_READ
           PNG_NO_SEQUENTIAL_READ
           PNG_NO_WRITE_TRANSFORMS
           PNG_NO_READ_ANCILLARY_CHUNKS
           PNG_NO_WRITE_ANCILLARY_CHUNKS

       PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.  It has been deprecated
       since libpng-1.0.16 and libpng-1.2.6.

       The function
           png_check_sig(sig, num) was replaced with
           !png_sig_cmp(sig, 0, num) It has been deprecated since libpng-0.90.

       The function
           png_set_gray_1_2_4_to_8()  which also expands tRNS to alpha was re-
       placed with
           png_set_expand_gray_1_2_4_to_8() which does not. It has been depre-
       cated since libpng-1.0.18 and 1.2.9.

X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
       Private  libpng  prototypes and macro definitions were moved from png.h
       and pngconf.h into a new pngpriv.h header file.

       Functions     png_set_benign_errors(),     png_benign_error(),      and
       png_chunk_benign_error() were added.

       Support  for  setting the maximum amount of memory that the application
       will allocate for reading chunks was added, as a security measure.  The
       functions  png_set_chunk_cache_max() and png_get_chunk_cache_max() were
       added to the library.

       We implemented support for I/O states by adding png_ptr member io_state
       and   functions   png_get_io_chunk_name()   and  png_get_io_state()  in
       pngget.c

       We added PNG_TRANSFORM_GRAY_TO_RGB to the  available  high-level  input
       transforms.

       Checking  for  and  reporting of errors in the IHDR chunk is more thor-
       ough.

       Support for global arrays was removed, to improve thread safety.

       Some obsolete/deprecated macros and functions have been removed.

       Typecasted NULL definitions such as
          #define png_voidp_NULL            (png_voidp)NULL  were  eliminated.
       If you used these in your application, just use NULL instead.

       The  png_struct and info_struct members "trans" and "trans_values" were
       changed to "trans_alpha" and "trans_color", respectively.

       The obsolete, unused pnggccrd.c and pngvcrd.c files and  related  make-
       files were removed.

       The PNG_1_0_X and PNG_1_2_X macros were eliminated.

       The PNG_LEGACY_SUPPORTED macro was eliminated.

       Many WIN32_WCE #ifdefs were removed.

       The    functions   png_read_init(info_ptr),   png_write_init(info_ptr),
       png_info_init(info_ptr),  png_read_destroy(),  and  png_write_destroy()
       have been removed.  They have been deprecated since libpng-0.95.

       The  png_permit_empty_plte()  was removed. It has been deprecated since
       libpng-1.0.9.  Use png_permit_mng_features() instead.

       We  removed  the  obsolete   stub   functions   png_get_mmx_flagmask(),
       png_set_mmx_thresholds(),     png_get_asm_flags(),     png_get_mmx_bit-
       depth_threshold(),                    png_get_mmx_rowbytes_threshold(),
       png_set_asm_flags(), and png_mmx_supported()

       We   removed  the  obsolete  png_check_sig(),  png_memcpy_check(),  and
       png_memset_check() functions.  Instead  use  !png_sig_cmp(),  memcpy(),
       and memset(), respectively.

       The  function png_set_gray_1_2_4_to_8() was removed. It has been depre-
       cated  since  libpng-1.0.18  and  1.2.9,  when  it  was  replaced  with
       png_set_expand_gray_1_2_4_to_8()  because  the former function also ex-
       panded any tRNS chunk to an alpha channel.

       Macros for png_get_uint_16, png_get_uint_32,  and  png_get_int_32  were
       added  and  are used by default instead of the corresponding functions.
       Unfortunately, from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro
       (but   not   the   function)  incorrectly  returned  a  value  of  type
       png_uint_32.

       We changed the prototype for png_malloc() from
           png_malloc(png_structp png_ptr, png_uint_32 size) to
           png_malloc(png_structp png_ptr, png_alloc_size_t size)

       This also applies to  the  prototype  for  the  user  replacement  mal-
       loc_fn().

       The  png_calloc()  function  was  added  and  is  used  in  place of of
       "png_malloc(); memset();" except in the case  in  png_read_png()  where
       the array consists of pointers; in this case a "for" loop is used after
       the png_malloc() to set the pointers to NULL, to give robust.  behavior
       in  case  the  application  runs  out  of  memory  part-way through the
       process.

       We changed  the  prototypes  of  png_get_compression_buffer_size()  and
       png_set_compression_buffer_size()   to  work  with  size_t  instead  of
       png_uint_32.

       Support for numbered error messages was removed by  default,  since  we
       never got around to actually numbering the error messages. The function
       png_set_strip_error_numbers() was removed from the library by default.

       The png_zalloc() and png_zfree() functions are no longer exported.  The
       png_zalloc()  function  no  longer  zeroes out the memory that it allo-
       cates.  Applications that called png_zalloc(png_ptr, number, size)  can
       call  png_calloc(png_ptr, number*size) instead, and can call png_free()
       instead of png_zfree().

       Support for dithering was disabled by default in libpng-1.4.0,  because
       it  has  not  been well tested and doesn't actually "dither".  The code
       was not removed, however, and could be enabled by building libpng  with
       PNG_READ_DITHER_SUPPORTED  defined.   In libpng-1.4.2, this support was
       re-enabled, but the function was renamed png_set_quantize() to  reflect
       more  accurately  what  it  actually  does.   At  the  same  time,  the
       PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to  PNG_QUAN-
       TIZE_[RED,GREEN,BLUE]_BITS,  and  PNG_READ_DITHER_SUPPORTED was renamed
       to PNG_READ_QUANTIZE_SUPPORTED.

       We removed the trailing '.' from the warning and error messages.

XI. Changes to Libpng from version 1.4.x to 1.5.x
       From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but  not  the
       function) incorrectly returned a value of type png_uint_32.  The incor-
       rect macro was removed from libpng-1.4.5.

       Checking for invalid palette index on write was added at libpng 1.5.10.
       If a pixel contains an invalid (out-of-range) index libpng issues a be-
       nign error.  This is enabled by default because this  condition  is  an
       error  according to the PNG specification, Clause 11.3.2, but the error
       can be ignored in each png_ptr with

          png_set_check_for_invalid_index(png_ptr, allowed);

             allowed  - one of
                        0: disable benign error (accept the
                           invalid data without warning).
                        1: enable benign error (treat the
                           invalid data as an error or a
                           warning).

       If the error is ignored, or if png_benign_error() treats it as a  warn-
       ing,  any invalid pixels are decoded as opaque black by the decoder and
       written as-is by the encoder.

       Retrieving the maximum palette index found was added at  libpng-1.5.15.
       This  statement  must  appear  after png_read_png() or png_read_image()
       while reading, and after  png_write_png()  or  png_write_image()  while
       writing.

          int max_palette = png_get_palette_max(png_ptr, info_ptr);

       This  will return the maximum palette index found in the image, or "-1"
       if the palette was not checked, or "0" if no palette was  found.   Note
       that  this  does  not  account  for any palette index used by ancillary
       chunks such as the bKGD chunk; you must check those separately  to  de-
       termine the maximum palette index actually used.

       There  are  no substantial API changes between the non-deprecated parts
       of the 1.4.5 API and the 1.5.0 API; however, the  ability  to  directly
       access  members  of  the main libpng control structures, png_struct and
       png_info, deprecated in earlier versions of libpng, has been completely
       removed  from  libpng  1.5, and new private "pngstruct.h", "pnginfo.h",
       and "pngdebug.h" header files were created.

       We no longer include zlib.h in png.h.  The include statement  has  been
       moved  to  pngstruct.h, where it is not accessible by applications. Ap-
       plications that need access to information in zlib.h will need  to  add
       the  '#include "zlib.h"' directive.  It does not matter whether this is
       placed prior to or after the '"#include png.h"' directive.

       The png_sprintf(), png_strcpy(), and png_strncpy() macros are no longer
       used and were removed.

       We moved the png_strlen(), png_memcpy(), png_memset(), and png_memcmp()
       macros into a private header file (pngpriv.h) that is not accessible to
       applications.

       In  png_get_iCCP,  the type of "profile" was changed from png_charpp to
       png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep.

       There are changes of form in png.h, including new and changed macros to
       declare  parts  of the API.  Some API functions with arguments that are
       pointers to data not modified within the function have  been  corrected
       to declare these arguments with const.

       Much  of  the internal use of C macros to control the library build has
       also changed and some of this is visible in the exported header  files,
       in  particular the use of macros to control data and API elements visi-
       ble during application compilation may require significant revision  to
       application  code.   (It  is  extremely  rare  for an application to do
       this.)

       Any program that compiled against libpng 1.4 and did not use deprecated
       features  or access internal library structures should compile and work
       against libpng  1.5,  except  for  the  change  in  the  prototype  for
       png_get_iCCP() and png_set_iCCP() API functions mentioned above.

       libpng  1.5.0  adds PNG_ PASS macros to help in the reading and writing
       of interlaced images.  The macros return the number of rows and columns
       in  each  pass and information that can be used to de-interlace and (if
       absolutely necessary) interlace an image.

       libpng 1.5.0 adds an API png_longjmp(png_ptr, value).  This  API  calls
       the  application-provided png_longjmp_ptr on the internal, but applica-
       tion initialized, longjmp buffer.  It is provided as a  convenience  to
       avoid  the  need to use the png_jmpbuf macro, which had the unnecessary
       side effect of resetting the internal png_longjmp_ptr value.

       libpng 1.5.0 includes a complete fixed point API.  By default  this  is
       present  along  with  the corresponding floating point API.  In general
       the fixed point API is faster and smaller than the floating  point  one
       because the PNG file format used fixed point, not floating point.  This
       applies even if the library uses floating point  in  internal  calcula-
       tions.  A new macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether
       the library uses floating point arithmetic (the default) or fixed point
       arithmetic  internally  for  performance  critical calculations such as
       gamma correction.  In some cases, the gamma  calculations  may  produce
       slightly   different   results.    This  has  changed  the  results  in
       png_rgb_to_gray and in alpha composition (png_set_background for  exam-
       ple). This applies even if the original image was already linear (gamma
       == 1.0) and, therefore, it is not necessary  to  linearize  the  image.
       This  is  because  libpng  has *not* been changed to optimize that case
       correctly, yet.

       Fixed point support for the sCAL chunk comes with an important  caveat;
       the sCAL specification uses a decimal encoding of floating point values
       and the accuracy of PNG fixed point values is insufficient  for  repre-
       sentation  of these values. Consequently a "string" API (png_get_sCAL_s
       and png_set_sCAL_s) is the only reliable way of reading arbitrary  sCAL
       chunks  in  the  absence  of  either the floating point API or internal
       floating point calculations.  Starting with libpng-1.5.0, both of these
       functions  are  present  when  PNG_sCAL_SUPPORTED is defined.  Prior to
       libpng-1.5.0, their presence also  depended  upon  PNG_FIXED_POINT_SUP-
       PORTED  being  defined  and  PNG_FLOATING_POINT_SUPPORTED not being de-
       fined.

       Applications no longer need to include the optional distribution header
       file  pngusr.h  or  define  the corresponding macros during application
       build in order to see the correct variant  of  the  libpng  API.   From
       1.5.0  application  code  can  check  for  the corresponding _SUPPORTED
       macro:

       #ifdef PNG_INCH_CONVERSIONS_SUPPORTED
          /* code that uses the inch conversion APIs. */ #endif

       This macro will only be defined if the inch conversion  functions  have
       been  compiled into libpng.  The full set of macros, and whether or not
       support has been compiled in, are available in the header file  pnglib-
       conf.h.  This header file is specific to the libpng build.  Notice that
       prior to 1.5.0 the _SUPPORTED macros would always have the default def-
       inition  unless  reset  by pngusr.h or by explicit settings on the com-
       piler command line.  These settings may produce  compiler  warnings  or
       errors in 1.5.0 because of macro redefinition.

       Applications  can now choose whether to use these macros or to call the
       corresponding   function    by    defining    PNG_USE_READ_MACROS    or
       PNG_NO_USE_READ_MACROS  before  including  png.h.   Notice that this is
       only supported from 1.5.0;  defining  PNG_NO_USE_READ_MACROS  prior  to
       1.5.0 will lead to a link failure.

       Prior to libpng-1.5.4, the zlib compressor used the same set of parame-
       ters when compressing the IDAT data and textual data such as  zTXt  and
       iCCP.   In  libpng-1.5.4 we reinitialized the zlib stream for each type
       of data.  We added five png_set_text_*() functions for setting the  pa-
       rameters to use with textual data.

       Prior  to  libpng-1.5.4,  the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
       option was off by default, and slightly  inaccurate  scaling  occurred.
       This  option can no longer be turned off, and the choice of accurate or
       inaccurate 16-to-8 scaling is by using the new  png_set_scale_16_to_8()
       API  for  accurate  scaling  or the old png_set_strip_16_to_8() API for
       simple   chopping.    In   libpng-1.5.4,   the   PNG_READ_16_TO_8_ACCU-
       RATE_SCALE_SUPPORTED macro became PNG_READ_SCALE_16_TO_8_SUPPORTED, and
       the PNG_READ_16_TO_8 macro became PNG_READ_STRIP_16_TO_8_SUPPORTED,  to
       enable the two png_set_*_16_to_8() functions separately.

       Prior to libpng-1.5.4, the png_set_user_limits() function could only be
       used  to  reduce  the  width  and  height  limits  from  the  value  of
       PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said
       that it could be used to override them.  Now this function will  reduce
       or increase the limits.

       Starting  in libpng-1.5.22, default user limits were established. These
       can  be  overridden  by  application  calls  to  png_set_user_limits(),
       png_set_user_chunk_cache_max(),  and/or png_set_user_malloc_max().  The
       limits are now
                                    max possible  default
          png_user_width_max        0x7fffffff    1,000,000
          png_user_height_max       0x7fffffff    1,000,000
          png_user_chunk_cache_max  0 (unlimited) 1000
          png_user_chunk_malloc_max 0 (unlimited) 8,000,000

       The png_set_option() function (and the  "options"  member  of  the  png
       struct) was added to libpng-1.5.15, with option PNG_ARM_NEON.

       The  library now supports a complete fixed point implementation and can
       thus be used on systems that have no floating  point  support  or  very
       limited  or  slow  support.   Previously gamma correction, an essential
       part of complete PNG support, required reasonably fast floating point.

       As part of this the choice of internal implementation has been made in-
       dependent of the choice of fixed versus floating point APIs and all the
       missing fixed point APIs have been implemented.

       The exact mechanism used to control attributes  of  API  functions  has
       changed, as described in the INSTALL file.

       A new test program, pngvalid, is provided in addition to pngtest.  png-
       valid validates the arithmetic accuracy of the gamma correction  calcu-
       lations  and  includes  a  number of validations of the file format.  A
       subset of the full range of tests is run when "make check" is done  (in
       the  'configure'  build.)   pngvalid also allows total allocated memory
       usage to be evaluated and performs additional memory overwrite  valida-
       tion.

       Many changes to individual feature macros have been made. The following
       are the changes most likely to be noticed by library builders who  con-
       figure libpng:

       1) All feature macros now have consistent naming:

       #define  PNG_NO_feature  turns the feature off #define PNG_feature_SUP-
       PORTED turns the feature on

       pnglibconf.h contains one line for each feature macro which is either:

       #define PNG_feature_SUPPORTED

       if the feature is supported or:

       /*#undef PNG_feature_SUPPORTED*/

       if it is not.  Library code consistently  checks  for  the  'SUPPORTED'
       macro.   It does not, and libpng applications should not, check for the
       'NO' macro which will not normally be defined even if  the  feature  is
       not supported.  The 'NO' macros are only used internally for setting or
       not setting the corresponding 'SUPPORTED' macros.

       Compatibility with the old names is provided as follows:

       PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED

       And the following definitions disable the corresponding feature:

       PNG_SETJMP_NOT_SUPPORTED disables  SETJMP  PNG_READ_TRANSFORMS_NOT_SUP-
       PORTED  disables  READ_TRANSFORMS PNG_NO_READ_COMPOSITED_NODIV disables
       READ_COMPOSITE_NODIV    PNG_WRITE_TRANSFORMS_NOT_SUPPORTED     disables
       WRITE_TRANSFORMS    PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED    disables
       READ_ANCILLARY_CHUNKS PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables
       WRITE_ANCILLARY_CHUNKS

       Library builders should remove use of the above, inconsistent, names.

       2)  Warning  and error message formatting was previously conditional on
       the STDIO feature. The library has been changed to use  the  CONSOLE_IO
       feature  instead. This means that if CONSOLE_IO is disabled the library
       no longer  uses  the  printf(3)  functions,  even  though  the  default
       read/write implementations use (FILE) style stdio.h functions.

       3) Three feature macros now control the fixed/floating point decisions:

       PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs

       PNG_FIXED_POINT_SUPPORTED  enables  the  fixed  point APIs; however, in
       practice these are normally required internally anyway (because the PNG
       file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT
       merely stops the function from being exported.

       PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating
       point implementation or the fixed point one.  Typically the fixed point
       implementation is larger and slower than the floating point implementa-
       tion  on  a  system  that  supports  floating point; however, it may be
       faster on a system which lacks floating point  hardware  and  therefore
       uses a software emulation.

       4)  Added  PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED.   This  allows the
       functions to read and  write  ints  to  be  disabled  independently  of
       PNG_USE_READ_MACROS, which allows libpng to be built with the functions
       even though the default is to use the macros - this allows applications
       to choose at app buildtime whether or not to use macros (previously im-
       possible because the functions weren't in the default build.)

XII. Changes to Libpng from version 1.5.x to 1.6.x
       A "simplified API" has been added (see documentation  in  png.h  and  a
       simple example in contrib/examples/pngtopng.c).  The new publicly visi-
       ble API includes the following:

          macros:
            PNG_FORMAT_*
            PNG_IMAGE_*
          structures:
            png_control
            png_image
          read functions
            png_image_begin_read_from_file()
            png_image_begin_read_from_stdio()
            png_image_begin_read_from_memory()
            png_image_finish_read()
            png_image_free()
          write functions
            png_image_write_to_file()
            png_image_write_to_memory()
            png_image_write_to_stdio()

       Starting with libpng-1.6.0, you can configure libpng to prefix all  ex-
       ported symbols, using the PNG_PREFIX macro.

       We no longer include string.h in png.h.  The include statement has been
       moved to pngpriv.h, where it is not accessible by applications.  Appli-
       cations  that  need access to information in string.h must add an '#in-
       clude <string.h>' directive.  It does not matter whether this is placed
       prior to or after the '#include "png.h"' directive.

       The following API are now DEPRECATED:
          png_info_init_3()
          png_convert_to_rfc1123() which has been replaced
            with png_convert_to_rfc1123_buffer()
          png_malloc_default()
          png_free_default()
          png_reset_zstream()

       The following have been removed:
          png_get_io_chunk_name(), which has been replaced
            with png_get_io_chunk_type().  The new
            function returns a 32-bit integer instead of
            a string.
          The png_sizeof(), png_strlen(), png_memcpy(), png_memcmp(), and
            png_memset() macros are no longer used in the libpng sources and
            have  been  removed.  These had already been made invisible to ap-
       plications
            (i.e.,  defined  in  the  private  pngpriv.h  header  file)  since
       libpng-1.5.0.

       The signatures of many exported functions were changed, such that
          png_structp became png_structrp or png_const_structrp
          png_infop became png_inforp or png_const_inforp where "rp" indicates
       a "restricted pointer".

       Dropped support for 16-bit platforms. The support for FAR/far types has
       been  eliminated  and  the  definition  of png_alloc_size_t is now con-
       trolled by a flag so that 'small size_t' systems can select it if  nec-
       essary.

       Error  detection  in  some  chunks has improved; in particular the iCCP
       chunk reader now does pretty complete validation of the  basic  format.
       Some bad profiles that were previously accepted are now accepted with a
       warning or rejected, depending upon  the  png_set_benign_errors()  set-
       ting,  in  particular  the  very old broken Microsoft/HP 3144-byte sRGB
       profile.  Starting with libpng-1.6.11, recognizing  and  checking  sRGB
       profiles can be avoided by means of

           #if     defined(PNG_SKIP_sRGB_CHECK_PROFILE)     &&             de-
       fined(PNG_SET_OPTION_SUPPORTED)
              png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE,
                  PNG_OPTION_ON);
           #endif

       It's not a good idea to do this if you are using the "simplified  API",
       which needs to be able to recognize sRGB profiles conveyed via the iCCP
       chunk.

       The PNG spec requirement that only grayscale profiles may appear in im-
       ages  with  color  type 0 or 4 and that even if the image only contains
       gray pixels, only RGB profiles may appear in images with color type  2,
       3,  or  6, is now enforced.  The sRGB chunk is allowed to appear in im-
       ages with any color type and is interpreted by libpng to convey a  one-
       tracer-curve gray profile or a three-tracer-curve RGB profile as appro-
       priate.

       Libpng 1.5.x erroneously used /MD for Debug DLL builds; if you used the
       debug  builds  in your app and you changed your app to use /MD you will
       need to change it back to /MDd for libpng 1.6.x.

       Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk  con-
       tained an empty language field or an empty translated keyword.  Both of
       these are allowed by the PNG specification, so these  warnings  are  no
       longer issued.

       The  library  now  issues an error if the application attempts to set a
       transform after it calls png_read_update_info() or if  it  attempts  to
       call  both png_read_update_info() and png_start_read_image() or to call
       either of them more than once.

       The default condition for benign_errors is now to treat  benign  errors
       as warnings while reading and as errors while writing.

       The  library now issues a warning if both background processing and RGB
       to gray are used when gamma correction happens. As with  previous  ver-
       sions of the library the results are numerically very incorrect in this
       case.

       There are some minor arithmetic changes  in  some  transforms  such  as
       png_set_background(),  that  might  be  detected  by certain regression
       tests.

       Unknown chunk handling has been improved internally,  without  any  API
       change.  This adds more correct option control of the unknown handling,
       corrects a pre-existing bug where the per-chunk 'keep' setting  is  ig-
       nored,  and  makes  it  possible  to skip IDAT chunks in the sequential
       reader.

       The  machine-generated  configure  files  are  no  longer  included  in
       branches libpng16 and later of the GIT repository.  They continue to be
       included in the tarball releases, however.

       Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning  of  the
       IDAT  stream  to set the size of the sliding window for reading instead
       of using the default 32-kbyte sliding window size.  It  was  discovered
       that  there  are  hundreds of PNG files in the wild that have incorrect
       CMF bytes that caused zlib to issue the "invalid distance too far back"
       error  and reject the file.  Libpng-1.6.3 and later calculate their own
       safe CMF from the image dimensions, provide a  way  to  revert  to  the
       libpng-1.5.x  behavior  (ignoring  the  CMF  bytes and using a 32-kbyte
       sliding window), by using

           png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW,
               PNG_OPTION_ON);

       and provide a tool (contrib/tools/pngfix)  for  rewriting  a  PNG  file
       while optimizing the CMF bytes in its IDAT chunk correctly.

       Libpng-1.6.0  and  libpng-1.6.1 wrote uncompressed iTXt chunks with the
       wrong length, which resulted in PNG files that cannot  be  read  beyond
       the  bad  iTXt chunk.  This error was fixed in libpng-1.6.3, and a tool
       (called contrib/tools/png-fix-itxt) has been added to the  libpng  dis-
       tribution.

       Starting  with  libpng-1.6.17, the PNG_SAFE_LIMITS macro was eliminated
       and safe limits are used by default (users who need larger  limits  can
       still override them at compile time or run time, as described above).

       The new limits are
                                       default   spec limit
          png_user_width_max         1,000,000  2,147,483,647
          png_user_height_max        1,000,000  2,147,483,647
          png_user_chunk_cache_max         128  unlimited
          png_user_chunk_malloc_max  8,000,000  unlimited

       Starting with libpng-1.6.18, a PNG_RELEASE_BUILD macro was added, which
       allows library builders to control compilation for an installed  system
       (a  release  build).  It can be set for testing debug or beta builds to
       ensure that they will compile when the build type is switched to RC  or
       STABLE.  In essence this overrides the PNG_LIBPNG_BUILD_BASE_TYPE defi-
       nition which is not directly user controllable.

       Starting with libpng-1.6.19, attempting  to  set  an  over-length  PLTE
       chunk is an error. Previously this requirement of the PNG specification
       was not enforced, and the palette was always limited to 256 entries. An
       over-length PLTE chunk found in an input PNG is silently truncated.

       Starting  with  libpng-1.6.31, the eXIf chunk is supported. Libpng does
       not attempt to decode the Exif profile; it simply returns a byte  array
       containing the profile to the calling application which must do its own
       decoding.

XIII. Detecting libpng
       The png_get_io_ptr() function has been present since  libpng-0.88,  has
       never changed, and is unaffected by conditional compilation macros.  It
       is the best choice for use in configure scripts for detecting the pres-
       ence  of  any libpng version since 0.88.  In an autoconf "configure.in"
       you could use

           AC_CHECK_LIB(png, png_get_io_ptr, ...

XV. Source code repository
       Since about February 2009, version 1.2.34, libpng has been under  "git"
       source  control.   The  git  repository  was  built  from  old  libpng-
       x.y.z.tar.gz files going back to version 0.70.  You can access the  git
       repository (read only) at

           https://github.com/glennrp/libpng or
           https://git.code.sf.net/p/libpng/code.git

       or you can browse it with a web browser at

           https://github.com/glennrp/libpng or
           https://sourceforge.net/p/libpng/code/ci/libpng16/tree/

       Patches  can  be  sent to png-mng-implement at lists.sourceforge.net or
       uploaded to the libpng bug tracker at

           https://libpng.sourceforge.io/

       or as a "pull request" to

           https://github.com/glennrp/libpng/pulls

       We also accept patches built from the tar  or  zip  distributions,  and
       simple verbal descriptions of bug fixes, reported either to the Source-
       Forge bug tracker, to the  png-mng-implement  at  lists.sf.net  mailing
       list, as github issues.

XV. Coding style
       Our   coding   style   is   similar   to   the   "Allman"   style  (See
       https://en.wikipedia.org/wiki/Indent_style#Allman_style),  with   curly
       braces on separate lines:

           if (condition)
           {
              action;
           }

           else if (another condition)
           {
              another action;
           }

       The braces can be omitted from simple one-line actions:

           if (condition)
              return 0;

       We  use  3-space indentation, except for continued statements which are
       usually indented the same as the first line of the statement plus  four
       more spaces.

       For  macro  definitions  we use 2-space indentation, always leaving the
       "#" in the first column.

           #ifndef PNG_NO_FEATURE
           #  ifndef PNG_FEATURE_SUPPORTED
           #    define PNG_FEATURE_SUPPORTED
           #  endif
           #endif

       Comments appear with the leading "/*" at the same  indentation  as  the
       statement that follows the comment:

           /* Single-line comment */
           statement;

           /* This is a multiple-line
            * comment.
            */
           statement;

       Very  short  comments  can  be placed after the end of the statement to
       which they pertain:

           statement;    /* comment */

       We don't use C++ style ("//") comments. We have, however, used them  in
       the past in some now-abandoned MMX assembler code.

       Functions  and  their curly braces are not indented, and exported func-
       tions are marked with PNGAPI:

        /* This is a public function that is visible to
         * application programmers. It does thus-and-so.
         */
        void PNGAPI
        png_exported_function(png_ptr, png_info, foo)
        {
           body;
        }

       The return type and decorations are placed on a separate line ahead  of
       the function name, as illustrated above.

       The  prototypes  for  all exported functions appear in png.h, above the
       comment that says

           /* Maintainer: Put new public prototypes here ... */

       We mark all non-exported functions with "/* PRIVATE */"":

        void /* PRIVATE */
        png_non_exported_function(png_ptr, png_info, foo)
        {
           body;
        }

       The prototypes for non-exported functions (except for those in pngtest)
       appear in pngpriv.h above the comment that says

         /* Maintainer: Put new private prototypes here ^ */

       To  avoid  polluting  the  global  namespace, the names of all exported
       functions and variables begin with "png_", and all publicly  visible  C
       preprocessor  macros  begin  with  "PNG".  We request that applications
       that use libpng *not* begin any of their own  symbols  with  either  of
       these strings.

       We  put  a  space  after the "sizeof" operator and we omit the optional
       parentheses around its argument when the argument is an expression, not
       a  type name, and we always enclose the sizeof operator, with its argu-
       ment, in parentheses:

         (sizeof (png_uint_32))
         (sizeof array)

       Prior to libpng-1.6.0 we used  a  "png_sizeof()"  macro,  formatted  as
       though it were a function.

       Control  keywords  if,  for, while, and switch are always followed by a
       space to distinguish them from function calls, which have  no  trailing
       space.

       We  put  a  space  after  each  comma and after each semicolon in "for"
       statements, and we put spaces before and after each C  binary  operator
       and  after  "for" or "while", and before "?".  We don't put a space be-
       tween a typecast and the expression being cast, nor do we put  one  be-
       tween a function name and the left parenthesis that follows it:

           for (i = 2; i > 0; --i)
              y[i] = a(x) + (int)b;

       We  prefer  #ifdef and #ifndef to #if defined() and #if !defined() when
       there is only one macro being tested.  We always use  parentheses  with
       "defined".

       We  express integer constants that are used as bit masks in hex format,
       with an even number of lower-case hex digits, and to make them unsigned
       (e.g.,  0x00U, 0xffU, 0x0100U) and long if they are greater than 0x7fff
       (e.g., 0xffffUL).

       We prefer to use underscores rather than camelCase in names, except for
       a few type names that we inherit from zlib.h.

       We  prefer  "if  (something  !=  0)" and "if (something == 0)" over "if
       (something)" and if "(!something)", respectively, and for  pointers  we
       prefer "if (some_pointer != NULL)" or "if (some_pointer == NULL)".

       We do not use the TAB character for indentation in the C sources.

       Lines do not exceed 80 characters.

       Other rules can be inferred by inspecting the libpng source.

NOTE
       Note about libpng version numbers:

       Due to various miscommunications, unforeseen code incompatibilities and
       occasional factors outside the authors' control, version  numbering  on
       the  library  has  not always been consistent and straightforward.  The
       following table summarizes matters since version 0.89c, which  was  the
       first widely used release:

        source               png.h    png.h  shared-lib
        version              string   int    version
        -------              ------   -----  ----------
        0.89c "1.0 beta 3"     0.89      89  1.0.89
        0.90  "1.0 beta 4"     0.90      90  0.90  [should have been 2.0.90]
        0.95  "1.0 beta 5"     0.95      95  0.95  [should have been 2.0.95]
        0.96  "1.0 beta 6"     0.96      96  0.96  [should have been 2.0.96]
        0.97b "1.00.97 beta 7" 1.00.97   97  1.0.1 [should have been 2.0.97]
        0.97c                  0.97      97  2.0.97
        0.98                   0.98      98  2.0.98
        0.99                   0.99      98  2.0.99
        0.99a-m                0.99      99  2.0.99
        1.00                   1.00     100  2.1.0 [100 should be 10000]
        1.0.0      (from here on, the   100  2.1.0 [100 should be 10000]
        1.0.1       png.h string is   10001  2.1.0
        1.0.1a-e    identical to the  10002  from here on, the shared library
        1.0.2       source version)   10002  is 2.V where V is the source code
        1.0.2a-b                      10003  version, except as noted.
        1.0.3                         10003
        1.0.3a-d                      10004
        1.0.4                         10004
        1.0.4a-f                      10005
        1.0.5 (+ 2 patches)           10005
        1.0.5a-d                      10006
        1.0.5e-r                      10100 (not source compatible)
        1.0.5s-v                      10006 (not binary compatible)
        1.0.6 (+ 3 patches)           10006 (still binary incompatible)
        1.0.6d-f                      10007 (still binary incompatible)
        1.0.6g                        10007
        1.0.6h                        10007  10.6h (testing xy.z so-numbering)
        1.0.6i                        10007  10.6i
        1.0.6j                          10007    2.1.0.6j  (incompatible  with
       1.0.0)
        1.0.7beta11-14        DLLNUM  10007  2.1.0.7beta11-14 (binary compati-
       ble)
        1.0.7beta15-18           1    10007  2.1.0.7beta15-18 (binary compati-
       ble)
        1.0.7rc1-2               1    10007  2.1.0.7rc1-2 (binary compatible)
        1.0.7                    1    10007  (still compatible)
        ...
        1.0.69                  10    10069  10.so.0.69[.0]
        ...
        1.2.59                  13    10259  12.so.0.59[.0]
        ...
        1.4.20                  14    10420  14.so.0.20[.0]
        ...
        1.5.30                  15    10530  15.so.15.30[.0]
        ...
        1.6.35                  16    10635  16.so.16.35[.0]

       Henceforth the source version will match the shared-library  minor  and
       patch numbers; the shared-library major version number will be used for
       changes  in  backward  compatibility,   as   it   is   intended.    The
       PNG_PNGLIB_VER  macro, which is not used within libpng but is available
       for applications, is an unsigned integer of the form XYYZZ  correspond-
       ing  to the source version X.Y.Z (leading zeros in Y and Z).  Beta ver-
       sions were given the previous public release number plus a letter,  un-
       til  version  1.0.6j;  from then on they were given the upcoming public
       release number plus "betaNN" or "rcNN".

SEE ALSO
       libpngpf(3), png(5)

       libpng:

              https://libpng.sourceforge.io/  (follow  the  [DOWNLOAD]   link)
              http://www.libpng.org/pub/png

       zlib:

              (generally) at the same location as libpng or at
              https://zlib.net/

       PNGspecification:RFC2083

              (generally) at the same location as libpng or at
              https://www.ietf.org/rfc/rfc2083.txt
              or (as a W3C Recommendation) at
              https://www.w3.org/TR/REC-png.html

       In the case of any inconsistency between the PNG specification and this
       library, the specification takes precedence.

AUTHORS
       This man page: Initially created by Glenn Randers-Pehrson.   Maintained
       by Cosmin Truta.

       The  contributing authors would like to thank all those who helped with
       testing, bug fixes, and patience.  This  wouldn't  have  been  possible
       without all of you.

       Thanks to Frank J. T. Wojcik for helping with the documentation.

       Libpng:  Initially  created in 1995 by Guy Eric Schalnat, then of Group
       42, Inc.  Maintained by Cosmin Truta.

       Supported by the PNG development group
       png-mng-implement  at  lists.sourceforge.net  (subscription   required;
       visit https://lists.sourceforge.net/lists/listinfo/png-mng-implement to
       subscribe).

                               November 20, 2022                     LIBPNG(3)

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