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)
Generated by dwww version 1.15 on Sat Jun 29 01:53:23 CEST 2024.