SDL_PixelFormat(3) SDL API Reference SDL_PixelFormat(3)
NAME
SDL_PixelFormat - Stores surface format information
STRUCTURE DEFINITION
typedef struct SDL_PixelFormat {
SDL_Palette *palette;
Uint8 BitsPerPixel;
Uint8 BytesPerPixel;
Uint8 Rloss, Gloss, Bloss, Aloss;
Uint8 Rshift, Gshift, Bshift, Ashift;
Uint32 Rmask, Gmask, Bmask, Amask;
Uint32 colorkey;
Uint8 alpha;
} SDL_PixelFormat;
STRUCTURE DATA
palette Pointer to the palette, or NULL if the BitsPer-
Pixel>8
BitsPerPixel The number of bits used to represent each pixel in
a surface. Usually 8, 16, 24 or 32.
BytesPerPixel The number of bytes used to represent each pixel in
a surface. Usually one to four.
[RGBA]mask Binary mask used to retrieve individual color val-
ues
[RGBA]loss Precision loss of each color component
(2^[RGBA]loss)
[RGBA]shift Binary left shift of each color component in the
pixel value
colorkey Pixel value of transparent pixels
alpha Overall surface alpha value
DESCRIPTION
A SDL_PixelFormat describes the format of the pixel data stored at the
pixels field of a SDL_Surface. Every surface stores a SDL_PixelFormat
in the format field.
If you wish to do pixel level modifications on a surface, then under-
standing how SDL stores its color information is essential.
8-bit pixel formats are the easiest to understand. Since its an 8-bit
format, we have 8 BitsPerPixel and 1 BytesPerPixel. Since BytesPerPixel
is 1, all pixels are represented by a Uint8 which contains an index
into palette->colors. So, to determine the color of a pixel in a 8-bit
surface: we read the color index from surface->pixels and we use that
index to read the SDL_Color structure from surface->format->pal-
ette->colors. Like so:
SDL_Surface *surface;
SDL_PixelFormat *fmt;
SDL_Color *color;
Uint8 index;
.
.
/* Create surface */
.
.
fmt=surface->format;
/* Check the bitdepth of the surface */
if(fmt->BitsPerPixel!=8){
fprintf(stderr, "Not an 8-bit surface.
");
return(-1);
}
/* Lock the surface */
SDL_LockSurface(surface);
/* Get the topleft pixel */
index=*(Uint8 *)surface->pixels;
color=fmt->palette->colors[index];
/* Unlock the surface */
SDL_UnlockSurface(surface);
printf("Pixel Color-> Red: %d, Green: %d, Blue: %d. Index: %d
",
color->r, color->g, color->b, index);
.
.
Pixel formats above 8-bit are an entirely different experience. They
are considered to be "TrueColor" formats and the color information is
stored in the pixels themselves, not in a palette. The mask, shift and
loss fields tell us how the color information is encoded. The mask
fields allow us to isolate each color component, the shift fields tell
us the number of bits to the right of each component in the pixel value
and the loss fields tell us the number of bits lost from each component
when packing 8-bit color component in a pixel.
/* Extracting color components from a 32-bit color value */
SDL_PixelFormat *fmt;
SDL_Surface *surface;
Uint32 temp, pixel;
Uint8 red, green, blue, alpha;
.
.
.
fmt=surface->format;
SDL_LockSurface(surface);
pixel=*((Uint32*)surface->pixels);
SDL_UnlockSurface(surface);
/* Get Red component */
temp=pixel&fmt->Rmask; /* Isolate red component */
temp=temp>>fmt->Rshift;/* Shift it down to 8-bit */
temp=temp<<fmt->Rloss; /* Expand to a full 8-bit number */
red=(Uint8)temp;
/* Get Green component */
temp=pixel&fmt->Gmask; /* Isolate green component */
temp=temp>>fmt->Gshift;/* Shift it down to 8-bit */
temp=temp<<fmt->Gloss; /* Expand to a full 8-bit number */
green=(Uint8)temp;
/* Get Blue component */
temp=pixel&fmt->Bmask; /* Isolate blue component */
temp=temp>>fmt->Bshift;/* Shift it down to 8-bit */
temp=temp<<fmt->Bloss; /* Expand to a full 8-bit number */
blue=(Uint8)temp;
/* Get Alpha component */
temp=pixel&fmt->Amask; /* Isolate alpha component */
temp=temp>>fmt->Ashift;/* Shift it down to 8-bit */
temp=temp<<fmt->Aloss; /* Expand to a full 8-bit number */
alpha=(Uint8)temp;
printf("Pixel Color -> R: %d, G: %d, B: %d, A: %d
", red, green, blue, alpha);
.
.
.
SEE ALSO
SDL_Surface, SDL_MapRGB
SDL Tue 11 Sep 2001, 23:01 SDL_PixelFormat(3)
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