SDL_PixelFormat(3) SDL API Reference SDL_PixelFormat(3)NAME
SDL_PixelFormat- Stores surface format information
STRUCTURE DEFINITION
typedef struct{
SDL_Palette *palette;
Uint8 BitsPerPixel;
Uint8 BytesPerPixel;
Uint32 Rmask, Gmask, Bmask, Amask;
Uint8 Rshift, Gshift, Bshift, Ashift;
Uint8 Rloss, Gloss, Bloss, Aloss;
Uint32 colorkey;
Uint8 alpha;
} SDL_PixelFormat;
STRUCTURE DATA
palette Pointer to the palette, or NULL if the
BitsPerPixel>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 individ-
ual color values
[RGBA]loss Precision loss of each color component
(2^[RGBA]loss)
[RGBA]shift Binary left shift of each color compo-
nent 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 understanding 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 Bytes-
PerPixel. Since BytesPerPixel is 1, all pixels are repre-
sented 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 sur-
face->pixels and we use that index to read the SDL_Color
structure from surface->format->palette->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 expe-
rience. 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)
