redhat man page for sdl_pixelformat

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 individual color values

       [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_PixelFor-
       mat 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 BytesPerPixel.  Since  BytesPer-
       Pixel  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->for-
       mat->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 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)