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RedHat 9 (Linux i386) - man page for sdl_pixelformat (redhat section 3)

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_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 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->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)


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