Manual Pages for UNIX Operating System command usage for man SDL_PixelFormat

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
                         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  com-
                         ponent 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



SDL            Last change: Tue 11 Sep 2001, 23:01              1






SDL API Reference                              SDL_PixelFormat(3)



     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  experi-
     ence.  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.




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SDL API Reference                              SDL_PixelFormat(3)



     /* 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<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<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<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<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            Last change: Tue 11 Sep 2001, 23:01              3