Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help 
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 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 pix-
     els 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 understand-
     ing how SDL stores its color information is essential.

     8-bit pixel formats are the easiest to understand. Since its an 8-bit  for-
     mat,  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)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=SDL_PixelFormat&sektion=3&manpath=FreeBSD+Ports+15.1.quarterly>

home | help