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 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)