SDL_PixelFormat - Stores surface format information
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;
- 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
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);
.
.
.
SDL_Surface,
SDL_MapRGB