CAIROFORMAT(3) 1 CAIROFORMAT(3)The CairoFormat classINTRODUCTION
CairoFormat enums are used to identify the memory format of the image data.
CLASS SYNOPSIS
CairoFormat
CairoFormat
Constants
o const integer$CairoFormat::ARGB320
o const integer$CairoFormat::RGB241
o const integer$CairoFormat::A82
o const integer$CairoFormat::A13
Methods
o publicstatic int CairoFormat::strideForWidth (int $format, int $width)
PREDEFINED CONSTANTS
o CairoFormat::ARGB32 -Each pixel is a 32-bit quantity, with alpha in the upper 8 bits, then red, then green, then blue. The 32-bit
quantities are stored native-endian. Pre-multiplied alpha is used. (That is, 50% transparent red is 0x80800000, not 0x80ff0000.)
o CairoFormat::RGB24 - Each pixel is a 32-bit quantity, with the upper 8 bits unused. Red, Green, and Blue are stored in the remain-
ing 24 bits in that order.
o CairoFormat::A8 -Each pixel is a 8-bit quantity holding an alpha value.
o CairoFormat::A1 -Each pixel is a 1-bit quantity holding an alpha value. Pixels are packed together into 32-bit quantities. The
ordering of the bits matches the endianess of the platform. On a big-endian machine, the first pixel is in the uppermost bit, on a
little-endian machine the first pixel is in the least-significant bit.
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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_PixelFor-
mat 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 BytesPer-
Pixel 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->for-
mat->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)