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.\n");
+ 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\n",
+ 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\n", red, green, blue, alpha);
+.
+.
+.