// Example of direct pixel access with SDL.#include <SDL/SDL.h>#include <stdio.h>#include <stdlib.h>Uint16 CreateHicolorPixel(SDL_PixelFormat * fmt, Uint8 red, Uint8 green,			  Uint8 blue){    Uint16 value;    // This series of bit shifts uses the information from the SDL_Format    //   structure to correctly compose a 16-bit pixel value from 8-bit red,    //   green, and blue data.    value = ((red >> fmt->Rloss) << fmt->Rshift) +	((green >> fmt->Gloss) << fmt->Gshift) +	((blue >> fmt->Bloss) << fmt->Bshift);    return value;}int main(){    SDL_Surface *screen;    Uint16 *raw_pixels;    int x, y;    // Initialize SDL's video system and check for errors.    if (SDL_Init(SDL_INIT_VIDEO) != 0) {	printf("Unable to initialize SDL: %s\n", SDL_GetError());	return 1;    }    // Make sure SDL_Quit gets called when the program exits!     atexit(SDL_Quit);    // Attempt to set a 256x256 hicolor (16-bit) video mode.    //   This will set some type of 16-bit mode, but we won't    //   know which particular pixel format ahead of time. If    //   the video card can't handle hicolor modes, SDL will    //   emulate it.    screen = SDL_SetVideoMode(256, 256, 16, 0);    if (screen == NULL) {	printf("Unable to set video mode: %s\n", SDL_GetError());	return 1;    }    // Video memory can be strange, and it's sometimes necessary to    //   "lock" it before it can be modified. SDL abstracts this with    //   the SDL_LockSurface function.    SDL_LockSurface(screen);    // Get a pointer to the video surface's memory.     raw_pixels = (Uint16 *) screen->pixels;    // We can now safely write to the video surface. We'll draw a nice    //   gradient pattern by varying our red and blue components along    //   the X and Y axes. Notice the formula used to calculate the    //   offset into the framebuffer for each pixel.    //   (The pitch is the number of bytes per scanline in memory.)     for (x = 0; x < 256; x++) {	for (y = 0; y < 256; y++) {	    Uint16 pixel_color;	    int offset;	    pixel_color = CreateHicolorPixel(screen->format, x, 0, y);	    offset = (screen->pitch / 2 * y + x);	    raw_pixels[offset] = pixel_color;	}    }    // We're finished drawing, so unlock the surface.     SDL_UnlockSurface(screen);    // Inform SDL that the screen has been changed. This is necessary    //   because SDL's screen surface is not always the real framebuffer;    //   it is sometimes emulated behind the scenes.    SDL_UpdateRect(screen, 0, 0, 0, 0);    // Pause for a few seconds as the viewer gasps in awe.    SDL_Delay(3000);    return 0;}