@lay:4401l2

Listing 2. Drawing Individual Pixels on the Screen

#include "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;

     /* Initialization code goes here. It should
	create a 256x256, 16-bit display and save
	the surface in the screen pointer. See
	the previous example. */

     /* "Lock" the screen surface so we can draw to
	it directly. */
    SDL_LockSurface(screen);

     /* Get a pointer to the video surface's i
	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. */
     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);
            /* Calculate the memory offset of the
		pixel we wish to change. */
	    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;
}