/* 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;
	}

	atexit(SDL_Quit);

	/* Attempt to set a 256x256 hicolor */
	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 patern by varying our red and blue components
	 * along the X and Y axes. Notice the forumula 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 chnaged. 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;
}