Funky SDL effects

After only about an hour of playing with SDL (Simple DirectMedia Layer), I came up with the following interesting looking effect (note: it's slow as molasses, probably from it being mostly example code... but, ianacm (I am not a Code Monkey), infact I don't even have any formal coding training. So, just deal with it)

#include <stdlib.h>
#include "SDL.h"
#include "SDL_endian.h" /* Used for the endian-dependent 24 bpp mode */

/* The following function is from the SDL introduction available on
 libsdl.org, so blame them for the lack of comments */
void DrawPixel(SDL_Surface *screen, int x, int y, Uint8 R, Uint8 G, Uint8 B){
	Uint32 color=SDL_MapRGB(screen->format,R,G,B);

	if(SDL_MUSTLOCK(screen)){
		if(SDL_LockSurface(screen) < 0){
			return;
		}
	}

	switch(screen->format->BytesPerPixel){
		case 1:{ /* Assuming 8bpp */
			Uint8 *bufp;

			bufp=(Uint8 *)screen->pixels + y * screen->pitch + x;
			*bufp=color;
		      }
		break;
		
		case 2:{ /* Probably 15 or 16 bpp */
			Uint16 *bufp;

			bufp=(Uint16 *)screen->pixels + y * screen->pitch / 2 + x;
			*bufp=color;
		       }
		break;

		case 3:{ /* Slow 24 bpp mode, not usually used */
			Uint8 *bufp;

			bufp=(Uint8 *)screen->pixels + y * screen->pitch + x * 3;
			if(SDL_BYTEORDER == SDL_LIL_ENDIAN){
				bufp[0]=color;
				bufp[1]=color>>8;
				bufp[2]=color>>16;
			} else {
				bufp[2]=color;
				bufp[1]=color>>8;
				bufp[0]=color>>16;
			}
		       }
		break;

		case 4:{ /* probably 32bpp */
			Uint32 *bufp;

			bufp=(Uint32 *)screen->pixels + y * screen->pitch / 4 + x;
			*bufp=color;
		       }
		break;
	}
	if(SDL_MUSTLOCK(screen)){
		SDL_UnlockSurface(screen);
	}
	SDL_UpdateRect(screen,x,y,1,1);
}
		       
			       

int main(int argc, char *argv[]){
	SDL_Surface *screen;
	Uint8 R,G,B;
	int x,y;
	
        /* initialise SDL */
	if(SDL_Init(SDL_INIT_VIDEO) < 0){
		fprintf(stderr, "Unable to init SDL: %s\n", SDL_GetError());
		exit(1);
	}
	atexit(SDL_Quit);

        /* initialise screen */
	screen=SDL_SetVideoMode(640,480,16,SDL_SWSURFACE);
	if(screen==NULL){
		fprintf(stderr,"Unable to set 640x480 video: %s\n",SDL_GetError());
		exit(1);
	}

        /* main loop: draw one pixel to the screen at a time (which would be the
cause of the really slow performance, but gives a chance to see the effect
before it closes as soon as it finishes */
	for(y=0;y<480;y++){
		for(x=0;x<640;x++){
			if((x)&&(y)){
				R=(Uint8)(x/y)*x;
				B=(Uint8)(x/y)*y;
				G=(Uint8)(((y/x)*x)/2)+(((y/x)*y)/2);
			} else {

				R=0;B=0;
			}
			DrawPixel(screen,x,y,R,G,B);
		}
	}

	return(0);
}

And, for those of who just can't live without a makefile (meaning you'll probably want to save the above as sdleffect.c):

all:  sdleffect

sdleffect: sdleffect.o
        gcc -o sdleffect `sdl-config --libs` -Wall sdleffect.o

sdleffect.o: sdleffect.c
        gcc -o sdleffect.o -c `sdl-config --cflags` -Wall sdleffect.c

Or for those who just want a one line compile without calling make (pointed out by Profoundly_Slack):

$ gcc `sdl-config --cflags --libs` -Wall sdleffect.c

There, now have fun.
Also, if anyone wants to post any more interesting effects, go ahead