Listing 3. Draw three parallel lines of pixels.
  for(int cnt = 0;cnt < 400;cnt += 3){
    putpixel(screen,cnt,400-cnt,makecol(255,0,0));;
    putpixel(screen,cnt-1,400-cnt-1,makecol(0,255,0));
    putpixel(screen,cnt+1,400-cnt+1,makecol(0,0,255));
  }//end for loop