let cleanblack surface x y =
        let (w,h) = get_dims surface in
        Sdlvideo.put_pixel_color surface x y (255,255,255);
        let x = ref 0 and y = ref 0 in
        while ((y < (h-1)) && (Sdlvideo.get_pixel_color surface x y = (0,0,0)) do
                while ((x< (w-1)) && (Sdlvideo.get_pixel_color surface x y = (0,0,0)) do
                        Sdlvideo.put_pixel_color surface x y (255,255,255);
                        x := (!x + 1)
                done;
        x := 0;
        y := (!y +1);
        done;

        x := (w-1);
        y:= 0;
        while ((y < (h-1)) && (Sdlvideo.get_pixel_color surface x y = (0,0,0)) do
                while ((x > 0) && (Sdlvideo.get_pixel_color surface x y = (0,0,0)) do
                        Sdlvideo.put_pixel_color surface x y (255,255,255);
                        x := (!x - 1)
                done;
        x := (w -1);
        y := (!y -1);
        done