static ItIt<Pt> pixelSpiral(int sx, int sy, int imageWidth, int imageHeight) {
  ret iff_endOnNull(new F0<Pt> {
    int x, y, dx, dy = -1;
    int maxI = sqr(max4(sx+1, imageWidth-sx, sy+1, imageHeight-sy));
    int i;

    Pt get() {
      while (i++ < maxI) {
        int px = sx+x, py = sy+y;

        if ((x == y) || ((x < 0) && (x == -y)) || ((x > 0) && (x == 1-y))) {
            int t=dx; dx=-dy; dy=t;
        }   
        x += dx; y += dy;
        
        if (px >= 0 && py >= 0 && px < imageWidth && py < imageHeight)
          ret pt(px, py);
      }
      null;
    }
  });
}