// start from center
static ItIt<Pt> pixelSpiral(int w, int h) {
  ret pixelSpiral(w/2, h/2, w, h);
}

// thanks to https://stackoverflow.com/questions/398299/looping-in-a-spiral
static ItIt<Pt> pixelSpiral(int sx, int sy, int imageWidth, int imageHeight, int maxPixels default Int.MAX_VALUE) {
  ret iff_endOnNull(new F0<Pt> {
    int x, y, dx, dy = -1;
    int maxPixels2 = min(maxPixels, sqr(max4(sx+1, imageWidth-sx, sy+1, imageHeight-sy)*2));
    int i;

    Pt get() {
      while (i++ < maxPixels2) {
        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;
    }
  });
}