static IBWImage clipIBWImage(IBWImage img, int x1, int y1, int w, int h) {
  if (x1 == 0 && y1 == 0 && w == img.getWidth() && h == img.getHeight()) ret img;
  ret new IBWImage {
    public int getWidth() { ret w; }
    public int getHeight() { ret h; }
    
    public float getFloatPixel(int x, int y) {
      ret img.getFloatPixel(x+x1, y+y1);
    }
  };
}

static IBWImage clipIBWImage(IBWImage img, Rect r) {
  ret clipIBWImage(img, r.x, r.y, r.w, r.h);
}