static BWImage clipBWImage(BWImage img, int x1, int y1, int w, int h) {
  if (x1 == 0 && y1 == 0 && w == img.getWidth() && h == img.getHeight()) ret img;
  BWImage img2 = new(w, h);
  copyBWImage(img, x1, y1, img2, 0, 0, w, h);
  ret img2;
}

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