static Rect rectOfDifferentPixels(RGBImage img1, RGBImage img2) {
  rgbImageAssertSameSize(img1, img2);
  Rect r = null;
  int w = img1.getWidth(), h = img1.getHeight();
  for y to h: for x to w: {
    if (img1.getInt(x, y) != img2.getInt(x, y))
      if (r == null)
        r = singlePointRect(x, y);
      else
        extendRectToIncludePoint_modifyOriginal(r, x, y);
  }
  ret r;
}