static S md5OfRGBImage(RGBImage img) ctex {
  MessageDigest m = MessageDigest.getInstance("MD5");
  m.update(intToBytes(img.getWidth()));
  int[] pixels = img.getPixels();
  for (int i = 0; i < l(pixels); i++)
    m.update(intToBytes(pixels[i]));
  ret bytesToHex(m.digest());
}