static S md5OfTransparentImage(BufferedImage img) ctex {
  MessageDigest m = MessageDigest.getInstance("MD5");
  m.update(intToBytes(img.getWidth()));
  int[] pixels = pixelsOfBufferedImage(img);
  for (int i = 0; i < l(pixels); i++)
    m.update(intToBytes(pixels[i] ^ 0xFF000000));
  ret bytesToHex(m.digest());
}