docs: public static String watermark(String imageUrl, int x, int y, int transparency) {
      if (imageUrl == null || imageUrl.length() == 0) {
        throw new IllegalArgumentException("Image URL must not be blank.");
      }
      if (transparency < 0 || transparency > 100) {
        throw new IllegalArgumentException("Transparency must be between 0 and 100, inclusive.");
      }
      return FILTER_WATERMARK + "(" + imageUrl + "," + x + "," + y + "," + transparency + ")";
    }