docs: public static String blur(int radius, int sigma) {
      if (radius < 1) {
        throw new IllegalArgumentException("Radius must be greater than zero.");
      }
      if (radius > 150) {
        throw new IllegalArgumentException("Radius must be lower or equal than 150.");
      }
      if (sigma < 0) {
        throw new IllegalArgumentException("Sigma must be greater than zero.");
      }
      return FILTER_BLUR + "(" + radius + "," + sigma + ")";
    }