code: function blur (sigma) {
    if (!is.defined(sigma)) {
      // No arguments: default to mild blur
      this.options.blurSigma = -1;
    } else if (is.bool(sigma)) {
      // Boolean argument: apply mild blur?
      this.options.blurSigma = sigma ? -1 : 0;
    } else if (is.number(sigma) && is.inRange(sigma, 0.3, 1000)) {
      // Numeric argument: specific sigma
      this.options.blurSigma = sigma;
    } else {
      throw new Error('Invalid blur sigma (0.3 - 1000.0) ' + sigma);
    }
    return this;
  }
docs: Blur the image.
  When used without parameters, performs a fast, mild blur of the output image.
  When a `sigma` is provided, performs a slower, more accurate Gaussian blur.
  @param {Number} [sigma] a value between 0.3 and 1000 representing the sigma of the Gaussian mask, where `sigma = 1 + radius / 2`.
  @returns {Sharp}
  @throws {Error} Invalid parameters