code: function linear (a, b) {
    if (!is.defined(a)) {
      this.options.linearA = 1.0;
    } else if (is.number(a)) {
      this.options.linearA = a;
    } else {
      throw new Error('Invalid linear transform multiplier ' + a);
    }
  
    if (!is.defined(b)) {
      this.options.linearB = 0.0;
    } else if (is.number(b)) {
      this.options.linearB = b;
    } else {
      throw new Error('Invalid linear transform offset ' + b);
    }
  
    return this;
  }
docs: Apply the linear formula a * input + b to the image (levels adjustment)
  @param {Number} [a=1.0] multiplier
  @param {Number} [b=0.0] offset
  @returns {Sharp}
  @throws {Error} Invalid parameters