code: function flatten (options) {
    this.options.flatten = is.bool(options) ? options : true;
    if (is.object(options)) {
      this._setColourOption('flattenBackground', options.background);
    }
    return this;
  }
docs: Merge alpha transparency channel, if any, with a background.
  @param {Object} [options]
  @param {String|Object} [options.background={r: 0, g: 0, b: 0}] - background colour, parsed by the [color](https://www.npmjs.org/package/color) module, defaults to black.
  @returns {Sharp}