code: function flipFn(horizontal, vertical, cb) {
    if (typeof horizontal !== 'boolean' || typeof vertical !== 'boolean')
      return throwError.call(
        this,
        'horizontal and vertical must be Booleans',
        cb
      );
  
    if (horizontal && vertical) {
      // shortcut
      return this.rotate(180, true, cb);
    }
  
    const bitmap = Buffer.alloc(this.bitmap.data.length);
    this.scanQuiet(0, 0, this.bitmap.width, this.bitmap.height, function(
      x,
      y,
      idx
    ) {
      const _x = horizontal ? this.bitmap.width - 1 - x : x;
      const _y = vertical ? this.bitmap.height - 1 - y : y;
      const _idx = (this.bitmap.width * _y + _x) << 2;
      const data = this.bitmap.data.readUInt32BE(idx);
  
      bitmap.writeUInt32BE(data, _idx);
    });
  
    this.bitmap.data = Buffer.from(bitmap);
  
    if (isNodePattern(cb)) {
      cb.call(this, null, this);
    }
  
    return this;
  }
docs: Flip the image horizontally
  @param {boolean} horizontal a Boolean, if true the image will be flipped horizontally
  @param {boolean} vertical a Boolean, if true the image will be flipped vertically
  @param {function(Error, Jimp)} cb (optional) a callback for when complete
  @returns {Jimp} this for chaining of methods