function unflatten () {
  this.options.unflatten = true;
  return this;
}
Ensure the image has an alpha channel
with all white pixel values made fully transparent.

Existing alpha channel values for non-white pixels remain unchanged.

This feature is experimental and the API may change.

@since 0.32.1

@example
await sharp(rgbInput)
  .unflatten()
  .toBuffer();

@example
await sharp(rgbInput)
  .threshold(128, { grayscale: false }) // converter bright pixels to white
  .unflatten()
  .toBuffer();