a property

num get a

@override
num get a => palette == null
    ? numChannels > 3
        ? data[_index + 3]
        : 255
    : palette!.getAlpha(data[_index]);

set a (num a)

@override
set a(num a) {
  if (image.numChannels > 3) {
    data[_index + 3] = a.clamp(0, 255).toInt();
  }
}