Is there a way to copy the luminance information onto the alpha and have the layer look as if it's been unchanged. The reason for my question is that it doesn't seem that redgiant has updated the unmult plugin neither has fandev with their xmult. The set or shift channels won't work as they don't compensate for the attenuation of the applied alpha channel. Who knows redgiant might upgrade the unmult but I need a solotion pretty quick.
I don't exactly follow... You can use Shift Channel just like you would a true Unmult effect if you add additional Levels controls. Just the same, you can use the technique described here or several other channel effects...