There are two related things I want to do:
One thing is for a layer turn white into transparent and white-ish things into partially transparent.
Specifically, for every pixel in that layer with H, S, V, A (w, x, y, z), I want to replace it with a pixel with H, S, V, A (w, x, 0, (100-y)%*z)
Another thing is I want to be able to control one layer's transparency with another layer.
So basically the idea is I have a layer on top in greyscale, and a layer below it with an image at full opacity, and I want to change the image's alpha values to the color values of the layer on top.
Is there any plugin or layer blend mode that would let me do this?