Pretty straight forward question, say in Gimp I have an image with 2 layers in question, one is temporary, I want to use it as the mask for the other. How can I transfer a layer into the mask of another? I get as far as having a grey scale floating selection which I can't place on the mask. I know how to paint on a mask, or use a layer's grey scale as it's own mask, but not a layer's grey scale for a different layer's mask.
Hope that makes sense