[lepton3 (Ivan)]

I'm playing around with some of my planetary images with GIMP, and it occurs to me that I don't know how to create an LRGB image properly.

I've always done it by loading R,G and B as separate layers, then colorifying R,G,B as Red, Green, Blue respectively, and changing the layer modes to "addition". Then I merge them, which gets me a RGB layer which looks great.

Now I load the L image as a separate layer, and if I change the mode of the RGB layer to "colour", and place it over the L layer, I get what I thought was an LRGB image. But it's not, because the RGB layer has luminance information in it already, so what I really get is a higher contrast RGB (rgb with 2 doses of luminance).

What I think I need is a way to convert an RGB to Hue+Saturation (no Value), and combine it with the L layer using that for Value, which would yield a HSV image. I've experimented with the decomposition into HSV, and the various Hue, Saturation and Value layer modes, but haven't achieved any sensible results.

So how is it done? Can GIMP even do this? Can Photoshop?

-Ivan