LRGB Images in GIMP

[lepton3]

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

[troypiggo]

It's been a while since I used GIMP, but what you're doing with the RGB sounds like it'll work, and using the layer property of "colour". Maybe what you need to do is also force the L layer to luminance instead of normal. IIRC GIMP calls it "value" as a layer property. So do what you were already doing, but change the L layer mode to "value" instead of "normal". See how that goes.

[lepton3]

Thanks for the suggestion Troy. I tried that, L as "value" on top of the RGB, and it does seem to be a bit better than RGB as "colour" on top of L.

I'm still researching, but that may be as good as it gets for GIMP. I guess I can play with the curves to make the contrast a bit less severe.

-Ivan