GIMP Version: 2.8.4
Operating System: Windows
OS Version: 7
GIMP Experience: Beginner Level

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What I have is something like this:


The red is the background layer and is a nice image of glowing lava.
The black is the foreground layer and is a image of a series of rocks with non-trivial borders.

What I want to do is blend the edges of the 2 layers such that it would appear that the rocks have a glowing red edge because of the light from the lava, but I don't want any of the rock pixels to blend into the lava layer.

How can I achieve this one-way blend effect?

How about making a path of the edge and stroking it with a brush?
Subject: Stroking paths with Brush Dynamics – GIMP 2.6

You could also maybe duplicate your black foreground and apply and "outer glow" to it using a lighter or darker color of the background layer. To create the glow.

Looking into it, but I was hoping for a more automatic method



The background layer is not homogenous, it has lighter areas and darker areas; I'd like the glow to reflect that

Copy some of the lava onto a new layer and do your blending there.

I tried duplicating the layer 3 times, then merging them down then duplicated another time and changed the top layer to multiply but I don't think it is what you are looking for.

Thanks to both of you for your inputs.
I actually ended up doing a combination of what you guys said:

1. Trace the border of the foreground
2. Grow the border inward and outward to get a strip of selection
3. Use the selection to cutout a piece of the background
4. Gaussian blur the cutout
5. Trace the border of the foreground again and use it to remove areas of the cutout that go beyond the foreground boundaries
6. Choose Layer->Overlay and adjust opacity

The result looks decent enough, though it looks very similar to just blurring I'm afraid:

BEFORE

AFTER