GIMP Version: 2.10.12
Operating System: Windows
OS Version: Windows 10
GIMP Experience: Basic Level

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I'm looking for help figuring out how to make a selection between two arbitrary figures. This is best explained with an example.



How can I create a selection halfway between the red/green border and the green/blue border?

Thanks.

I should have been clearer. The image I'm actually working with is sufficiently complex and large that I need a way to find the midway point automatically, rather than doing it freehand.

I think Peglegg had skills that can do this... like transition one shape, then transition another shape and then cut off where they meet...some how i forgot.

Also, maybe you can benefit of:
Gimp Scripts and Plugins Topic: path-inbetweener
You have to transform first the two border selections into paths.

viewtopic.php?f=9&t=4568

dinasset,
that path-inbetweener're requirement is must have same anchor of points.
so i don't think it works in this case.

OK, Tin; I'm not an expert of paths.

Probably have to do it manually using path tool.

The problem is defining "half-way", for instance in areas like the "tail" in the bottom right of the example. There is a very equivalent problem elsewhere in Gimp: the "shaped" gradients, but if you try these you will see that they cop out:)

I also envisioned path-inbetweener but it requires the same number of anchors (and in the same order), so it can work for simple shapes but it
not terribly useful for arbitrary complex shapes.

Also trie to blur the thing and figure out the pixels where R=B but results aren't that good.

I don't know or not familiar with centerline trace but you could try selecting the blue area and centerline-tracing that.
Maybe others can help better with their centerline trace experience.

Just a suggestion Trandoductin (instead of completely manually) - as a start to the expected result (color to selection, on the new layer fill in the selection, selection none, plugin-trace: centerline x 2)

Centerline trace seems to do the best that I can think of.
I exported your blue and green as separate black images over transparent (let's call them large.png[blue converted to black] and small.png[green converted to black]).



Then used this online site to centerline-trace the black images.
https://online.rapidresizer.com/tracer.php
and got these as svg, I just load them up in GIMP and convert using default settings:



I then brought these centerlines in as layers on top of original image.
and make minor adjustments such as closing of the area, if it's not yet a closed loop (like in this case the large centerline was closed off, but the small [green area] center line wasn't closed off so I draw extra pixels to close it off.
Then select the area inside and colored it like below.
with the centerlines layers turned on (and colored as white)

with the centerlines layers turned off.

Then I chose the area, grew selection by 2 pixel and shrunk it back down by 2 pixels so that it would get rid of those cuts/lines that go inside the shape.


That's it.