GIMP Change pixel to transparent only if above/below layer color match

Question

I have an image - game map. It was generated a long time ago and not by me. I wan to reverse-engeneer this map to create my own tileset. E.g. I have a tile with grass with tree: And I have a tile with grass only:

I want with GIMP to detect only difference between each pixel. If they match - make them transparent. So that I can get tree tile. So the desired result should be like this three without grass:

How can I do this with GIMP?

This works but with wiered effects: Copy grass -> paste into new layer on top of three with grass -> apply blend mode for grass layer as color erase

Answer 1

It's hard to distinguish the green "native" to the tree and the green from the grass so Color erase is going to make things a bit transparent all over.

Possible technique:

1. Put the top layer in "Difference" mode.
2. In the Channels list, drag the Green channel to the general area to make a copy of it
3. Make a copy of the three+grass
4. Add a layer mask, initialize to the copied channel
5. Color > Invert the mask
6. Threshold the mask (very low value)

If needed you can paint the mask to restore pixels.

(the layer group isn't absolutely necessary, it is only used to show what the Difference looks like)

Edit: a slighty simpler/faster workflow is to replace steps 2 & 4 by:

• In 2., right-click the Green channel and Channel to selection
• In 4., initialize the mask with the selection.

Also, make sure that you aren't de-selecting the Green channel in the process (which is what happened in the screenshot above). You may have to explicitly re-select it.