GIMP 2.8 Python Plug-in.
This plugin allows you to create layer mask from another layer mask.
Or create layer mask from grayscale copy of layer.

Installation:
1. Extract "create-layer-mask-from.py" from zip archive to the ".../.gimp-2.x/plug-ins" directory.
2. If you're on a UNIX system, make sure "create-layer-mask-from.py" is executable.
3. Once you have saved the file in your plug-ins folder, if you are already running GIMP, close and reopen it.
4. If you have done this correctly, the scripts will appear in the menu under: "<Image>/Layer/Mask/Create Layer Mask from..." and "<Layers>/Create Layer Mask from...".

Download:
Download from my deviantart page:
slybug.deviantart.com

Tested with GIMP 2.8.
I hope this plug-in will be useful for someone else.

Version History:
v1.0 (30.08.2012)
- Initial release.

My other plugins:
Duplicate to another Image
Layer via Copy/Cut
Scale Layer to Image Size

Thanks Slybug.

i worked a lot with layermasks, they are a very useful technique, but i have no idea what this script does and what i could with it

why would i want to create a layermask from another layermask ??

apologies, if thats a stupid question
but any explanation would be really helpful

I am glad you asked it Esper, cuz I thought the same so now there is two with the same stupid question.

... and what is wrong with copy/paste of the layer mask between layers?

Not sure what may be wrong but every time i tried didn't worked

Make mask of source layer the active drawable
Edit/Copy
Add empty mask to target layer
Make mask of target layer the active drawable
Edit/Paste -> creates floating selection
Layer/Anchor -> Floating selection becomes mask of target layer

Complete no-brainer if both layers are the size of the canvas. If the layers are equal size but smaller than the canvas and at different coordinates, the paste adjust itself to the target...

You're welcome Rod!


The main feature of this plug-in - is an option that allows you to change the interaction between two masks (Add, Subtract...).



Otherwise, the plug-in just a little speeds up the process of copying mask from one layer to another.
So... each to his own
Sorry for my english...

i appreciate that speed up i hate copy, paste ,anchor ,also because even if is a trivial process always something go wrong when i do with layer mask (basically it refuse to anchor on the mask but only accept to get pasted to the layer .) ,so i am happy for a script speeding up the process.

Because you have to be editing the target mask (and not the layer) at the time when when you paste.

not sure if i was editing but i am sure i took care it was the active thing in that moment ...but of course you are right is doable and i should have goofed and done some dumb deviation from the correct steps...
but still i am happy to have not to try again because a script that could take care