Make alpha dependent on RGB

[Helios]

Hello everyone. I have this simple normal image. What i want to do is to edit alpha channel as a curve, but based on Red and Green channel. RGB channels must remain unchanged ("colour to alpha" change them). I know Colours - Levels/Curve tools, thats ability i want, but i haven't find out how to use input channel different from output channel. So i can edit for example Red in different shades of Red, but not based on Green. 
 I really need it similar to curve, in my example image there is only few shades, but i want to learn proper workflow for complex images too.

I'm using GIMP 2.10. I'm total gimp newbie, i use it only for simple edits of my textures.. So please detailed answer is appreciated.

[rich2005]

Can you post your image in a format that contains the alpha channel you want to edit. Gimp .xcf or maybe .png

Quote:What i want to do is to edit alpha channel as a curve, but based on Red and Green channel

Is this possible ? An alpha channel is just a greyscale mask, nothing in there to adjust red or green ?

[Helios]

Well right now its simple full opacity. I have yet to create proper alpha, but thats what i need, base it on RG channels.

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(10-10-2020, 09:26 AM)rich2005 Wrote:

Quote:What i want to do is to edit alpha channel as a curve, but based on Red and Green channel

Is this possible ?  An alpha channel is just a greyscale mask, nothing in there to adjust red or green ?

I have green channel 0-100. I want my alpha to be 0-50, 50 where green is 100, alpha 25 where green is 50...etc etc. Or even non linear - curve

[rich2005]

Is this for one of those games engines that uses the alpha channel independent of the image?

I do not know, this might work

Take a duplicate of the image.
Colours -> Colour curves and adjust the green curve to suit.
Layers -> Mask -> Add layer mask with greyscale copy of layer option
Copy the original image and paste into the modified
As far as I can see that now has the original RGB and an alpha channel based on green.
some screenshots of that sequence. https://i.imgur.com/PNvwxOQ.jpg
You can apply the layer mask Layer -> Mask -> Apply or I assume exporting in whatever format ( tga ? ) will do.

[Helios]

Ok, thank you. From your advice i learnt that greyscale copy gives me alpha i want after i invert red channel. So not exactly what i was looking for, but works good enough i guess. I will see at more complicated images.

Yes, its for game which uses alpha channel in normal image to calculate snow coverage.