Compose not seeing layers

[Rowdy Wallbanger]

Folks,
I'm a self taught GIMP user, please be gentle!
I'm painting car templates for a racing simulator called iRacing.  iRacing has Spec Maps which add Chrome and Roughness to the image.
The template contains layers for RG and B Channels.  They are setup to work in Photoshop which can handle layers in Greyscale.  These layers work in such a way that they merge to make a combination, which doesn't work in GIMPS RGB only way.
However, there is a work around which is to copy the image, make it's mode Greyscale and then Compose the R,G and B channels, adjusting the greyscale to your liking.  We export the file into the program as a TGA.

The Problem is this: If I make a fresh project this process seems to work fine.  However my old projects don't always work, when composing I only see a "Mask Layer" option.  Why is it not seeing the other layers that exist in the XCF file?
Kind Regards,
Rowdy.

[Ofnuts]

How do you save your "old" projects? If you don' t save them as XCF the layers are lost.

Otherwise you can work with a setup like this:

• The Groups and "Blue" are in "Addition" mode
  
• The Groups are made of a Color layer in "Multiply" mode, plus your actual work layer (in "Normal" mode)
  
• At the bottom the layer mask of the Alpha layer dictates the alpha channel of the whole image
  

   

To work on a specific layer more comfortably you can make most of the rest invisible:

   

You can also split an existing TGA into a similar setup if necessary (but I have no time to describe how right now).

See XCF attached.

[Rowdy Wallbanger]

OfNuts, thanks for the response, it does seem to work as a merging rather than a hierarchical layering.  Is the solution to that by changing the layer property to "Multiply" and the Groups to addition?  I do save them as XCF's.
I will try to send either my problematic file or an example of the way the Spec map layers are formatted.

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Here are the spec layers and groups as they come...

Please note that below these layers there are more paintable layers for the livery. I gave making groups Additional and the layers multiply a go, but with the parts layers multiply seemed to ignore them or add to them. Also changing them from Normal had the lower Painted layers come through. Currently the Spec Map layers trump everything once made visible (although I could turn those layers off.)

[Ofnuts]

(11-23-2021, 11:12 AM)Rowdy Wallbanger Wrote: OfNuts, thanks for the response, it does seem to work as a merging rather than a hierarchical layering.  Is the solution to that by changing the layer property to "Multiply" and the Groups to addition?  I do save them as XCF's.
I will try to send either my problematic file or an example of the way the Spec map layers are formatted.

---

Here are the spec layers and groups as they come...

Please note that below these layers there are more paintable layers for the livery.  I gave making groups Additional and the layers multiply a go,  but with the parts layers multiply seemed to ignore them or add to them.  Also changing them from Normal had the lower Painted layers come through.  Currently the Spec Map layers trump everything once made visible (although I could turn those layers off.)

Yes the layer & group blend mode are important: addition for groups, multiplication for color layers

Yes, you can have several layers per feature (all in "normal" mode in the group, below the color layer).

Your Blue layer should be blue or black. Otherwise,  its R&G components will saturate the result.

But what is the purpose of the top level group you added?

[Rowdy Wallbanger]

Sorry, which Top Level group?

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Here is iRacing's rather long winded tutorial 
https://www.youtube.com/watch?v=H-oSKG8d...el=iRacing  

Gimp users ignore the "use the red channel for chrome, green for roughness" and blend the colours using RGB values. 
https://www.youtube.com/watch?v=fTmyujok...rkerRacing.

I was trying to find a way to bring both groups together and have fallen down this hole.  I like the idea of using the greyscales, but that increases the number of duplicate layers i.e. you need a Logos layer for gloss and roughness.  Whereas, with Gimp you blend Red and Green at the same time.

With your latest example how would I change the amount of Gloss for a Logo layer?

I'm trying to send you a better example but they're all to big.