Now the only thing I have to do is to recreate a 4 layer PNG. That's my goal for today!
In retrospective I shouldn't have played Oolite so much today...
I'm still stuck somehow. Gimp creates new files in every step concerning the splitting and combining of the layers, and in the end I have a nice 3 channel PNG but the alpha channel is lost.
That's what I did so far:
1) Open the original file "... diffuse.png"
2) Colours-> Components-> Decompose-> RGBA. This creates a file with 4 layers R, G, B, and A.
3) Colours-> Components-> Compose-> RGB (I'm not sure what the menu is called in the English version of Gimp, as I'm using the German version). This joins the R, G, and B layers and creates a "normal" RGB bitmap as a separate PNG file.
4) I can change this file by simply painting to it or by adding new layers with decals, the last step being to join all layers. Everything said about decals, surface structure and opacity is part of this step.
But now what? I probably have to import the alpha channel from the decomposed file created in step 2. But I can't figure out how to do it. I also tried try to import my decal layer into the original file or into the file created in step 2) but so far I haven't succeeded in getting the desired result.
It has become late. Maybe tomorrow evening...