GIMP Version: 2.10.28
Operating System: Windows
GIMP Experience: Intermediate Level

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Fonts.conf has a notice saying do not edit, as it is automatically overwritten. It directs one to store local changes in local.conf.

Where can I find documentation necessary to produce a 51-local.conf that excludes the Windows font folders and only uses fonts in the Gimp folders? (this is ALL I want to do with it currently)

I can edit fonts.conf easily enough to get this result, but it gets overwritten regularly. I'm commenting out lines, and it's not clear how to do that from 51-local.conf. The fontconfig documentation was huge, abtruse, and didn't really address local.conf at all, except for ADDING fonts.

Any pointers? There is an existing script to delete and/or replace the Windows folders from fonts.conf, but that's not really ideal.

I got this far, but this doesn't answer the question of what goes IN the file. From Stackoverflow:
"I found the solution for this, Basically we have to create a local.conf and add customized configurations to it. It should be read automatically by font-config, But it is not the case then check that if conf.d have some file which will include local.conf (by default that file is 51-local.conf)."

That's what I found. It's a hack to automate editing of the fonts.conf file. I've done that, and it works, more or less.

I also found this, a Gentoo Linux page that describes where to put local.conf, and shows how to enable and disable individual fonts.
https://wiki.gentoo.org/wiki/Fontconfig

And another old linux page with a bunch of local.conf examples. In these examples, one can ADD font folders to local.conf, but can't REMOVE them. And gimp setup adds the windows folders on every update. And all those pages mention that fontconfig documentation is sparse.

If I can't find anything better, I'll just use a batch mode text editor to add and remove the windows font folders from fonts.conf, and have batch files to start gim with and without windows fonts. I'd rather have it outside gimp myself, but if you needed it THAT often, and if gimp rescanned the fonts when you hit the toggle. a plugin toggle would be cool.

Ultimately the entire PURPOSE of fontconfig is something I don't, in general, want done. "The intent is to make font configurations relatively static, and shared by as many applications as possible." I don't want all my specialist fonts available to all my software, and I don't want the system to substitute for missing fonts, or guess at what it thinks might be close, but to tell me they're missing and let me choose a substitute. So I really don't expect to get it to do what I want.