UserFilters works for Gimp-2.10 (for Win and LInux) - but will not work for Gimp-2.99/3.0 unlike G’MIC.
The G’MIC - User-Defined filter seems pretty close, but for *.guf the code requires a lot more conversion.
I eventually wrote a Photoshop plugin host for Paint.NET (based on GIMP’s PSPI) to allow it to use Filter Factory plugins directly, that was easier than trying to fix any bugs in the UserFilter code or trying to write my own parser/interpreter for that language.
My first (fuzzy and incomplete, making me susceptible to Dunning–Kruger effects) impressions, formed primarily from the programming guide, is that if G’MIC-QT never had come along, this would be pretty exciting stuff — and in the late nineties, I am sure it was. but GMIC-QT is here, with the underlying gmic pipeline processor and its allied math expression processor, and, to my possibly DK affected mind, those have occupied the mind-share which the UserFilter plug-in first had.
I have that impression as well; but for notation differences, In the G’MIC-QT User-Defined filter, I write math expressions to populate various channels with procedurally-generated imagery. What I’m missing, I think, is the analogue to the .guf file which can be named, written out, and retrieved by name later. A G’MIC-QT filter which writes out its own filters, then reads them back in later (meaning: a GUI file dialog too). That’s a lot of coding pain; it would have to have a Really Exciting Endpoint!!! to furnish sufficient motivation. I’d rather go case-by-case, look at the behavior of *.guf filters in the original plug-in, and if one seems especially engaging, re-engineer it as a straight GMIC-QT filter. That falls short of @MrQ request, I think, but @PDN_GMIC raises the practical issue of cost-v.-benefit.
Honestly, I think G’MIC-QT could use a upgrade in its interface option. I had suggested a way to append GUI items next to GUI items to allow for grid mode. And this would make things more reasonable interactive. A upgrade would make something like that possible. I wouldn’t mind tabs either.
Still use a few userfilters (inc. Ink Rubber (like how it darkens blues and greens in a unique way while brighten reds. Use it with other blendings to fake an IR). Sad that it won’t be carried over when 3.X comes out. I remember when I was keeping 1.2x around for the unique filters it had that never carried over to 2.x world. Decided not to continue using it when I got my new PC. lol
Will keep 2.10X around so hopefully, the developers won’t force giving up it when 3.x gets released (like they did when 2.6x went to 2.8x as I recall).
