After I ugraded form Ubuntu 16.04 to Ubuntu Mate 18.04, I wanted to try out GIMP 2.10, so I installed the flatpak GIMP 2.10.2 from flathub. (The snap containers only offers 2.10.0 at the moment)
sudo apt install flatpak
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak search gimp
flatpak install flathub org.gimp.GIMP
Running GIMP:
flatpak run org.gimp.GIMP
Works fine, but now I need some plugins:
How do I install plugins like Liquid Rescale or G'MIC. The old way won't work, it will install GIMP as a dependency.
sudo apt install gimp-plugin-registry
For scripts and Python plugins, a simple solution is to put them in some plain directory (outside of the flatpak packaging) and use
Edit>Preferences>Folders>{Scripts|Plugins}to add them to the directories checked by flatpak Gimp (this is also a good way to share them between your Gimp 2.8 and Gimp 2.10).Binaries are a different matter, binaries for 2.8 may not work with 2.10 (and in many case will have to be at least recompiled to take advantage of the new bit depths in 2.10), and the new binaries may have dependencies on libraries that are only available in the flatpak environment.
There is also the (rare) occurence of a Python script that depends on a python library which isn't part of the Python installed in the flatpak environment (which isn't the same as your system Python).
These problems are at the core of many open issues.
GIMP Flatpak offers these paths for plugins (Preferences -> Folders -> Plug-ins)
Copy a plugin into the first folder and restart GIMP.
You could download the source package, and copy over the needed scm files manually to above folder