I have both GIMP and Raw Therapee installed on Linux Mint as Flatpak and they don't see each other.

Consequently GIMP won’t open my Sony ARW raw files. I also have Flatseal installed and have opened my file system to both apps but still nothing. What do I need to allow access to so these two can talk to each other?

Gimp in flatpak is sandboxed and doesn’t see the standard file system. Popular binary plugins (GMIC, resynthesize) are also provided as flatpak packages because of this.

OK so GMIC will open GIMP up?

I used Flatseal to open my file system so RT would see my SSD. I’m not sure it’s possible between two apps though.

No, but if you install the GMIC flatpak it magically shows up as a GIMP plugin. SO maybe there is a RT flatpak that does the same.

Personally, I have never seen the point of the demosaicing plugins. Either you spend very little tile in the plugin, and directly feed the image to Gimp, in which case the Jpeg from the camera (or with modern cameras, the 10-bit HEIF) will do just as well (or possibly better, because it could also have applied lens corrections), of you do some careful processing in the demosaicing app, in which case you want to save the intermediate results, or try several things, and then working as the Gimp file loading plugin is more of a hindrance than a help.

That’s the thing, I installed RT as flatpak so both GIMP and RT are flatpaks but they don’t see each other. It’s not a huge deal to open the raw image first in RT and convert to TIF or JPG and then open in GIMP but it should be a little simpler.

The RT flatpak use to carry a bit of code to send to the gimp flatpak, not sure I’d it is still there, check the external editor options in RT.

RT shows GIMP as the external editor. I’ll have to play around with it some more.