Any on-screen live effects widget for Linux?

[ChameleonScales]

Looking for a way to bypass GIMP's lack of dynamic filters and color adjustments in some situations, I wonder if there exists any real-time widget software that performs image effects and adjustments on a given region of the screen.
This could be useful to quickly analyse an image or to preview the result of an adjustment without having to merge all the layers and apply the filter in GIMP.

To give you a better idea of what I'm looking for, there is this app, "Scopes", that displays the vectorscope of a given screen region. What I would like is something that lets you preview filters or effects such as curves or levels on that region (could be other effects too).

[CHJ85]

There's a plugin for Gimp called G'Mic. It's essentially a repository with 100s of filters and effects. You can preview most of them on-the-fly.
Gmic is great because you can apply the filters to specific layers, selected areas or the entire thing.
If you're on a Debian/Ubuntu based system, the repo version of gmic is a bit dated and you should download it from their website instead (gmic.eu).
But if you're on Arch or Manjaro, you can just grab it from the AUR.

[ChameleonScales]

Thanks but I use G'mic and that's not what I'm looking for here. I'd like something that would behave as an overlay on the screen so that I could continue working on my image.
Essentially, this would be the same as having a filter layer in Krita, except it would obviously not appear in an export.