@Artisto:
i completely agree with you that the whole Masking/Alpha stuff is way too complicated, still struggling after months of Natron usage…
However, try it with this tut: https://www.youtube.com/watch?v=9XxCc2mjS8g (its in french, but you get what to click).
Tried with a a single pic and I think its what you need:
Btw, there is another VFX tool on Linux you might like:
To be honest, stuff like this tracking and blur, I currently solve in a proprietary software called DaVinci Resolve, which kind of works on Linux, but has no official support when you are on Ubuntu.
More and more I do everything in Blender, see this tut: https://www.youtube.com/watch?v=vQtg1AoE1xk
Blender is getting more powerful every week and I guess in a few months I’ll also do all the video/vfx stuff in there…
Feel free to ask to get some inputs while switching your creative workflow to Linux.
It took me quite a while to be able to solve all the stuff in there.
Best thing is that when you report a bug or ask a question, the dev himself will help you. Its just a completely other world…