vapoursynth is a cross-platform implementation mostly analogous to avisynth in usage.
There are many plugins (I have some basic ebuilds I can file later), and it is since a while supported as powerful video-filter in mpv (in mpv-ebuild at the moment forcefully disabled, as vapoursynth is not packaged => main reason for this ebuild).
I only attach a live-ebuild for now - all releases have issues, the latest one has problems detecting python 3 (and python 3 is required). I wrote the ebuild in a way also the releases should be supported (once there is a working one).
Please review carefully - this is my first ebuild written for portage submission, feel free to RTFM me to your liking.
Created attachment 394366 [details]
live ebuild for vapoursynth
Sorry, but I just realized this was not clear from my original report:
The live-ebuild is working stable for me on amd64, I have no other platform to test on.
This suggests that also their next release will build fine, but sadly releases are non-regular, about each 0.5 years.
Created attachment 394438 [details]
live ebuild with support for media-libs/ffmpegsource
I updated the ebuild to also support media-libs/ffmpegsource if the corresponding use-flag is set (symlinking the library for auto-loading by vapoursynth).
For all users coming here in expectations of a more "finished product": The very active "vapoursynth" overlay at https://github.com/4re/vapoursynth-portage provides a very good vapoursynth experience.
I will also direct my efforts in improving further ebuilds to this common place.