I was wondering if you could implement optical flow kinda like adobe did with their video editor software, upon a quick google search it appears OpenCV is used for this. From my understanding, this gives a very clean frame doubling, better than most methods at least.
In the meantime, here is an alternative using FFmpeg command line:
And here is why that filter isn’t in Shotcut already:
For my personal workflow, I create slow-mo segments outside of my timeline. Optical flow can generate a lot of unsightly artefacts (even using Premiere), and I’d rather not be surprised by them in my final Shotcut export. Given how slow optical flow is to render, I like to pre-process segments outside of Shotcut, review it, tweak it, and when it’s acceptable, import it into my timeline. I also get real-time preview playback that way. But that’s just my personal preference.
For an example of optical flow artefacts generated by Premiere, see the screenshots in this thread: