Make "boomerang" video

I think I'd like to see the ability to have it "boomerang" the video, i.e. go from min to max back to min.

Is this something we need to encode in the video, or can we tell formats to play backwards? Do we encode the frames again, or reverse the video and concatenate it, or maybe is there a fancy filter for it?

A super quick search found this: https://stackoverflow.com/questions/42257354/concat-a-video-with-itself-but-in-reverse-using-ffmpeg

If we did this, we'd want to tell the formats they can loop, if that's something we can do.