Thanks. Yep, a pure GPU approach is probably the way to go for this stuff in general. One reason I used FFmpeg here was the goal of applying the effect to stand-alone videos/images, which you can't conveniently/easily do with shaders (AFAIK).

The other reason is that my shader coding experience is pretty much zero, something I should fix at some point. ;) But if someone more knowledgeable gets inspired by this in the meantime and cooks something up, that'd be cool for sure.