Combining high res photo with low res video in a static shot Suppose I have a 1080p video of a static shot with minimal motion and little change in lighting, and a higher res (~ 18 megapixels) still photograph of the same setup.

How would I go best about using the extra resolution of the photo if I wanted an ever higher res video output (say, 4K)?

My naive idea would be to scale up the 1080p video in an AF composition and use a blending mode for it to influence the still photograph below. I'd have to do rotoscoping to isolate the moving parts and put them on top (upscaled)..

Is there a better way?
To extract the moving things, I'd try a difference matte. There's a tutorial on doing it in After Effects here and another one here. You'll need to somehow extract the static background, probably by averaging a few (or several) frames of it together. Then use the techniques described above to subtract out the static stuff from the moving stuff. (Maybe a combo of garbage mattes and difference matte could do it? Depends on the footage.)

Once you've done that, combining the moving matted footage over the hi-res image (probably using a "Normal" blend mode) should get you what you want. I'd be interested to know how it looks in the end. I've seen composites that looked bad because the composited object had a different amount or type of grain from the background. I suspect being at a different resolution may be just as bad, given that the grain will be huge compared with the background, but since I've never done it, I can't say for sure. Good luck!