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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?

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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!

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It's an interesting idea, and I'd also like to see the results.

My first thought is that it would probably look very odd. My feeling is that the eye is drawn to movement first, and detail second.

For instance, if your example is a locked-off interview shot, where everything is 4K resolution, except for the interesting part - e.g. the talking head, the viewer is barely going to notice that the background is higher quality, because all their attention is going to be drawn to the part of the image which is lower resolution. Does that make any kind of sense?

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