user10458: 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?

user6207: 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!

10458: <p>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.</p>

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

<p>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)..</p>

<p>Is there a better way?</p>
 6207: <p>To extract the moving things, I'd try a difference matte. <a href="http://motionworks.net/after-effects-difference-matte/" rel="nofollow">There's a tutorial on doing it in After Effects here</a> and <a href="http://www.motion-graphics-exchange.com/after-effects/Difference-Matte-Effect-Tutorial/474f283e66ac2" rel="nofollow">another one here</a>. 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.)</p>

<p>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!</p>
