Replacing Footage With Pixel Accuracy in After Effects or FCP

Say you need to replace low-res footage with a new, high-res version while maintaining pixel accuracy. First, create a new layer. Stack the new footage of top of the old.

In Final Cut, right-click on the top layer and choose Composite Mode>Difference. In After Effects, use the layer's drop-down Mode menu.

Your footage will take on an inverted and somewhat psychedelic appearance. But here's the great part: when the two versions are perfectly aligned, they will cancel each other out and your Viewer will become black.

Credit to Chad Perkin's and his excellent After Effects CS4 Beyond the Basics on lynda.com.