Could anyone please explain how I can create a difference matte between a clean frame and a frame with an actor moving through it so that I can isolate his reflection in a window? The window was reproduced and cleaned up as a matte painting, and I want to overlay his reflection on the painting.
Masking out the old window, revealing the new window with the reflection, is probably the easiest and fastest way to do it.
AE does have a difference key: it compares a clean plate to the same shot with a new subject in it, removing the duplicated pixels. However, it's designed to work with absolutely perfect footage -- the clean plate and the new shot's background must match almost pixel-for-pixel. Thus, it rarely works well: hardly anyone shoots absolutely perfect footage, even shooting comparatively lightly-compressed footage. Any video shot on a DSLR or HDV camera is quite heavily-compressed.
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I've had a bit of success with this. I'm precomping the output from the difference matte, cleaning up the mess, then layering it on top of my painting layer. Seems to be doing most of what I was hoping.