I should know how to do this, but after trying several keys and many different modes, it has me stumped. I have a movie already made in Mo3. It has lots of white on black stills rotating and whatnot. The project, for better or worse, was created with a transparent background. In an art installation I projected this movie onto a screen made of blueprints... I now have a photo of the blueprint screen and want to composite it with the movie so the "new" movie documents the installation view - ie where the movie is lighter, the screen shows through more. I've tried everything I know (which is obviously not enough!) and would appreciate your expertise on this.