Process invented by Ray Harryhausen to combine model shots with live action in stop-motion animation. The live footage is back-projected onto a screen behind the stop-motion elements, with a plate placed between that and the camera. Onto the plate is painted the silhouette of anything from the live footage (say, buildings) that should appear in front of the stop-motion stuff, so that anything that is shot at this stage has these areas blocked out. Then the stop-motion setup is cleared out, the blocked out areas on the plate reversed, and the film from the first stage is exposed again, filling in the blocked areas from the first pass.