Rear projection

Rear projection (background projection, process photography, etc.) is one of many in-camera effects cinematic techniques in film production for combining foreground performances with pre-filmed backgrounds. It was widely used for many years in driving scenes, or to show other forms of "distant" background motion.

Technique

Actors stand in front of a screen while a projector positioned behind the screen casts a reversed image of the background. This requires a large space, as the projector needs to be placed some distance from the back of the screen. Frequently the background image may initially appear faint and washed out compared to the foreground. The image that is projected can be still or moving, but is always called the plate. One might hear the command "Roll plate" to instruct stage crew to begin projecting.

These so-called process shots were widely used to film actors as if they were inside a moving vehicle, who in reality are in a vehicle mock-up on a sound stage. In these cases the motion of the backdrop film and foreground actors and props were often different due to the lack of camera stabilizing in the filming from the moving vehicles used to produce the plate. This was most noticeable as bumps and jarring motions of the background image that would not be duplicated by the actors.

A major problem with rear projection use was that the image projected on the screen was always slightly less crisp than the action in front of it, an effect which was especially noticeable in sequences where footage with rear projection alternates with non-projection shots.