Rear projection effect
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Rear projection effect is an in-camera special effects technique in film production for combining foreground performances with pre-filmed backgrounds. Rear projection was conceived long before its actual usage; however, it was only made possible in the 1930s due to three necessary technical developments. Namely, simple camera and projector motors which could be linked up for synchronization of their shutters were not available until the demands of sound technology made constant-speed motors for film equipment necessary. Secondly, Eastman Kodak's introduction of panchromatic film stock in 1928 allowed for the camera to expose the projected background more than orthochromatic stocks, making it look less faint than it would have before. Finally, the larger film gauges beginning to emerge in the late 1920s demanded more powerful projection lamps, which were subsequently available for making the rear projection screen brighter and thus more properly exposed. Twentieth Century Fox was the first to use the technique, in 1930 with their films Liliom and then Just Imagine, and were subsequently awarded a technical Oscar for their work the next year. Shortly following this, Paramount Studios' Farciot Edouart refined the technique starting in 1933, and developed several new methods, such as syncing three projectors with the same background plate for more even and bright exposure.
The actor stands in front of a screen while a projector positioned behind the screen casts a reversed image of the background. The system was used in many old Hollywood movies, but required a large space to film as the projector had to be placed some distance from the back of the screen. Frequently the background image would appear faint and washed out compared to the foreground.
Alfred Hitchcock used the process successfully to show Cary Grant being attacked by a crop duster plane in North by Northwest, but was criticized when he used it extensively in Marnie.
Innovations such as bluescreen and front projection have rendered rear projection obsolete, though Quentin Tarantino used the process for the taxi ride sequence of Pulp Fiction, and James Cameron used rear projection for several special effects shots within Aliens, including the crash of the Dropship.
Such so-called "process shots" were widely used to film actors as if they were inside a moving vehicle, but who were, in reality, in a vehicle mock-up on a soundstage.
The film that is projected can be still or moving. It is called the "plate." One might hear the command "Roll plate." to instruct stage crew to begin projecting.
