Rear projection is an in-camera special effects technique 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. The presence of a movie screen between the background image and foreground objects leads to a distinctive washed-out look that makes these "process shots" recognizable.

Technique

The actors stand in front of a screen while a projector positioned behind the screen casts a reversed image of the background. This 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. The film 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 were, in reality, in a vehicle mock-up on a soundstage. In these cases the motion of the backdrop film and foreground actors and props were often different due to the lack of steadicam-like imaging 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 advance over rear projection is front projection, which uses a special screen material to allow the plate to be projected from the front of the screen. This results in a much sharper and more saturated image. Although the technique had been used experimentally for some time, it was during the filming of 2001: A Space Odyssey that the modern version was fully developed. In this case it was used to avoid costly on-location shots in Africa during the opening scenes of the movie, but the effect was also used throughout the film for a variety of shots into or out of the windows of spacecraft. 2001 also used rear projection to produce computer screen effects.

As front projection and bluescreen effects became more widespread and less costly, rear projection has been rendered largely obsolete. 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. Also, the Austin Powers film series frequently used rear projection to help give it the feel of old spy movies.

History

Rear projection was conceived long before its actual usage; however, it was only made possible in the 1930s due to three necessary technical developments. The most important was the development of camera and projector motors which could be linked up for synchronization of their shutters, which were developed out of the unrelated needs of "talking" movies whose timing had to be carefully controlled. 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 did 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.