Datenbank Glossar – Compositing

Physical compositing

In physical compositing the separate parts of the image are placed together in the photographic frame and recorded in a single exposure. The components are aligned so that they give the appearance of a single image. The most common physical compositing elements are partial models and glass paintings.

Partial models are typically used as set extensions such as ceilings or the upper stories of buildings. The model, built to match the actual set but on a much smaller scale, is hung in front of the camera, aligned so that it appears to be part of the set. Models are often quite large because they must be placed far enough from the camera so that both they and the set far beyond them are in sharp focus.

Glass shots are made by positioning a large pane of glass so that it fills the camera frame, and is far enough away to be held in focus along with the background visible through it. The entire scene is painted on the glass, except for the area revealing the background where action is to take place. This area is left clear. Photographed through the glass, the live action is composited with the painted area. A typical glass shot is the approach to Ashley Wilkes’ plantation in Gone with the Wind. The plantation and fields are all painted, while the road and the moving figures on it are photographed through the glass area left clear.

A variant uses the opposite technique: most of the area is clear, except for individual elements (photo cutouts or paintings) affixed to the glass. For example, a ranch house could be added to an empty valley by placing an appropriately scaled and positioned picture of it between the valley and the camera.

Multiple exposure

An in-camera multiple exposure is made by recording on only one part of each film frame, rewinding the film to exactly the same start point, exposing a second part, and repeating the process as needed. The resulting negative is a composite of all the individual exposures. (By contrast, a “double exposure” records multiple images on the entire frame area, so that all are partially visible through one another.) Exposing one section at a time is made possible by enclosing the camera lens (or the whole camera) in a light-tight box fitted with maskable openings, each one corresponding to one of the action areas. Only one opening is revealed per exposure, to record just the action positioned in front of it.

Multiple exposure is difficult because the action in each recording must match that of the others; so multiple exposure composites typically contain only two or three elements. However, in the 1921 film The Playhouse, Buster Keaton used multiple exposures to appear simultaneously as nine different actors on a stage, perfectly synchronizing all nine performances.

Background projection

Background projection throws the background image on a screen behind the subjects in the foreground while the camera makes a composite by photographing both at once. The foreground elements conceal the parts of the background image behind them. Sometimes, the background is projected from the front, reflecting off the screen but not the foreground subjects because the screen is made of highly directional, exceptionally reflective material. (The prehistoric opening of 2001: A Space Odyssey uses front projection.) However, rear projection has been a far more common technique.

In rear projection, background images (called “plates” whether they are still pictures or moving) are photographed first. For example a camera car may drive along streets or roads while photographing the changing scene behind it. In the studio, the resulting “background plate” is loaded into a projector with the film “flopped” (reversed), because it will be projected onto (and through) the back of a translucent screen. A car containing the performers is aligned in front of the screen so that the scenery appears through its rear and/or side windows. A camera in front of the car records both the foreground action and the projected scenery, as the performers pretend to drive.

Like multiple exposure, rear projection is technically difficult. The projector and camera motors must be synchronized to avoid flicker and perfectly aligned behind and before the screen. The foreground must be lit to prevent light spill onto the screen behind it. (For night driving scenes, the foreground lights are usually varied as the car “moves” along.) The projector must use a very strong light source so that the projected background is as bright as the foreground. Color filming presents additional difficulties, but can be quite convincing, as in the famous crop duster sequence in Alfred Hitchcock’s North by Northwest. Because of its complexity, rear projection has been largely replaced by digital compositing with, for example, the car positioned in front of a blue or green screen.

Matting

Traditional matting is the process of compositing two different film elements by printing them, one at a time, onto a duplicate strip of film. After one component is printed on the duplicate, the film is re-wound and the other component is added. Since the film cannot be exposed twice without creating a double exposure, the blank second area must be masked while the first is printed; then the freshly-exposed first area must be masked while the second area is printed. Each masking is performed by a “traveling matte:” a specially altered duplicate shot which lies on top of the copy film stock.

Like its digital successor, traditional matte photography uses a uniformly colored backing – usually, but not always a special blue or green (fig. 1). Because a matching filter on the camera lens screens out only the backing color, the background area records as black, which, on the camera’s negative film, will develop clear (fig. 2).

First, a print from the original negative is made on high-contrast film, which records the backing as opaque and the foreground subject as clear (fig. 3). A second high-contrast copy is then made from the first, rendering the backing clear and the foreground opaque (fig. 4) .

Next, a three-layer sandwich of film is run through an optical printer. On the bottom is the unexposed copy film. Above it is the first matte, whose opaque backing color masks the background. On top is the negative of the foreground action. On this pass, the foreground is copied while the background is shielded from exposure by the matte (fig. 5).

Then the process is repeated; but this time, the copy film is masked by the reverse matte, which excludes light from the foreground area already exposed (fig. 6). The top layer contains the background scene (fig. 7) , which is now exposed only in the areas protected during the previous pass. The result is a positive print of the combined background and foreground (fig. 8). A copy of this composite print yields a “dupe negative” (fig. 9) that will replace the original foreground shot in the film’s edited negative.