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 negatives.
Read more about this topic: Compositing