How Blue Screens Work

Static Mattes
The original scene: Actors on a plain on a nice day              Not very spooky.
The original scene: Actors on a plain on a nice day Not very spooky.

Mattes have been used in the film industry practically forever to create special effects. A very common effect can be easily created using a double-exposure matte. Here is how this technique works.

Let's say that the director would like to create a spooky scene where the actors are walking across a large, flat plain while the sky boils with dark clouds. To create this effect, the cameraman can first shoot the actors on the plain. When this shot is created, however, a piece of black paper or tape is used on the lens so that the area of the sky is masked out and left unexposed on the film. The scene is shot normally, but in the camera the film is exposed on only one half of the frame. Then, the camera operator rewinds the film in the camera, puts a piece of black paper on the lens to mask out the portion of the film already exposed, and films the clouds of a thunderstorm. Perhaps the clouds are filmed with a slow film speed, so that when played at normal speed they look like they are boiling across the sky.

Here are the steps:

The sky is matted out with black paper placed over the sky on the camera's lens. The sky portion of the film is not exposed in the first shot.
The film is rewound and a dark, cloudy sky is filmed with a matte placed over the previously exposed portion of the film.
When the film is developed, the two shots appear as one. Spooky.

There are two variations on this technique that are very common:

  • The sky might be computer generated rather than a real, outside sky.
  • The two scenes might be shot separately on two pieces of film and then brought into the special effects department to be combined onto a third piece of film using a technique called optical compositing. The two pieces of film are projected onto the third piece of film in a compositing machine that handles the film very precisely one frame at a time. Or, in a digital shop, the two pieces of film are digitized, combined frame by frame in the computer's memory and then written out to a third piece of film with a film printer.

This sort of matting is one of the oldest special-effect techniques used in the industry.