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How did blue-screen photography allow actors to appear in imaginary settings?

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Director Steven Spielberg and camera operator Michael Chapman film next to a large mechanical shark on the set of the 1975 movie Jaws. — motion-picture technology Director Steven Spielberg holding the movie camera, assisted by camera operator Michael Chapman, during the filming of *Jaws* (1975).

Blue-screen photography allows actors to appear in imaginary settings by using a process called a traveling matte to composite foreground action onto a separate background.1

The technique exploits film’s sensitivity to blue light. Actors are filmed in front of a blue background, which is illuminated to avoid shadows. Because the film’s blue-sensitive layer yields a dense image in the area of the screen, filmmakers can create an "action matte"—a silhouette of the actor—and a corresponding "countermatte" of the background.1

Using an optical printer, these elements are combined in multiple passes:
* The background is printed onto the film.1
* The actor, isolated by the matte, is printed into the foreground.1

This process effectively replaces the blue screen with any desired scenery, allowing for complex visual effects like characters flying through the air. While the term "blue-screen" is standard, other colors—such as yellow sodium vapor light—can be used to achieve the same effect of creating a transparent background for compositing.1