Chroma Key - History

History

In filmmaking, a complex and time-consuming process known as "travelling matte" was used prior to the introduction of digital compositing. The blue screen and traveling matte method were developed in the 1930s at RKO Radio Pictures and other studios, and were used to create special effects for The Thief of Bagdad (1940 film). At RKO, Linwood Dunn used a travelling matte to create "wipes" – where there were transitions like a windshield wiper in films such as Flying Down to Rio (1933).

The development of the bluescreen is credited to Larry Butler, who won an Academy Award for special effects in The Thief of Bagdad.

In 1950, Warner Brothers employee and ex-Kodak researcher Arthur Widmer began working on an ultraviolet travelling matte process. He also began developing bluescreen techniques: one of the first films to use them was the 1958 adaptation of the Ernest Hemingway novella, The Old Man and the Sea, starring Spencer Tracy.

The background footage is shot first, and the actor or model is filmed carrying out their actions against a bluescreen. To simply place the foreground shot overlapping the background shot, as added light, would create a ghostly image with a blue-tinged background. The actor or model must be separated from the background and placed into a specially-made "hole" in the background footage. The bluescreen shot was first rephotographed through a blue filter so that only the background is exposed. A special film is used that creates a black-and-white negative image – a black background with a subject-shaped hole in the middle. This is called a 'female matte'. The bluescreen shot was then rephotographed again, this time through a red and green filter so that only the foreground image was cast on film, creating a black silhouette on an unexposed (clear) background. This is called a 'male matte'.

The background image is then rephotographed through the male matte, and the shot rephotographed through the female matte. An optical printer with two projectors, a film camera and a 'beam splitter' combines the images together one frame at a time. This part of the process must be very carefully controlled to ensure the absence of 'black lines'. During the 1980s, minicomputers were used to control the optical printer. For the film The Empire Strikes Back, Richard Edlund created a 'quad optical printer' that accelerated the process considerably and saved money. He received a special Academy Award for his innovation.

One drawback to the traditional traveling matte is that the cameras shooting the images to be composited cannot be easily synchronized. For decades, such matte shots had to be done "locked-down" so that neither the matted subject nor the background could shift their camera perspective at all. Later, computer-timed, motion-control cameras alleviated this problem, as both the foreground and background could be filmed with the same camera moves.

Petro Vlahos was awarded an Academy Award for his development of these techniques. His technique exploits the fact that most objects in real-world scenes have a color whose blue color component is similar in intensity to their green color component. Zbigniew Rybczyński also contributed to bluescreen technology.

For Star Trek: The Next Generation, an ultraviolet light matting process was proposed by Don Lee of CIS and developed by Gary Hutzel and the staff of Image G. This involved a fluorescent orange backdrop which made it easier to generate a holdout matte, thus allowing the effects team to produce effects in a quarter of the time needed for other methods.

Some films make heavy use of chroma key to add backgrounds that are constructed entirely using computer-generated imagery (CGI). Performances from different takes can even be composited together, which allows actors to be filmed separately and then placed together in the same scene. Chroma key allows performers to appear to be in any location without even leaving the studio.

Computer development also made it easier to incorporate motion into composited shots, even when using handheld cameras. Reference-points can now be placed onto the colored background (usually as a painted grid, X's marked with tape, or equally spaced tennis balls attached to the wall). In post-production, a computer can use the references to adjust the position of the background, making it match the movement of the foreground perfectly. Modern advances in software and computational power have even eliminated the need to use grids or tracking marks – the software analyzes the relative motion of colored pixels against other colored pixels and calculates the 'motion' to create a camera-motion algorithm which can be used in compositing software to match the motion of composited elements to a moving background plate.

Meteorologists on television often use a field monitor, to the side of the screen, to see where they are putting their hands, against the background images. A newer technique is to project a faint image onto the screen.