History of Chroma Key

Prior to the introduction of digital compositing, the process was complex and time consuming known as "traveling matte". The blue screen and and traveling matte method were developed in the 1930s and were used to create special effects for The Thief of Bagdad.

The credit for development of the blue screen is given to Larry Butler, who won the Academy Award for Special Effects for the Thief of Baghdad in 1940. He had invented the blue screen and traveling matte technique in order to achieve the visual effects which were unprecedented in 1940. He was also the first special effects man to have created these effects in Technicolor, which was in its infancy at the time.

In 1950, Warner Bros. employee and ex-Kodak researcher Arthur Widmer began working on an ultra violet traveling 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. ^[2]

The background footage was shot first and the actor or model was filmed against a bluescreen carrying out their actions. To simply place the foreground shot over the background shot would create a ghostly image over 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 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 can't be easily synchronized. For decades, such matte shots had to be done "locked-down" so that neither the matted subject nor the background would move 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. Zbig Rybczynski also contributed to bluescreen technology.

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.

In the past decade, the use of green has become dominant in film special effects. The main reason for this is that green not only has a higher luminance value than blue but also in early digital formats the green channel was sampled twice as often as the blue, making it easier to work with. The choice of color is up to the effects artists and the needs of the specific shot. Red is usually avoided due to its prevalence in normal human skin pigments, but can be often used for objects and scenes which do not involve people.

Weathermen often use a field monitor to the side of the screen to see where they are putting their hands. A newer technique is to project a faint image onto the screen.