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Blue / Green Screen

Blue screen (known in television as chroma key) is a term for the filmmaking technique of using an evenly-lit monochromatic background for the purpose of replacing it with a different image or scene. The term also refers to the visual effect resulting from this technique as well as the colored screen itself (although it is often not blue: for example, with green screen).

Traveling matte

Prior to the CGI revolution, blue screen was a complex, time consuming process called 'traveling matte'.

Developed by Warner Bros. employee and ex-Kodak researcher Arthur Widmer in 1950, he began working with an ultra violet traveling matte process. Widmer also developed and refined technologies for other motion picture processes including 3D and widescreen. He began developing blue screen techniques, with one of the first films to use them being the 1958 adaptation of the novella by Ernest Hemingway written in Cuba in 1951, The Old Man and the Sea, starring Spencer Tracy.

The background footage was shot first and the actor or model was filmed against a blue screen 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 blue screen shot was first re-photographed 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 man/spaceship-shaped hole in the middle. This is called a 'female matte'.

The blue screen shot was then re-photographed, 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 re-photographed through the male matte, and the blue screen shot re-photographed 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 sped up the process considerably, and thus saved the production money. He received a special Academy Award for his innovation.

Petro Vlahos was awarded an Academy Award for his development of blue screen 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 blue screen technology.

Chroma key

The key background color in the video signal is processed out and overlaid with content from a different video signal — such as from a separate camera, a recorded video playback, or a digital source — a process called 'compositing'. Both digital and analogue techniques exist for doing this. The image replacement may be done in production or in post-production.

A classic example of the technique is the television news weatherman who on-screen appears to point at a map, but is in fact being recorded standing in front of a blank screen. On the sides of this screen are smaller televisions projecting a front view of the weathercaster, so they know where and when to place their hands. This technique is illustrated in an early scene in the film Groundhog Day. These early television effects were originally accomplished by a technique called chroma keying, but older analogue methods have been increasingly supplanted by modern digital compositing techniques.

Sometimes a television presenter's clothing will happen to have a region, such as a logo or other decoration, whose color is close enough to the chroma key being used that it gets included in the mask and the background shows through. If the production staff fails to notice this before the program goes on the air, it will then look to viewers as though there is a small hole in the body of the presenter through which the background is visible.

Towards the end of 2004, Drew Carey hosted the TV show Drew Carey's Green Screen Show, where comedians act against a green screen background with live audience interaction. After post-production, viewers watching the show would see animation interlaced with the live acting.

At the 78th Academy Awards (2006) Ben Stiller, introducing the Academy Award for Visual Effects, parodied the effect by appearing in a green jump suit which he claimed would appear invisible on television, making him appear as a disembodied floating head. In fact it was clearly visible, since it was not shot using a green screen effect.

Other colors

Yellow screens were used in the Sodium Vapor Process developed by Disney for the film Song of the South.

There are some modern screens that at first sight appear grey, but are in fact coated with tiny half-silvered glass beads to give a significant degree of retro reflectivity. A ring of colored lights (usually LEDs) is placed around the camera lens, and the screen reflects this color back to the camera. This technique reduces problems from performers casting shadows on the screen, and allows operation at low lighting levels. As the screen color is defined by the color of the ring light, it is easier to change the screen color quickly, and to use a color with a narrow range, making it easier to distinguish between the color of the screen and colors on the subject.

Other colors are sometimes used instead of blue, including magenta (The Matrix), yellow (some 1970s episodes of Doctor Who), orange (Apollo 13) and red (Air Force One). The choice of color depends on the subject and specific technique used. Blue is normally used for people because human skin has very little blue color to it. Green is used because digital cameras retain more detail in the green color channel and it requires less light. Magenta screens are often used with model photography where the model contains both blue and green components.

Blue screen in the digital age

Some films make heavy use of blue screen and add backgrounds that are constructed entirely using computer-generated imaging (CGI). In the early 2000s several movies were made using this technique, including Immortel: Ad Vitam, Sky Captain and the World of Tomorrow, Casshern, Star Wars Episode III and Sin City. Performances from different takes can even be composted together, which allows actors to be filmed separately and then placed together in the same scene. Blue screen allows performers to appear to be in any location without even leaving the studio. They could appear to be anywhere on Earth, or any other world that could be depicted.

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 format the green channel was sampled twice as often as the blue, making it easier to work with. But the choice of color is up to the effects artists and the needs of the specific shot.


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