Bluescreen
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Bluescreen (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 greenscreen).
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[edit] Traveling matte
Prior to the introduction of digital compositing, bluescreen was a complex, time consuming process called traveling matte.
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. [1]
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, 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 bluescreen 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 exact same camera moves.
Petro Vlahos was awarded an Academy Award for his development of bluescreen 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.
[edit] 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 actually 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 fail 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 greenscreen 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 greenscreen effect.
[edit] Other colors
Some modern screens appear grey, but are coated with tiny half-silvered glass beads to give a significant degree of retroreflectivity. 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. The screen color is defined by the color of the ring light, making it easier to change the screen color quickly. This technique facilitates the use of 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, and Song of the South), orange (Apollo 13) and red (Air Force One or Predator). 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, and yellow was used in Doctor Who because a number of props and costume items contained both blue and green.
[edit] Bluescreen in the digital age
Some films make heavy use of bluescreen and add backgrounds that are constructed entirely using 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; I, Robot; and Sin City. Performances from different takes can even be composited 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.
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 always avoided due to its prevalence in normal human skin pigments.
Weatherman 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 light image onto the screen.
[edit] See also
- Drew Carey's Green Screen Show
- Film production
- Front projection effect
- Matte (filmmaking)
- Optical printer
- Rear projection effect
- Reverse bluescreen
- Signal processing
- Sodium vapor process
- Video
[edit] References
[edit] External links
- How to do basic color keying in Adobe After Effects
- How to build a mobile green/blue screen
- The Blue Screen / Chromakey Page
- How Blue Screens Work
- Free bluescreen video files
- How to create transparent video for use in a website from Blue Screen / Chromakey film
[edit] Further reading
- T. Porter and T. Duff, "Compositing Digital Images", Proceedings of SIGGRAPH '84, 18 (1984).
- The Art and Science of Digital Compositing (ISBN 0-12-133960-2)
