Chroma Key Compositing
H O P P E R P E D I A ©
-Brian Hammons
Chroma key (green, blue, or magenta) compositing is a technique for compositing two images or frames together. A color from one image is removed (or made transparent), revealing another image behind it. Computer development has made it easier to incorporate motion into composited shots. The portions of the video which match the preselected color are replaced by the alternate background video. This process is commonly known as "keying", "keying out" or simply a "key".
Reference-points can now be placed onto the green screen 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 solves 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.
Green
Green (and blue) are used because it is considered to be the furthest away from skin tone. Green (especially in digital) is currently used as a backdrop more than blue because image sensors in digital video cameras are most sensitive to green, due to the Bayer pattern allocating more pixels to the green channel, this mimicks the human increased sensitivity to green light. The green camera channel contains the least "noise" and can produce the cleanest key/matte/mask. Additionally, less light is needed to illuminate green, again because of the higher sensitivity to green in image sensors.
Why blue at all?
A bluescreen will be used if the subject is predominately green (for example plants). The use of blue is also tied to the fact that the blue emulsion layer of film has the finest crystals and thus good detail and minimal grain (in comparison to the red and green layers of the emulsion.)
Magenta
Occasionally, a magenta background is used. Some software applications refer to it as "magic pink".
Difficutlies: "even lighting"
The biggest challenge when setting up a bluescreen or greenscreen is even lighting and the avoidance of shadow. A shadow would present itself as a darker color to the camera and might not register for replacement. This can sometimes be seen in low-budget or live broadcasts where the errors cannot be manually repaired. The material being used affects the quality and ease of having it evenly lit. Materials which are shiny will be far less successful than those that are not. A matte surface will diffuse the reflected light and have a more even color range.
New Techniques
A newer technique is to use a retroreflective curtain in the background, along with a ring of bright LEDs around the camera lens. This requires no light to shine on the background other than the LEDs, which use an extremely small amount of power and space unlike big stage lights, and require no rigging. This advance was made possible by the invention of practical blue LEDs in the 1990s, which also allow for emerald green LEDs. There is also a form of color keying that uses a light spectrum invisible to human eye. Called Thermo-Key, it uses infrared as the key color, which would not be replaced by background image during postprocessing.
Virtual Studio
The diference between a virtual studio and Chroma key (green, blue, or magenta) compositing is that the computer graphics are rendered in realtime, removing the need for any post production work, and allowing it to be used in live television broadcasts. A key point of a virtual studio is that the real camera can move in 3D space, while the image of the virtual camera is being rendered in real-time from the same perspective. There exist many technical solutions for creating virtual studios, but most of them include the following components:
Camera tracking, that uses either optical or mechanical measurements to create a live stream of data describing the exact perspective of the camera.
Realtime rendering software, that uses the camera tracking data and generates a synthetic image of a television studio.
A video mixer, which combines the video from the camera with the video from the realtime rendering software to produce a final video output.
