Q) I'm looking for information on the use of chroma-key for backgrounds—why it's used, and when to use the green and when to use the blue.
Roger E. Johnston Jr.
A) Chroma-keying is the process whereby all of the pixels of a certain color on one image are replaced with pixels on another image. If you've ever watched a weather report on television, you've seen a chroma-key. The meteorologist appears to be standing in front of a weather map, but, in reality, is standing in front of a colored background. By chroma-keying (keying on a certain color), the actual background behind the person is replaced by the map.
A chroma-key background is a solid, highly saturated surface that's typically either blue or green in color. In the past, chroma-keying usually was relegated to moving images, namely video and motion pictures. As still-image-editing software has advanced, the chroma-key process has become available in the digital darkroom. You either can make use of the manual color selection in your image editor or use more sophisticated third-party tools to achieve good results (check out www.digitalanarchy.com).
The colors green and blue were originally chosen because they contrast with skin tones (which have a lot of red; if red was used, a person's face would start to disappear as the color was replaced by the new image). Blue was preferred because of color spill on flesh tones. The blue spill could be removed with less objectionable color casting. Now the answer of green or blue usually is dependent on what color won't be in the scene. In other words, if someone is wearing a blue shirt, you should use a green background so the shirt doesn't disappear.