A chroma key is the removal of a color from one image to reveal another image "behind" it. The removed color becomes transparent. The presenter appears to be standing in front of a large map, but in the studio it is actually a large blue or green background. I use a full-length green screen together with software by Serious Magic to create all of the chroma-keyed videos seen on this site.

The process

The principal subject is photographed/filmed against a background having a single color or a relatively narrow range of colors, usually in the blue or green because these colors are considered to be the furthest away from skin tone. When the phase of the chroma signal corresponds to the preprogrammed state or states associated with the desired color(s), the signal of the alternate background is composited and presented as the output. When the phase of the chroma signal deviates from the chosen color, the principal video is presented at the output. This process is commonly known as "keying", "keying out" or simply a "key."

The background

Blue is used for weather maps and special effects because it is complementary to human skin tone. However, in many instances, green has become the favored color because digital cameras retain more detail in the green channel and it requires less light than blue. Although green and blue are the most common, any color can be used.