Chroma Keying

A chroma key is the removal of a color (or small color range) from one image to reveal another image "behind" it. The removed color becomes transparent. This technique is also referred to as "color keying", "colour-separation overlay" ("CSO") (primarily by the BBC[1]), "greenscreen" and "bluescreen". It is commonly used for weather forecast broadcasts. 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.

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."

A chroma key subject must not wear clothing similar in color to the chroma key color(s) (unless intentional), because the clothing may be replaced with the background video. An example of intentional use of this is when an actor wears a blue covering over a part of their body to make it invisible in the final shot.

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. Occasionally a magenta background is used.

With better imaging and hardware, many companies are avoiding the confusion often experienced by weather presenters, who must otherwise watch themselves on a monitor to see the image shown behind them, by lightly projecting a copy of the background image image onto the blue-/greenscreen. This allows the presenter to accurately point and look at the map without referring to the monitors.