Chroma keys use a special color to cut a video hole.

Special Effects Keys — Chroma Keys

Chroma keys also cut a hole out of the video, but unlike luminance keys, the triggering device is not contrast. It's a particular color in the subject video (the video receiving the special effect). In a chroma key, the system detects the chosen color in the subject video and wherever it sees that color replaces it with information from another video source. In the first figure the person (or talent) is in front of a chroma key window. The camera that shoots the chroma key window is called the source camera. Any other video source can supply the fill video, but in this example camera 2 has been used. Where the system sees the selected color (the window), it replaces that information with the corresponding information from the fill camera. Thus, you get a rocket launch in the studio. If you were to make the entire background the chroma key window, then that background would be filled. The second figure illustrates this. You now see the entire area behind the talent filled with the launch because the entire background has become the key window. The chroma key circuitry will try to fill in the source anywhere it sees the chosen color. It is for this reason that primary colors are used for the key window. If you chose yellow for your key window, for example, the system would not only lock up to yellow, but it would try to lock up to anything containing yellow's components, red and green. Thus, the chroma key would try to lock up to almost everything but the color blue.

Since people almost always appear in the chroma key source picture, red is not used for the key window very often. After all, there is quite a bit of red in the flesh tones of people. Blue and green are the colors most commonly used for chroma key windows. Thus, the industry terms are blue screen and green screen.

Anyone wearing clothing that is the same color as the chroma key window will present problems, since the clothing might be keyed out as well. Since white includes all colors of light, the chroma keyer may try to lock up to it, but this will usually appear as an incomplete key. Although black is the lack of colors, it too can create problems. Small patches of black, particularly shadows, may also be keyed over. When the system is scanning and it comes to a void of color (black), it may not be sure what to do, so it keys over the area. Larger areas of black may not cause a problem because they're large enough for the system to determine exactly what they are.

The chroma key also has a clip control. In addition, there is a hue (chroma) control that allows you to choose the color to be keyed out, and there is a gain control that controls the strength of the source picture.

1.  Small chroma key window.

2.  Full background chroma key window.

3.  Talent wearing coat same color as key window.