Chroma key is a video production technique that allows one image to be superimposed over another. Special effects are commonly produced with chroma key. And when you watch your local newscast and see a meteorologist standing in front of a moving radar image or an animated computer map, you're seeing chroma key in action.
Process
In a typical chroma key shot, actors recite their lines and hit their stage marks as usual, but they do it in front of a single-color backdrop. The backdrop color is the "key," and the video equipment is set so that the key color is interpreted as being transparent and drops out of the image. The remaining image---that of the actors and any props that aren't in the key color---can then be laid over a background image.
Uses
Chroma key allows actors to "appear" in all sorts of locations without leaving the studio. By having them act against a blue or green screen, filmmakers can insert any background they like, be it video of a city street, a still photo of the Grand Canyon or a computer-generated interior of a spaceship. Chroma key also allows you to make certain elements of the image disappear. This is how they "amputated" the legs of Gary Sinise's character, Lieutenant Dan, in the film "Forrest Gump": Sinise's legs were merely wrapped in fabric in the key color.
Color
The key color can be any color, but it's usually blue or green because those are the colors farthest away from the human skin tones that are the focal point of most shots. Lighting on the set may reflect some of the key color onto the performers, which is why some chroma-key effects, particularly in older productions, will produce a bluish "halo" around the edges of the actors. Nowadays, digital imaging technology is able to mostly eliminate the halo effect.
Actors, news anchors and others who appear on chroma key must take care not to wear clothing that matches the key color. If they do, those items will become transparent as well, and the audience will be able to see "through" them.
History
Special effects artist Larry Butler developed the chroma key process for the 1940 film "The Thief of Baghdad," for which he won an Academy Award. Back then, chroma key was a chemical process performed on film negatives. Today, it's all done digitally.
Terminology
The term "chroma key" is used mostly in television production. In films, the process is more commonly referred to as simply "blue-screen" or "green-screen," depending on the color of backdrop.
Tags: blue green, Special effects