Chroma key is an image-compositing technique that combines two images to create a single, finished image. The backdrop used to film the subject of an image that will be subjected to the chroma key process is sometimes referred to as blue or green screen, referring to the backdrop used to film the subject of an image that will be subjected to the chroma key process. Because this editing technique is widely used in film and television to create a variety of special effects, many film fans are familiar with the basic concept.
A director might want to create the illusion that two actors are sitting in a car together, driving down the road, in a classic example of a film segment created with the chroma key technique. While a film crew could go to a location, have the actors drive down a road, and film from inside and around the car, this is time-consuming and sometimes impossible. The actors and the car are instead placed on a sound stage surrounded by a traditional green backdrop, and the scene is then filmed.
The editor instructs the computer in the editing room to select the green background and “key it out” of the image, allowing the editor to overlay the segment over footage of a sweeping landscape. The actors appear to be driving down the road while conversing in the final image. The same method can be used to create weather maps for background on weather reports, to place actors in exotic locations without actually visiting them, and a variety of other special effects tricks.
Because they are as far away from human skin tones as possible, blue and green screens are traditionally used for chroma key. To avoid having parts of themselves keyed out, actors wear clothing that is similar in tone to the background, and the background is lit with an even wash of light to keep the tone as even as possible, making it easier to edit out in the editing room. An image created using the chroma key process can have jagged edges or halos due to uneven lighting if the background is not evenly lit.
The chroma key technique is sometimes used to create special effects like invisibility, with the actor wearing garments and makeup that match the tone of the chroma key background, allowing him or her to blend into the finished image. Producers can use chroma key to make it appear as if actors are in dangerous, crowded, or fantastical situations for a fraction of the cost and with less stress for cast and crew.