Augmented Reality Chroma Key System For Videos

What is Chroma Key?

Chroma key, or chroma key compositing, is a video compositing technique using video broadcast software. It’s used to splice two video streams together into a single image sequence. Typically, chroma key compositing is used to place a moving figure, such as a person, against a background where the person isn’t present in filming. It can either be something you do after the camera work is done, or, with today’s video encoding software, a way of modifying live stream video between the camera and the broadcast. Either way, it’s a processing technique that produces the final image.

 Common Uses of Chroma Key

Chroma key compositing is often used in weather broadcasting. The weather forecaster is shot against a blue or green screen, and the background plate is a satellite photo, weather chart, or other source of visual information.

Chroma key is also used to produce video special effects. For example, a video sequence in which people are riding in a car may be produced with this method. The car is actually stationary, but the background moves as if the car was in motion. (The chroma key method predates digital photography. It was originally implemented using analog production techniques. That was a lot more laborious, but generated similar effects to what can be done now with computers.)

Background

The name comes from the technique, which involves filming the subject against a monocolor background. Any color that isn’t found in the person himself can be used as the background, but typically either a blue or a green background is used. (Hence the names “blue screen” and “green screen,” which are often used for this process.) In compositing, the blue or green background is replaced by another image, which is called the “background plate.”

The replacement is done not by cutting and pasting but by “keying” the compositing software to the color (or “chroma”) of the screen, so that everything in the image of that particular color is removed. Chroma key is a variant on the older luminance key, which removes everything above or below a certain brightness. That technique is useful for a number of effects, but not so good for use with human figures, since the luminance of a human body along with clothing varies widely. Chroma key allows more nuanced and precise excision of a background without visibly impacting video footage of people.

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