Chroma keying is a video image punching technique in which any motif, photo, text, graphic or video sequence is superimposed as a background. In the realization, the object
or person to be provided with a background is placed in front of a monochrome wall. With chroma keying, the wall glows in a saturated blue or green. This is why it is called blue-keying or blue-screen and if green, it is called green-keying or green-screen. The wall color is the color used for cropping the object and punching. To avoid chroma keying within the object to be cut out, the punch color must not be present in the object or person. After chroma keying, the cropped subject can be blended into another background, image source, or scene, or into a computer-generated imagery
Chroma keying is used in all film and video studios and is part of the recording technique. Examples of this are the classic news broadcasts. If the person to be recorded sits too close to the blue screen or if the screen is poorly lit, halo effects occur. These appear as blue or green halos on the head and shoulders of the person to be recorded.