A weather forecaster stands in front of a map. The map is not really there. The forecaster is standing in front of a green screen. The computer replaces the green with the map. That is chroma key, a technique that replaces a colored background with another image. The color is usually green or blue, because those colors are far from human skin tones and do not appear in most clothing. The actor wears nothing that matches the screen, and the computer keys out the background.

The technique works by selecting a color range and making it transparent. The software then composites the actor over a new background. The process requires even lighting on the screen, sharp focus, and careful separation between the actor and the background. Spill, where green light reflects onto the actor, must be suppressed. Hair and translucent materials are difficult to key because they have fine edges.

Chroma key is used everywhere. It puts actors in fantastical worlds, in front of explosions, and on alien planets. It is used in news, sports, and online streaming. It is used in video games and virtual production, where the background is rendered in real time on an LED wall. The technique is invisible when it works and painfully obvious when it fails.