W3Schools

Green Matt Floor

Green Mat can transport a subject anywhere, limited only by the imagination. The green color used in the background is called as Chroma Keying. Green screen techniques were originally developed by the professional film industry to help separate actors shot in studio from the backgrounds that they were shot on. Later in post-processing, the green screen background was removed digitally and replaced with a new background, shot separately or created in a computer. Through the magic of video effects and technology, you can superimpose your subjects onto virtual backgrounds, place them over animated digital backdrops or transport them to a desert oasis. You can shrink down a full-grown man so he can stand on a tabletop, use visual effects to make him fly through the sky like a superhero or simply simulate your own TV weather report.

The secret to pulling your subject out of the real world and placing him or her into a digital domain is chromakey, and that means going green. Keying is the process of isolating a single color or brightness value in an electronic image and using software to make that value transparent, allowing another image to show through the affected areas. Luminance keying, or lumakeying, is the process of keying out a brightness value or range, like black or white. Luminance keys are often used for applying mattes. Color keying, or chromakeying, identifies a specific color to remove.