In movies and on television, actors walk — and sometimes fly — through elaborate and
fantastic landscapes that simply don't exist in the real world. They ride on dragons' backs, grow crops on distant planets or visit magical realms with towering citadels inhabited by bizarre creatures. Sometimes the story takes place in a familiar city, but in the distant past — or the far-off future. Sometimes characters stage epic battles that seem to pulverize landmarks or places that audiences know well or where they live. And sometimes, the characters themselves are physically transformed, or defy the laws of gravity.
All of this high-tech fakery happens with the help of backdrops of brightly colored fabric or paint, and a process called "chroma key," also referred to as "green screen" due to the backdrops' color, which is typically a vivid green.
Chroma keying allows media technicians to easily separate green screens and panels from the people standing in front of them and replace those backgrounds with pretty much anything — from animated weather maps to the skyline of 1930s-era New York City to the icy Wall guarded by the Night's Watch in HBO's hit TV series "Game of Thrones." [Science Fact or Fiction? The Plausibility of 10 Sci-Fi Concepts] https://www.livescience.com/55814-how-do-green-screens-work.html
