How does a blue screen work?

Answer 1

­In all of these cases, the illusion is created by a special effects technique known as traveling matte or blue screen. This technique allows actors and scale models to find themselves in totally imaginary situations -- in space ships, dangling from rope bridges over gorges, flying through the air (a la Superman) -- and have it look completely real in the theater. The technique is used so often now that you don't even realize it. News reporters are made to look like they are on location when they are not, and complete segments in TV shows can be created this way to make it look like the segment was filmed on location when, in fact, no one left Los Angeles.