How Green Screens Work

Green screen (like its predecessor, blue screen) is another term for an editing technique that the Hollywood pros call Chroma Key. The idea is that you tell your computer to replace every pixel of a certain color (like vibrant green) with new footage. In other words, anything that’s not green (like the actor) gets superimposed onto thebackground stuff. This is how Superman flew, how Neo dodged bullets in The Matrix, how The Daily Show correspondents seem to be in Paris or Iraq or Washington, and how TV actors never seem to hit anyone when they’re in driving scenes and paying no attention to the road. The actors performed their scenes with smooth green fabric filling the car windows; later, editors (and their computers) replaced all patches of green with passing scenery. Watching actors in front of a green screen can be quite funny. They have to pretend they’re being chased through the jungle by a dinosaur, when in fact they’re sitting in a nondescript studio with its walls painted green, without so much as a vine in sight. Green replaced blue as the most popular color for this technique because digital cameras are most sensitive to green. (Blue remains the runner-up, which is why it’s sometimes used in place of green. But in iMovie, green is your only option.) Of course, this also means you can’t wear green clothes in the shot, unless you are intentionally going for the floating-disembodied-head-thing.