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The Art of Digital Compositing: Blue-Screening

From Adrien-Luc Sanders,

Sometimes we see effects in television and movies that couldn’t possibly be real; actors move against surreal digital backgrounds, or perform strange bodily contortions that should be intensely painful to the average human being, going so far as to detach body parts. While sometimes this can be accomplished with digital, 3D modeled replicas of the actors, many times it’s quite obvious that these are real people in not-so-real settings. So if these scenes aren’t being shot against real-life backdrops....how are these actors being placed in these fantastic environments, performing these strange and unreal acts?

What's the Secret?
The answer is a technique called blue-screening. Using this technique, actors enact their scenes against a solid background of bright blue or green (in that case, termed “green-screening”) rather than on a regular movie set, usually with the aid of visual cues to help them keep their location and actions straight.

How Does Blue-Screening Work?
What the bright, blank background does is create a clear field around the actor, allowing for clean outlines of his or her body and motions. When the film of the actor against the blue screen is taken into a digital editing medium, the blue or green background can be edited out using a transparency filter that causes all things in each frame in that particular shade of blue or green to appear invisible. (For this reason, actors can’t wear any clothing in that shade, or even wear much white that might reflect the background shade, because the filter will catch it and create a “hole” in the actor’s clothing or body.)

But How Does That Create the Finished Scene?
With the blue screen edited out, nothing is left but the actor(s) seemingly floating in empty space; using layered compositing, they can then be laid over the background intended for the scene. Lighting effects to match the scene can even be added, so that they seem to blend in better rather than being wholly displaced because they were filmed in a different lighting environment.