The process of compositing, or placing actors in front of a background that’s not actually there, is as old as filmmaking itself — and it’s always been a pain. Netflix has a new technique that relies on machine learning to do some of the hard work, but it requires lighting actors in a garish magenta.
For decades the simplest method of compositing was chroma keying, in which actors stand against a brightly colored background (originally blue, later green) that can easily be identified and replaced with anything from a weather map to a battle with Thanos. The foreground is said to be “matted” and the background is a transparent “alpha” channel manipulated along with the red, green, and blue channels.
It’s easy and cheap, but there are a few downsides to this, among them problems with transparent objects, fine details like hair, and of course anything else with a similar color to the background. It’s usually good enough, though, that attempts to replace it with more so...
