system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can cgi be used to cast objects on thin air like they do in football matches without the need of a green screen which is used in movies

My understanding of a green screen is that the green screen defines the cast space on which an image or a video is cast on. Any space which isnt green isnt detected as cast space. However in football they just cast stuff on thin air without the use of a green screen. How does that work..... cause movie makers spends LOADS making entire green rooms.
gpt
The purpose of chroma keying is preserving the foreground. The things that go *in front of* the screen are preserved and the shot being composited in goes behind them. In compositing overlays for sports broadcasts and such the overlay is *supposed* to be the "topmost" thing in the final shot. That's what it's there for.