system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do live football field graphics appear beneath the players?

You know when it shows the scrimmage lines or whatever right on the field? I assume the graphics are already using pan/tilt/zoom data from the cameras, since it looks a lot stabler than simply motion tracking (please correct me if I'm wrong on that), but what really baffles me is how the graphics appear "beneath" the players. Chroma key? Nope. I see teams with green colours that still pass over it. Extracting a matte from thermal imaging? Wouldn't the helmets be cooler than their bodies? What then? How?
gpt
You had the right idea already. It is indeed chroma key. The cameras are simply good enough that they can distinguish between the green of the grass and the green in uniforms. They actually need to continually reset the chroma key as the color of the grass changes as the sun moves or when shadows show up on the field.