ELI5- what is the purpose of black or white “+” or “x” or “•” markers on a chroma/blue/green screen?

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My only thought would be perspective indicators to the VFX animators/editors, but why not make them in green/blue/same color as the screen? The human eye can see them but the computer could still wipe them.

What is their purpose?

In: 3

Those are tracking points used to get the movement of the camera and reproduce it for the CGI scene that will be replacing the green screen.

They aren’t the same color as the scene because they need to stand out to be easily detectable by the editing tools so that they could track them. They are not that hard to remove, the ones that are far from any actors/objects in the scene can be easily masked or just painted over with the same green shade as the greenscreen without much care (most likely using a clone tool in the editing software taking a part of the scene close to the tracker and replacing the tracker with it using the same motion it has), only the ones that are behind actors/objects take a bit more care when removing.