This kind of visualization is useful to understand why filming an actor in front of a green screen makes sense for special effects. If you look at such an imag

This kind of visualization is useful to understand why filming an actor in front of a green screen makes sense for special effects. If you look at such an image, you will have two clusters of pixels, one for the green screen, and one for the actor. By selecting the appropriate cluster you now know which pixels you need to replace for compositing your actor in front of a CGI background.