AFAIK, the Chroma key happens in the vision mixer or something?; after the camera picked up the green colored cloth(?) then the vision mixer applies the CGI images to the video.
Sorry for confusing you guys.. This is a snapshot from a video about Chroma keying. Though the adjacent scenes doesn't tell exactly that what in this frame is a green screening too, I thought it is because of the video's topic. So, I need to know if this is possible?