user14021: 
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? 

user14523: You are correct that the chroma key is typically performed in the vision mixer/video switcher. In order to assist camera operators in framing shots around chroma keys (and other downstream effects like DVE moves, lower thirds, and over the shoulder graphics), professional studio cameras have one or more program returns that allow the vision mixer/technical director or engineer to route an appropriate composited video source back to the camera. The camera operator then presses a button to select the return instead of the through the lens video.
On productions that don't have too many different framing setups, you can also go low-tech and use a grease pencil or apply tape to mark framing areas directly on the viewfinder. Some cameras also have simple box generators that can also mark framing references or safe areas.

14021: <p><a href="https://i.stack.imgur.com/2Qipf.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2Qipf.jpg" alt="enter image description here"></a></p>

<p>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. </p>

<p>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? </p>
 14523: <p>You are correct that the chroma key is typically performed in the vision mixer/video switcher. In order to assist camera operators in framing shots around chroma keys (and other downstream effects like DVE moves, lower thirds, and over the shoulder graphics), professional studio cameras have one or more program returns that allow the vision mixer/technical director or engineer to route an appropriate composited video source back to the camera. The camera operator then presses a button to select the return instead of the through the lens video.</p>

<p>On productions that don't have too many different framing setups, you can also go low-tech and use a grease pencil or apply tape to mark framing areas directly on the viewfinder. Some cameras also have simple box generators that can also mark framing references or safe areas.</p>
