C++, opencv - anlaysis screen (movie, game) in real time

Question

I have a specific problem - I wanna catch video directly from the image displayed on the monitor.

Explaining: When I want catch video from my camera, I use something like that:

VideoCapture capture = VideoCapture(0);
capture >> frm; 

Now in 'frm' I have one frame with camera.

What should I do, when in 'frm' I wanna get screenshot (or part of screenshot) of my monitor? With for example, movie I watch, or game I play?

I can't specify my question more precisely, so I even don't know which keywords should I use in searching.

Please for some clues.

Regards!