How to get the regions (rectangles with coordinates) by dense optical flow?

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Hi everyone:

     I am studying something about dense optical flow in opencv, but got some problems.
     If I want to get the regions (rectangles with coordinates) of moving objects by dense optical flow, how should I do? I have gotten the dense optical flow between frames, but do not know how to get the rectangles for moving objects.
     By the way, if I use GMM, how should I get the regions ?
     Can anyone help me?

     Thanks a lot!