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For setting ROI in Video you can have some thing like below.

      VideoCapture cap(0);   
      cv::Mat frame;   
      while(1)
      cap >> frame; //Get the frame from cam    
      // SetImageRoi    
      cv::Rect roi(x, y, width, height);
      cv::Mat image_roi = frame(roi);  // note: this assignment does not copy data, frame and   image_roi now share data

      // Do some processing on ROI region
      process(image_roi);  // any changes to image_roi will also be in frame

      // Reset ROI  
      //     -- nothing required

    }