title: OpenCV C++, getting Region Of Interest (ROI) using cv::Mat
body: <p>I'm very new to OpenCV (started using it two days ago), I'm trying to cut a hand image from a depth image got from Kinect, I need the hand image for gesture recognition. I have the image as a <code>cv::Mat</code> type. My questions are:</p>
  
  <ol>
  <li>Is there a way to convert <code>cv::Mat</code> to <code>cvMat</code> so that I can use <code>cvGetSubRect</code> method to get the Region of interest?</li>
  <li>Are there any methods in <code>cv::Mat</code> that I can use for getting the part of the image?</li>
  </ol>
  
  <p>I wanted to use <code>IplImage</code> but I read somewhere that <code>cv::Mat</code> is the preferred way now.</p>