title: Define image ROI with OpenCV in C
body: <p>I have a question about how to define a ROI with OpenCV. I know what the definition of the function <code>cvSetImageROI</code> but I want to know if is possible to define a ROI with OpenCV with a different shape than rectangle. For example, define a ROI in a circle or in different form.</p>
  
  <p>It is possible to do this?</p>
  
  <blockquote>
    <p>cvSetImageROI(img1, cvRect(a, b, c, d));</p>
  </blockquote>