title: Detect non-closed contour on opencv
body: <p>I'm doing computer vision project for automatic card detection.
  I need to separate the card from the background. I have applied the canny edge detection, using automatic parameter settings from this</p>
  
  <p><a href="https://stackoverflow.com/questions/4292249/automatic-calculation-of-low-and-high-thresholds-for-the-canny-operation-in-open">Automatic calculation of low and high thresholds for the Canny operation in opencv</a></p>
  
  <p>The result is excellent. However, sometimes the canny is not perfect like this
  <img src="https://i.stack.imgur.com/bnFtF.jpg" alt="Photo after canny"></p>
  
  <p>I have applied cvFindContour to detect the box. However, due to "hole" on the upper side, opencv failed to detect the contour.</p>
  
  <p>How do I tune the cvFindContour to detect the contour or should I tune the canny edge instead?</p>