title: python opencv - How to combine two contours
body: <p>I want cut up the captcha image.<br>
  the origin picture is here:
  <a href="http://railway.hinet.net/ImageOut.jsp" rel="nofollow noreferrer">captcha url</a></p>
  
  <p>There are my results:<br>
  <img src="https://i.stack.imgur.com/ZRswa.png" alt="picture 1"> </p>
  
  <p><img src="https://i.stack.imgur.com/YKVLB.png" alt="picture 2">  </p>
  
  <p>First,I remove noise and line from captcha.<br>
  Then find contours. 
  I draw contours in image using green line.<br>
  But some chars in image will separated in two piece.<br>
  How to determine two piece is belong to one char?    </p>
  
  <pre><code>contours, hierarchy = cv2.findContours(img, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
  cv2.drawContours(self.im, contours, -1, (0, 255, 0), 1)
  </code></pre>