title: Can not detect the marked square using OpenCV
body: <p>I need to detect the option marked using OpenCV. Currently, I have been able to detect all the squares but the one that is marked. I have done this using the following piece of code.</p>
  <pre><code>canny = (cv2.Canny(roi_box, 30, 100))
  cv2_imshow(canny)
  img = roi_box.copy()
  
  contours, heirarchy = cv2.findContours(canny, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
  cntsSorted = sorted(contours, key=lambda x:cv2.contourArea(x))
  
  print(&quot;contours %i&quot; % len(contours))
  for i in range(45, 0, -1):
      cv2.drawContours(img, cntsSorted[i], -1, (0, 255,0), 4)
      if (cv2.contourArea(cntsSorted[i]) &gt; 300):
          cv2_imshow(img)
  </code></pre>
  <p>The area of the square that is marked is around 50. Can someone suggest to me how can I solve this problem?</p>
  <p><a href="https://i.stack.imgur.com/npDro.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/npDro.png" alt="My work till now" /></a></p>