title: OpenCV findContours() just returning one external contour
body: <p>I'm trying to isolate letters in a captcha, I managed to filter a captcha and that result in this black and white image:</p>
  
  <p><a href="https://i.stack.imgur.com/qmldb.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/qmldb.png" alt="enter image description here"></a></p>
  
  <p>But when I tried to separate the letters with findContours method of OpenCV it just found a external contour that wraps my entire image, resulting in this image (black contour outside image).</p>
  
  <p><a href="https://i.stack.imgur.com/f0fo3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/f0fo3.png" alt="enter image description here"></a></p>
  
  <p>I'm using this code with Python 3 and OpenCV 3.4.2.17:</p>
  
  <pre><code>img = threshold_image(img)
  cv2.imwrite("images/threshold.png", img)
  
  image, contours, _ = cv2.findContours(img, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
  
  for i, contour in enumerate(contours):
      area = cv2.contourArea(contour)
      cv2.drawContours(img, contours, i, (0, 0, 0), 3)
  
  cv2.imwrite('images/output3.png', img)
  </code></pre>
  
  <p>I just want my final result is 5 contours outside each character.</p>