title: cv2.findContours doesn't find contours
body: <p>i need to find contours of brown regions. but befıre that, i tried to draw all contours. but i can't see any contour on</p>
  
  <p>i tried this:</p>
  
  <pre><code>contours, hierarchy = cv2.findContours(thresh_dummy, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE) 
  cv2.drawContours(thresh_dummy, contours, -1, (0, 255, 0), 3) 
  </code></pre>