title: cv2.drawContours doesn't display all the contours
body: <p>I have an image and I want to display all the contours using cv.drawContours but it misses one between two overlapped cells and I don't know why even though cv2.Canny shows clearly all the contours.
  This is the code I used:</p>
  <pre><code>import cv2
  import imageio
  from skimage import io, color
  import numpy as np
  from google.colab.patches import cv2_imshow
  
  
  image = imageio.imread(path+'3.png')
  print(image.shape)
  image = color.gray2rgb(image)
  print(image.shape)
  
  cv2_imshow(image)
  cv2.waitKey(0)
  
  # Grayscale
  gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
  
  # Find Canny edges
  edged = cv2.Canny(gray, 30, 200)
  cv2.waitKey(0)
  
  # Finding Contours
  # Use a copy of the image e.g. edged.copy()
  # since findContours alters the image
  contours, hierarchy = cv2.findContours(edged.copy(),
      cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
  
  cv2_imshow(edged)
  cv2.waitKey(0)
  
  print(&quot;Number of Contours found = &quot; + str(len(contours)))
  
  # Draw all contours
  # -1 signifies drawing all contours
  img2=cv2.drawContours(image, contours, -1, (100,0,0), 2)
  print(len(contours))
  print(img2.shape)
  cv2_imshow(image)
  cv2.waitKey(0)
  cv2.destroyAllWindows()
  </code></pre>
  <p>and this is the input image: <a href="https://i.stack.imgur.com/X9S40.png" rel="nofollow noreferrer">input image</a>, this is the image after canny edging <a href="https://i.stack.imgur.com/zcXQm.png" rel="nofollow noreferrer">edged</a> and this is img2 the final image after cv2.drawCOntours <a href="https://i.stack.imgur.com/kf23D.png" rel="nofollow noreferrer">img2</a> and as you can see it misses the contour between the 2 overlapped cells in the middle. I will be grateful if you can help me solve this problem.</p>