title: Python opencv sorting contours
body: <p>I am following this question:</p>
  
  <p><a href="https://stackoverflow.com/questions/38654302/how-can-i-sort-contours-from-left-to-right-and-top-to-bottom">How can I sort contours from left to right and top to bottom?</a></p>
  
  <p>to sort contours from left-to-right and top-to-bottom. However, my contours are found using this (OpenCV 3):</p>
  
  <pre><code>im2, contours, hierarchy = cv2.findContours(threshold,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)
  </code></pre>
  
  <p>and they are formatted like this:</p>
  
  <pre><code>   array([[[ 1,  1]],
  
     [[ 1, 36]],
  
     [[63, 36]],
  
     [[64, 35]],
  
     [[88, 35]],
  
     [[89, 34]],
  
     [[94, 34]],
  
     [[94,  1]]], dtype=int32)]
  </code></pre>
  
  <p>When I run the code </p>
  
  <pre><code>max_width = max(contours, key=lambda r: r[0] + r[2])[0]
  max_height = max(contours, key=lambda r: r[3])[3]
  nearest = max_height * 1.4
  contours.sort(key=lambda r: (int(nearest * round(float(r[1])/nearest)) * max_width + r[0]))
  </code></pre>
  
  <p>I am getting the error </p>
  
  <pre><code>ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
  </code></pre>
  
  <p>so I changed it to this:</p>
  
  <pre><code>max_width = max(contours, key=lambda r:  np.max(r[0] + r[2]))[0]
  max_height = max(contours, key=lambda r:  np.max(r[3]))[3]
  nearest = max_height * 1.4
  contours.sort(key=lambda r: (int(nearest * round(float(r[1])/nearest)) * max_width + r[0]))
  </code></pre>
  
  <p>but now I am getting the error:</p>
  
  <p><code>TypeError: only length-1 arrays can be converted to Python scalars</code></p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>After reading the answer below I modified my code:</p>
  
  <p><strong>EDIT 2</strong></p>
  
  <p>This is the code that I use to "dilate" the characters and find the contours</p>
  
  <pre><code>kernel = cv2.getStructuringElement(cv2.MORPH_RECT,(35,35))
  
  # dilate the image to get text
  # binaryContour is just the black and white image shown below
  dilation = cv2.dilate(binaryContour,kernel,iterations = 2)
  </code></pre>
  
  <p><strong>END OF EDIT 2</strong></p>
  
  <pre><code>im2, contours, hierarchy = cv2.findContours(dilation,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)
  
  myContours = []
  
  # Process the raw contours to get bounding rectangles
  for cnt in reversed(contours):
  
      epsilon = 0.1*cv2.arcLength(cnt,True)
      approx = cv2.approxPolyDP(cnt,epsilon,True)
  
      if len(approx == 4):
  
          rectangle = cv2.boundingRect(cnt)
          myContours.append(rectangle)
  
  max_width = max(myContours, key=lambda r: r[0] + r[2])[0]
  max_height = max(myContours, key=lambda r: r[3])[3]
  nearest = max_height * 1.4
  myContours.sort(key=lambda r: (int(nearest * round(float(r[1])/nearest)) * max_width + r[0]))
  
  i=0
  for x,y,w,h in myContours:
  
      letter = binaryContour[y:y+h, x:x+w]
      cv2.rectangle(binaryContour,(x,y),(x+w,y+h),(255,255,255),2)
      cv2.imwrite("pictures/"+str(i)+'.png', letter) # save contour to file
      i+=1
  </code></pre>
  
  <p>Contours before sorting:</p>
  
  <pre><code>[(1, 1, 94, 36), (460, 223, 914, 427), (888, 722, 739, 239), (35,723, 522, 228), 
  (889, 1027, 242, 417), (70, 1028, 693, 423), (1138, 1028, 567, 643),     
  (781, 1030, 98, 413), (497, 1527, 303, 132), (892, 1527, 168, 130),  
  (37, 1719, 592, 130), (676, 1721, 413, 129), (1181, 1723, 206, 128), 
  (30, 1925, 997, 236), (1038, 1929, 170, 129), (140, 2232, 1285, 436)]
  </code></pre>
  
  <p>Contours after sorting:</p>
  
  <p>(<strong>NOTE:</strong> This is not the order I want the contours to be sorted in. Refer to image at the bottom)</p>
  
  <pre><code>[(1, 1, 94, 36), (460, 223, 914, 427), (35, 723, 522, 228), (70,1028, 693, 423), 
  (781, 1030, 98, 413), (888, 722, 739, 239), (889, 1027, 242, 417), 
  (1138, 1028, 567, 643), (30, 1925, 997, 236), (37, 1719, 592, 130), 
  (140, 2232, 1285, 436), (497, 1527, 303, 132), (676, 1721, 413, 129), 
  (892, 1527, 168, 130), (1038, 1929, 170, 129), (1181, 1723, 206, 128)]
  </code></pre>
  
  <p>Image I am working with</p>
  
  <p><a href="https://i.stack.imgur.com/9VayB.png" rel="noreferrer"><img src="https://i.stack.imgur.com/9VayB.png" alt="enter image description here"></a></p>
  
  <p>I want to find the contours in the following order:
  <a href="https://i.stack.imgur.com/s4jKm.png" rel="noreferrer"><img src="https://i.stack.imgur.com/s4jKm.png" alt="enter image description here"></a></p>
  
  <p>Dilation image used for finding contours
  <a href="https://i.stack.imgur.com/sTyvW.png" rel="noreferrer"><img src="https://i.stack.imgur.com/sTyvW.png" alt="enter image description here"></a></p>