title: Failure to use adaptiveThreshold: CV_8UC1 in function adaptiveThreshold
body: <p>I have used openCV python and encountered an error.</p>
  
  <pre><code>img_blur = cv2.medianBlur(self.cropped_img,5)
  img_thresh_Gaussian = cv2.adaptiveThreshold(img_blur, 255, cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY, 11, 2)
  
  plt.subplot(1,1,1),plt.imshow(img_thresh_Gaussian, cmap = 'gray')
  plt.title("Image"), plt.xticks([]), plt.yticks([])
  plt.show()
  </code></pre>
  
  <p>but I received:</p>
  
  <pre><code>cv2.error: /home/phuong/opencv_src/opencv/modules/imgproc/src/thresh.cpp:1280: error: (-215) src.type() == CV_8UC1 in function adaptiveThreshold
  </code></pre>
  
  <p>Do I have to install something else?</p>