title: Depth error in 2D image with OpenCV Python
body: <p>I am trying to compute the Canny Edges in an image (ndarray) using OpenCV with Python.</p>
  
  <pre><code>slice1 = slices[15,:,:]
  slice1 = slice1[40:80,60:100]
  print slice1.shape
  print slice1.dtype
  slicecanny = cv2.Canny(slice1, 1, 100)
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>(40, 40)
  float64
  ...
  error: /Users/jmerkow/code/opencv-2.4.6.1/modules/imgproc/src/canny.cpp:49: 
  error: (-215) src.depth() == CV_8U in function Canny
  </code></pre>
  
  <p>For some reason I get the above error.  Any ideas why?</p>