title: Arguments to cv2::imshow
body: <p><strong>Edit: original title "convert numpy array to cvmat" was a mistake - OpenCV's less than useful error message and my not reading the docs!</strong></p>
  
  <p>With <a href="http://en.wikipedia.org/wiki/OpenCV" rel="nofollow noreferrer">OpenCV</a> 2, <a href="http://en.wikipedia.org/wiki/IPython" rel="nofollow noreferrer">IPython</a> now uses NumPy arrays by default.</p>
  
  <pre><code>cvimage = cv2.imread("image.png") #using OpenCV 2
  type(cvimage)
  Out: numpy.ndarray  #dtype is uint8
  
  pltimage = plt.imread("image.png")  #using Matplotlib
  type(pltimage)
  Out: numpy.ndarray   #dtype is float
  
  plt.imshow(cvimage)  # works great
  
  cv2.imshow(cvimage)
  TypeError: Required argument 'mat' (pos 2) not found
  </code></pre>
  
  <p>Since <code>cv2</code> uses NumPy arrays by default, there is no longer any cv::Mat constructor and NumPy has no functions to convert to a <code>cv::Mat</code> array.</p>
  
  <p>Any ideas?</p>