title: cv2.imread and cv2.imshow return all zeros and black image
body: <p>I've create a *.png image using PPT. </p>
  
  <p>I want to use <code>cv2-python</code> to read and show it. But the <code>cv2.imshow</code> gave me a total black image with nothing on it. This is my code，I just don't know where was wrong:</p>
  
  <pre><code>img = cv2.imread(r"C:/Users/.../Test_shapes.png")
  print img.max(),img.shape
  cv2.imshow('color_image',img)
  cv2.waitKey()
  </code></pre>
  
  <p>p.s. the <code>img.shape</code> returns the correct shapes (881, 1803, 3). But <code>img.max()</code> returns 0.</p>
  
  <p>Sorry, I don't have enough reputation to add images here. But with Matlab, my *.png could be successfully read and the max is 108.</p>