title: why datatype has to be 'uint8' in Opencv python wrapper?
body: <p>I'd like to create an image of <strong>gray</strong> color using the following code:</p>
  
  <pre><code>import numpy as np
  import cv2
  cv=cv2
  
  s=np.zeros((240, 320, 3), dtype=int)
  s[s==0]=128
  cv.imshow('3', s)
  cv.waitKey()
  cv.destroyAllWindows()
  </code></pre>
  
  <p>but I get a totally <strong>black</strong> image. When I write the image to a file, it <strong>indeed</strong> is a gray image:</p>
  
  <pre><code>fn='example.jpg'
  cv.imwrite(fn, s)
  </code></pre>
  
  <p>So I have to change <code>int</code> to <code>uint8</code>, then everything works well. But I'm still curious that why I have to use <code>uint8</code> instead of <code>int</code>, is there any docs describing this?</p>