title: Opencv draws numpy.zeros as a gray image
body: <p>I'm struggling with understanding how opencv interprets numpy arrays.</p>
  
  <pre><code>import cv2
  import numpy as np
  
  if __name__ == '__main__':
      size = (w, h, channels) = (100, 100, 1)
      img = np.zeros(size, np.int8)
      cv2.imshow('result', img), cv2.waitKey(0)
      cv2.destroyAllWindows()
  </code></pre>
  
  <p>Grayscale black 100x100 image, right?
  No, it's showing me gray! Why's that?</p>