title: OpenCV Assertion Error when converting image to greyscale
body: <p>When trying to convert an image to grey scale in opencv I get the following error message which can be seen here:
  <a href="http://i.imgur.com/Av71SYz.png" rel="nofollow">http://i.imgur.com/Av71SYz.png</a></p>
  
  <p>Here's the code:</p>
  
  <pre><code>import cv2
  
  img = cv2.imread('pictures\chessBoard.png',0)
  gray_image = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
  </code></pre>
  
  <p>These error messages are pretty cryptic, especially to someone new to opencv. Thanks for the help!</p>