title: opencv VideoWriter under OSX producing no output
body: <p>I am trying to create a video from the python wrapper for OpenCV under OSX. I am using python 2.7.1, opencv 2.3.1a, and the python wrappers from willowgarage that come with that version of opencv. I have:</p>
  
  <pre><code>import cv,cv2
  w = cv2.VideoWriter('foo.avi', cv.FOURCC('M','J','P','G'), 25, (100,100))
  for i in range(100):
      w.write(np.ones((100,100,3), np.uint8))
  </code></pre>
  
  <p>OpenCV says</p>
  
  <pre><code>WARNING: Could not create empty movie file container.
  Didn't successfully update movie file.
  ... [ 100 repetitions]
  </code></pre>
  
  <p>I'm not sure what to try next</p>