title: Can't write video by opencv in Python
body: <p>I am an open cv beginner. I followed the steps in this tutorial to practice using it.
  <a href="http://opencv-python-tutroals.readthedocs.org/en/latest/py_tutorials/py_gui/py_video_display/py_video_display.html#display-video" rel="noreferrer">http://opencv-python-tutroals.readthedocs.org/en/latest/py_tutorials/py_gui/py_video_display/py_video_display.html#display-video</a></p>
  
  <p>I changed some lines for performance on my mac:</p>
  
  <pre><code>import numpy as np
  import cv2
  
  cap = cv2.VideoCapture(0)
  size = (int(cap.get(cv2.cv.CV_CAP_PROP_FRAME_WIDTH)),
          int(cap.get(cv2.cv.CV_CAP_PROP_FRAME_HEIGHT)))
  
  # Define the codec and create VideoWriter object
  fourcc = cv2.cv.CV_FOURCC(*'DIVX') # upper case - yl3
  out = cv2.VideoWriter('output.avi',fourcc, 20, size, 1) #20.0: number of frames per sec
  
  while(cap.isOpened()):
      ret, frame = cap.read()
      if ret==True:
          frame = cv2.flip(frame,1)
  
          # write the flipped frame
          out.write(frame)
  
          cv2.imshow('frame',frame)
          if cv2.waitKey(1) &amp; 0xFF == ord('q'):
              break
      else:
          break
  
  # Release everything if job is finished
  cap.release()
  out.release()
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>I am using Python 2.7.10, opencv 2.4.12 and mac OS X 10.10.5
  Now I can successfully get the camera to work and generate an <code>output.avi</code> file in the same directory, but the file was only 414k overtime and I can't open it. It seemed that it contained nothing but black.</p>
  
  <p>Can anyone help me with that?</p>