title: python opencv cv2.cv.CV_CAP_PROP_FRAME_COUNT get wrong numbers
body: <pre><code>import os
  import cv2
  path='/home/nlpr4/video-data/UCF-101/GolfSwing/v_GolfSwing_g24_c06.avi'
  cap=cv2.VideoCapture(path)
  video_length=int(cap.get(cv2.cv.CV_CAP_PROP_FRAME_COUNT))
  
  success=True
  count=0
  while success:
      success,image=cap.read()
      if success==False:
          break
      count=count+1
  
  print video_length,count
  </code></pre>
  
  <p>output:</p>
  
  <pre><code>149 
  146
  </code></pre>
  
  <p>why the two numbers different?
  what's wrong?</p>