title: Cannot open video files in openCV
body: <p>Please help to fix the video opening issue using opencv on raspberry pi 3. Almost 3 weeks I am fighting with this issue.<br>
  This is my code:</p>
  
  <pre><code>import cv2
  
  cap = cv2.VideoCapture("/home/pi/Documents/forRaspberry/small.mp4")
  print (cap.isOpened())
  </code></pre>
  
  <p>And when running, the cap isOpened returns false: 
  (same False result with cap.grab() )</p>
  
  <pre><code>Python 3.4.2 (/usr/bin/python3)
  &gt;&gt;&gt; %Run roundBound_modif_withVideo.py
  False
  </code></pre>
  
  <p>I am using 3.2.0 opencv version and 3.4.2 python version.
  <br/>I assumed the issue is connected with FFMPEG. And put all my efforts to fix it. These are what I have tried so far:<br/>
  1)  I build opencCV with FFMPEG on <br/>
   <code>WITH_FFMPEG                      ON</code>    <br/>
  Does not fix the issue<br/>
  2) FFPMEG is not installed in my raspbain. I read in forums that <code>libav-tools</code> replaces the FFMPEG. So installed libav-tools. Still not helped<br/>
  3) When I do cmake it prints following:<br/></p>
  
  <pre><code>--     FFMPEG:                      NO
  --       avcodec:                   NO
  --       avformat:                  NO
  --       avutil:                    NO
  --       swscale:                   NO
  --       avresample:                YES (ver 2.1.0)
  </code></pre>
  
  <p>Read in forum that I need to alias  ffmpeg to <code>avconv</code> (<a href="https://www.raspberrypi.org/forums/viewtopic.php?t=125367" rel="nofollow noreferrer">https://www.raspberrypi.org/forums/viewtopic.php?t=125367</a>). Still does not help<br/></p>
  
  <p>I think should I install ffmpeg in raspbain separately?<br/></p>
  
  <p>Can this issue be connected with version mismatches of some packages? Can someone please explain how I can know all matching pacakges versions list for my opencv and pyhton versions?</p>