title: extracting frames from a mp4.file
body: <p>i want to open a .mp4 file, so that i can access each frame.
  I tried to do it using javacv, but it didnt work.(see code below)
  Does anyone know why it didnt work or how else i could achieve my goal?</p>
  
  <p>i tried to call</p>
  
  <pre><code>string path = Environment.getExternalStorageDirectory().getAbsolutePath()
                  + File.separator + "Test";
  opencv_videoio.VideoCapture cap = new opencv_videoio.VideoCapture(path + File.separator + "test.mp4");
  </code></pre>
  
  <p>but then <code>cap.isOpend()</code> returns <code>false</code>. I have set the permission <code>WRITE_EXTERNAL_STORAGE</code>(which includes READ)</p>