title: opencv: understanding cvsetcaptureproperty function and argument
body: <p>I am starting to learn openCv but I have some difficulties understanding the last argument (double value) of this function</p>
  
  <pre><code>int cvSetCaptureProperty(CvCapture* capture, int property_id, double value)
  </code></pre>
  
  <p>I understand this is a propoerty used in c++ but how does it works in the code below ?</p>
  
  <pre><code>void onTrackSlide(int pos)
  {
      cvSetCaptureProperty(cap, CV_CAP_PROP_POS_FRAMES, pos);
  }
  </code></pre>
  
  <p>it is then called in the main program like this:</p>
  
  <pre><code>cvCreateTrackbar("position", "example3", &amp;slider, frames, onTrackSlide);
  </code></pre>
  
  <p>I don't understand why the pos is never filled or used in the argument of onTrackSlide function.</p>
  
  <p>here is the full program: </p>
  
  <pre><code>#include&lt;opencv\cv.h&gt;
  #include&lt;opencv\highgui.h&gt;
  #include "opencv2/opencv.hpp"
  //#include &lt;iostream&gt;
  
  using namespace cv;
  
  int slider=0;
  CvCapture* cap = NULL;
  
  void onTrackSlide(int pos)
  {
  
      cvSetCaptureProperty(cap, CV_CAP_PROP_POS_FRAMES, pos);
  }
  
  
  int main(int argc, char ** argv)
  {
  
      cvNamedWindow("example3", CV_WINDOW_AUTOSIZE);
      cap = cvCreateFileCapture(argv[1]);
      int frames = (int)cvGetCaptureProperty(cap, CV_CAP_PROP_FRAME_COUNT);
  
      if (frames != 0)
      {
          cvCreateTrackbar("position", "example3", &amp;slider, frames, onTrackSlide);
  
      }
  
      IplImage* frame;
      while (1)
  
      {
          frame = cvQueryFrame(cap);
          if (!frame)break;
          cvShowImage("example3", frame);
          char c = cvWaitKey(33);
          if (c == 27)  break;
  
      }
  
      cvReleaseCapture(&amp;cap);
      cvDestroyWindow("example3");
  
  
  }
  </code></pre>