OpenCV - getting the slider to update its position during video playback

Question

I\'ve picked up \'Learning OpenCV\' and have been trying some of the code examples/exercises. In this code snippet, I want to get the slider to update its position with each

Answer 1

You are incrementing g_slider_position twice in the code, so it will increment beyond its limit (set in cvCreateTrackbar as frames). This is likely causing your picture to freeze.

To fix, change this

    g_slider_position++;
    cvSetTrackbarPos(
        "Position", 
        "The Tom 'n Jerry Show",
        ++g_slider_position
        );

to

    cvSetTrackbarPos(
        "Position", 
        "The Tom 'n Jerry Show",
        ++g_slider_position
        );

---

Accounting for the edited code, I would check that OpenCV is properly reading the number of frames from your file. Look at Learning OpenCV's Chapter 2, example 2.3 for a method of generically retrieving the number of frames from your AVI (if that is what you are using).

In your code above, if the number of frames is 0, the trackbar is not created but the code still enters a loop that attempts to update the trackbar position (if it finds a frame). I would use this instead:

if (frames != 0)
{
    cvCreateTrackbar(
        "Position",
        "The Tom 'n Jerry Show",
        &g_slider_position,
        frames,
        onTrackbarSlide
        );
} 
else
{
    exit(1);
}