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 video frame change, but for some reason it won’t work (the picture freezes with the following code):

#include "cv.h"
#include "highgui.h"

int g_slider_position = 0;
CvCapture* g_capture = NULL;

void onTrackbarSlide(int pos)
{
    cvSetCaptureProperty(g_capture, CV_CAP_PROP_POS_FRAMES, pos);
}

int main(int argc, char** argv)
{
    cvNamedWindow("The Tom 'n Jerry Show", CV_WINDOW_AUTOSIZE);
    g_capture = cvCreateFileCapture(argv[1]);
    int frames = (int) cvGetCaptureProperty(
        g_capture, 
        CV_CAP_PROP_FRAME_COUNT
        );

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

    IplImage* frame;

    while (1)
    {
        frame = cvQueryFrame(g_capture);
        if (!frame) 
            break;

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

        cvShowImage("The Tom 'n Jerry Show", frame);
        char c = cvWaitKey(33);
        if (c == 27)
            break;
    }

    cvReleaseCapture(&g_capture);
    cvDestroyWindow("The Tom 'n Jerry Show");

    return 0;
}

Any idea how to get the slider and video to work as intended?

Answer 1

This is the actual working code



// PROGRAM TO ADD A UPDATING TRACKBAR TO A VIDEO

#include <cv.h>
#include <highgui.h>


int g_slider_position = 0;
CvCapture* video_capture = NULL;

void onTrackbarSlide(int current_frame)
{
    current_frame = g_slider_position;
    cvSetCaptureProperty(video_capture,CV_CAP_PROP_POS_FRAMES,current_frame);
}

int main( int argc, char** argv )
{
    cvNamedWindow( "Video", CV_WINDOW_AUTOSIZE );
    video_capture = cvCreateFileCapture( "Crowdy.avi");
    int no_of_frames = (int) cvGetCaptureProperty(video_capture,CV_CAP_PROP_FRAME_COUNT);
    if( no_of_frames!= 0 ) 
    {
        cvCreateTrackbar("Slider","Video",&g_slider_position,no_of_frames,onTrackbarSlide);
    }

    IplImage* frame;

    while(1) 
    {
        frame = cvQueryFrame( video_capture );
        if( !frame ) break;
        cvShowImage( "Video", frame );
        cvSetTrackbarPos("Slider","Video",++g_slider_position);
        char c = cvWaitKey(33);
        if( c == 27 ) break;
    }
    cvReleaseCapture( &video_capture );
    cvDestroyWindow( "Video" );

    return(0);
}