OpenCV - Video playback is slow if slider updates its position while playing - c

Question

I have 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 is slowing down the video playback speed.
The slider is updating the position during video playback using cvSetTrackbarPos() function but it is making the playback very slow.
#include <cv.h>
#include <highgui.h>
using namespace std;
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[])
{
if(argc<2)
{
printf("Usage: main <video-file-name>\n\7");
exit(0);
}
// create a window
cvNamedWindow("Playing Video With Slider", CV_WINDOW_AUTOSIZE);
g_capture = cvCreateFileCapture(argv[1]);
int frames = (int) cvGetCaptureProperty(g_capture, \
CV_CAP_PROP_FRAME_COUNT);
if(frames !=0)
{
cvCreateTrackbar("Slider","Playing Video With Slider", \
&g_slider_position,frames, onTrackbarSlide);
}
IplImage* frame = 0;
while(1)
{
frame = cvQueryFrame(g_capture);
if(!frame)
{
break;
}
cvShowImage("Playing Video With Slider", frame);
cvSetTrackbarPos("Slider","Playing Video With Slider", \
g_slider_position+1); //Slowing down playback
char c= cvWaitKey(33);
if(c == 27)
{
break;
}
}
// release the image
cvReleaseImage(&frame );
cvReleaseCapture(&g_capture);
// Destroy Window
cvDestroyWindow("Playing Video With Slider");
return 0;
}

Answer 1

This is an inefficiency in how opencv displays trackbars (the same problem occurs even if you don't update the slider, and in case when you refer to non-changing variable outside the processing loop).
A workaround might be to display the trackbar in a separate window.

Answer 2

Line char c= cvWaitKey(33); have problem.
It is in while(1) and every time it takes 33 milisecond to Wait for a pressed key.Make this number lesser.
EDITED LATER:
Make change as shown below
void onTrackbarSlide(int pos)
{
pos = g_slider_position;
cvSetCaptureProperty(g_capture,CV_CAP_PROP_POS_FRAMES,pos);
}

Answer 3

The problem is that every time You call cvSetTrackbarPos("Slider","Playing Video With Slider", g_slider_position+1); callback onTrackbarSlide changing the video position another time and slowing down the program flow.
The way I find out to avoid that is with a flag. It tells the callback if the change in the Trackbar is produced by the normal update flow or is produced by You.
int g_slider_position = 0;
int g_update_slider = 0;//flag
CvCapture *g_capture = NULL;
void onTrackbarSlide(int pos)
{
if (!g_update_slider)//if not changed by the video flow
{
cvSetCaptureProperty(
g_capture,
CV_CAP_PROP_POS_FRAMES,
pos
);
}
}
void updateSlider(int pos)
{
g_update_slider = 1; //Changed by the video flow
cvSetTrackbarPos("Position", "Example3", pos);
g_update_slider = 0; //Returns the flag when the change is performed
}
In the main I call for updateSlider instead of cvSetTrackbarPos.