opencv trackbar控制视频

[小村长]

今晚看了opencv用trackbar控制视频帧，不知为什么始终不能实现滚动条随着视频帧移动，求指教
代码:

#include <iostream>
#include "cxcore.h"
#include "cv.h"
#include "highgui.h"
using namespace std;

CvCapture *capture = NULL;
int frame_count = 0;
int g_capture;

void track_change(int pos){
	cvSetCaptureProperty(capture, CV_CAP_PROP_POS_FRAMES, pos);
	frame_count = pos;
}

int main()
{
	cvNamedWindow("show");
	capture = cvCreateFileCapture("F:\\download\\itudou\\xx5.avi");
	int frame_sum = cvGetCaptureProperty(capture, CV_CAP_PROP_FRAME_COUNT);
	if(frame_sum != 0){
		cvCreateTrackbar("track_name", "show", &g_capture, frame_sum, track_change);
	}
	IplImage *image = NULL;
	while(1){
		image = cvQueryFrame(capture);
		if(!image){
			break;
		}
		cvShowImage("show",image);
		char c = cvWaitKey(33);
		if(c == 27) 
		{
			break;
		}
		
		cvSetTrackbarPos("track_name","show", frame_count);  //运行两帧就结束了  为什么呢？
		frame_count ++;

	
	}
	cvReleaseCapture(&capture);
	cvDestroyWindow("show");
	return 0;
	
}

当我将cvSetTrackbarPos("track_name","show", frame_count); 改为重新创建滚动条时是可以的cvCreateTrackbar("track_name", "show", &frame_count, frame_sum, track_change);，为什么？ 求指教，谢谢