imshow() function freezes - c++

Question

I am developing some video analytic algorithms using openCV. However, after I process a frame and want to display it on a window, it hangs at imshow() function. I have searched for this issue online but still cannot find the problem! Here is the code where I am using multithread and openCV:
void CFeatureExtraction::extract(){
boost::thread OpFlowThread, BGSThread;
while (m_nRun){
ImgPtr frame_ptr;
m_pQueue->wait_and_pop(frame_ptr);
Mat frame1, frame2;
(*frame_ptr).copyTo(frame1);
(*frame_ptr).copyTo(frame2);
OpFlowThread = boost::thread(&COpFlow::op_flow, m_pAlgo,frame1);
BGSThread = boost::thread(&CBGSub::bgsub, m_pBGS, frame2);
OpFlowThread.join();
BGSThread.join();
}
}
And inside op_flow and bgsub function, I am using imshow() and cvWaitKey() together! But it keeps hanging the whole program.
If my question is still not clear for you, pls feel free to ask me for more detail.
Here is the detail code of calling imshow():
CFarnebackAlgo::CFarnebackAlgo() : COpFlow()
{
namedWindow("Optical Flow");
}
CFarnebackAlgo::~CFarnebackAlgo()
{
destroyWindow("Optical Flow");
}
int CFarnebackAlgo::op_flow(Mat frame)
{
Mat flow;
cvtColor(frame, gray, COLOR_BGR2GRAY);
if (prevgray.data)
{
calcOpticalFlowFarneback(prevgray, gray, flow, 0.5, 3, 15, 3, 5, 1.2, 0);
for (int y = 0; y < frame.rows; y += 8)
for (int x = 0; x < frame.cols; x += 8){
const Point2f& fxy = flow.at<Point2f>(y, x);
line(frame, Point(x, y), Point(cvRound(x + fxy.x), cvRound(y + fxy.y)), Scalar(0, 255, 0));
circle(frame, Point(x, y), 2, Scalar(0, 255, 0), -1);
}
if (frame.data == NULL){
cout << "No Img!" << endl;
exit(0);
}
imshow("Optical Flow", frame);
waitKey(50);
}
std::swap(prevgray, gray);
return 0;
}
If I put the namedWindow() in the op_flow(), it is working. But if I put it in the constructor, it's not working and freezes. Anyone knows why?