OpenCV: Image window keeps hanging and not responding - c++

Question

The following code draws a line connecting two points user clicks on an image:
using namespace cv;
using namespace std;
void onMouse(int evt, int x, int y, int flags, void* param) {
if(evt == CV_EVENT_LBUTTONDOWN) {
std::vector<cv::Point>* ptPtr = (std::vector<cv::Point>*)param;
ptPtr->push_back(cv::Point(x,y));
}
}
int main()
{
std::vector<Point> points;
cv::namedWindow("Output Window");
Mat frame = cv::imread("chhha.png");
cv::setMouseCallback("Output Window", onMouse, (void*)&points);
int X1=0, Y1=0, X2=0, Y2=0;
while(1)
{
cv::imshow("Output Window", frame);
if (points.size() > 1) //we have 2 points
{
for (auto it = points.begin(); it != points.end(); ++it)
{
}
break;
}
waitKey(10);
}
//just for testing that we are getting pixel values
X1=points[0].x;
X2=points[1].x;
Y1=points[0].y;
Y2=points[1].y;
cout<<"First and second X coordinates are given below"<<endl;
cout<<X1<<'\t'<<X2<<endl;
cout<<"First and second Y coordinates are given below"<<endl;
cout<<Y1<<'\t'<<Y2<<endl;
// Now let us draw a line on the image
line( frame, points[0], points[1], 'r', 2, 8 );
cv::imshow("Output Window", frame);
waitKey( 10 );
getch();
return 0;
}
The problem is the program is not exiting if I close (by clicking the cross button on the image on top right )the "Output Window", instead it hangs and says Not responding.
How do I remove this problem ?

Answer 1

The reason that your application does not exit is that you have an infinite loop, and clicking on the cross to close a window does nothing to break that loop.
One way to exit is to test for a key being pressed, e.g.
while(true)
{
...
char c = cv::waitKey(10);
if(c == 'q')
break;
}
BTW, assuming you are on windows, if a window is destroyed waitKey() intercepts a WM_DESTROY message. In this case waitKey() returns message.wParam, but the windows doco for WM_DESTROY says This parameter is not used. It looks like a bug to me, but it might be worth investigating whether waitKey() returns a consistent value when the window is closed - normally -1 is returned if no key is pressed.