OpenCV blocking my GTK interface - c++

Question

I've coded a very simple GUI with GTK3, with three buttons:
Start: calls OpenCV VideoCapture
Stop: stops the VideoCapture
Quit: destroys the window
The problem arises when I hit "Stop", the OpenCV process blocks the GUI and the command is executed after many seconds (typically, a minute).
this is my main:
bool stop = false;
static void start_webcam (GtkWidget *widget, gpointer data)
{
cout<<"Webcam On"<<endl;
int err = 0;
**err = opencv_webcam(); // this function loads the OpenCV Webcam**
// troubles?
if (err != 0)
{
cout<<"Unable to load the webcam. Error Code # "<<err<<endl;
}
}
static void stop_webcam (GtkWidget *widget, gpointer data)
{
stop = true;
cout<<"Webcam Off"<<endl;
}
int main (int argc, char *argv[])
{
// Widgets
GtkWidget *window;
GtkWidget *grid;
GtkWidget *button;
// GTK Init
gtk_init (&argc, &argv);
window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
gtk_window_set_title (GTK_WINDOW (window), "WEBCAM CONTROL PANEL");
g_signal_connect (window, "destroy", G_CALLBACK (gtk_main_quit), NULL);
gtk_container_set_border_width (GTK_CONTAINER (window), 10);
grid = gtk_grid_new ();
gtk_container_add (GTK_CONTAINER (window), grid);
/// START WEBCAM
button = gtk_button_new_with_label ("Start");
g_signal_connect (button, "clicked", G_CALLBACK (start_webcam), NULL);
gtk_grid_attach (GTK_GRID (grid), button, 0, 0, 1, 1);
/// STOP WEBCAM
button = gtk_button_new_with_label ("Stop");
g_signal_connect (button, "clicked", G_CALLBACK (stop_webcam), NULL);
gtk_grid_attach (GTK_GRID (grid), button, 1, 0, 1, 1);
/// QUIT
button = gtk_button_new_with_label ("Quit");
g_signal_connect (button, "clicked", G_CALLBACK (gtk_main_quit), NULL);
gtk_grid_attach (GTK_GRID (grid), button, 0, 1, 2, 1);
/// GO!
gtk_widget_show_all (window);
gtk_main ();
return 0;
}
I would like to set some high priority to the Quit button. I tried by setting the highest priority in my main by SetPriorityClass() in Windows, but I was unsuccessful.
Thank you very much!
EDIT
this is my openCV code:
extern bool stop;
int opencv_webcam()
{
/// Various stuff
cv::Mat frame;
cv::namedWindow("Webcam Session");
/// Webcam Opening
cv::VideoCapture clip(0);
while(stop != true)
{
// frames extraction
clip.grab();
clip.retrieve(frame);
// troubles?
if ((frame.empty()==1))
{
// errore!
return -1;
}
/// SHOW the frames
cv::imshow("Webcam Session", frame);
if (waitKey(30)>= 0)
{
break;
}
}
return 0;
}

Answer 1

GTK+ is NOT multithreaded. When one of your signal handlers is running, all other GUI processing stops. Your opencv_webcam() function is running on the GUI thread, and runs its main loop while processing the Start button signal. The reason why your Stop and Quit buttons aren't working is because they never get a chance to run (at least not until opencv_webcam() errors out).
I don't know what the appropriate solution is; I don't know enough about OpenCV to suggest a solution that lets you have stable timing while playing nice with GTK+. I know of two possibilities, though:
Doing all OpenCV processing in a gdk_threads_add_idle() callback (stable timing will be harder)
Running OpenCV on a different thread (you will need to communicate frame images across threads if you want to show them in your window)
You will need to search around and figure out what the best solution is. Try Googling for "gtk opencv" and seeing what you get.
Alternatively, if you're doing strict webcam work, you might want to look at libcheese, which is a GTK+ complement specifically intended for webcams.
As for button priorities, there's no signal prioritization that I know of. Given the diagnosis of your bug above, you should be able to see why prioritization wouldn't have helped anyway. In fact, I don't know of any GUI toolkit for any platform that has that ability. You should instead fix threading bugs that prevent the GUI from being snappy.