use signal&slot between two tabs

[stella1016]

I hope I did something wrong. Maybe you can help me solve my old problem. I post my code here. My app is to display the camera output images in QGLWidget.

MyThread: in its run method, it does the while-loop

run()
{
   while(flag)
   {
	IplImage *lp_color = t_camera.get_image();
	myQLWidget->setCurrentImageIpl(lp_color);
 	cvReleaseImage(&lp_color);
    }
}

run()
{
   while(flag)
   {
	IplImage *lp_color = t_camera.get_image();
	myQLWidget->setCurrentImageIpl(lp_color);
 	cvReleaseImage(&lp_color);
    }
}

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MyQLWidget: in charge of updating the texture when it receives any image.
I am using SIGNAL & SLOT in MyQLWidget, because in method setCurrentImageIpl(), I cannot directly call updateGL(). This is because OpenGl & Thread issue... So in setCurrentImageIpl() I emit a SIGNAL to a SLOT, the SLOT will do updateGL() for me.

MyQLWidget::setCurrentImageIpl()
{
 
    ...(set image)
    imageData = ...;
    update = true;
    emit newData();
 
}

MyQLWidget::setCurrentImageIpl()
{
     
    ...(set image)
    imageData = ...;
    update = true;
    emit newData();

}

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newData is connect to SLOT updateImage()

MyQLWidget::updateImage()
{
  this->updateGL();
}
 
MyQLWidget::paintGL()
{
  if(update)
       glTexSubImage2D = (.....imageData);
}

MyQLWidget::updateImage()
{
  this->updateGL();
}

MyQLWidget::paintGL()
{
  if(update)
       glTexSubImage2D = (.....imageData);
}

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During displaying the first few frames that grasped by camera, my program crashes. Not every time. I don't know where is the problem. When it crashes, the reason is that the imageData is not set jet, so when update texture it crashes.

I set breakpoint, and found that in the first few steps, ...emit... executes twice, but updateImage() only once. Maybe this causes the crash, I am not sure.

Hard to explain what I did in my code here.

[wysota]

I hope I did something wrong. Maybe you can help me solve my old problem. I post my code here. My app is to display the camera output images in QGLWidget.

MyThread: in its run method, it does the while-loop

run()
{
   while(flag)
   {
	IplImage *lp_color = t_camera.get_image();
	myQLWidget->setCurrentImageIpl(lp_color);
 	cvReleaseImage(&lp_color);
    }
}

run()
{
   while(flag)
   {
	IplImage *lp_color = t_camera.get_image();
	myQLWidget->setCurrentImageIpl(lp_color);
 	cvReleaseImage(&lp_color);
    }
}

To copy to clipboard, switch view to plain text mode 

This is bad. You can't access widgets from other threads and I can't stress that more. You should have emitted a signal from the thread (from the thread and not the thread object!) that would be caught by a slot in the widget.

So in setCurrentImageIpl() I emit a SIGNAL to a SLOT, the SLOT will do updateGL() for me.

This is wrong. You are emitting a signal on behalf of the widget from a worker thread. Qt checks which thread owns the sending and receiving objects, sees that it is the GUI thread, so makes a direct function call to updateGL() from the worker thread which is wrong and will cause segmentation faults sooner or later.

During displaying the first few frames that grasped by camera, my program crashes.

That's exactly what I mean

[stella1016]

This is bad. You can't access widgets from other threads and I can't stress that more. You should have emitted a signal from the thread (from the thread and not the thread object!) that would be caught by a slot in the widget.

So in setCurrentImageIpl() I emit a SIGNAL to a SLOT, the SLOT will do updateGL() for me.

This is wrong. You are emitting a signal on behalf of the widget from a worker thread. Qt checks which thread owns the sending and receiving objects, sees that it is the GUI thread, so makes a direct function call to updateGL() from the worker thread which is wrong and will cause segmentation faults sooner or later.



That's exactly what I mean

Thanks for your replay.
I don't fully understand what you mean...So how can I make things right?

[wysota]

Read about signals and multithreading, that's for a start. Then you need to make sure that:
1. you don't access any widget method from non-GUI thread directly
2. queued connections are used for signal-slot connections between threads (and I don't mean passing the optional argument to connect() but to make sure Qt will choose queued connections by itself).