Show webcam through Qt - progam doesn't respond to any user input

Hi all,
I try to get the output of my webcam with opencv and display it through qt. Therefore I convert the IplImage into a QImage and call the function setPixmap() to draw the image onto a QLabel. Afterwards I repaint the QLabel to update it. Generally the output works fine but the program isn't responsive to any command. I can't close it by push esc or clicking on the "X" to close the window. My last option is to break the program by "ctrl-alt-del".

Does anybody has a hint or know where the problem lie?
I would be very excited for any help.
Thanks in advance and kind regards,
Saul

Thats my whole code:

Code:

---

// main.cpp
 
#include "webcam.h"
#include <QtGui/QApplication>
 
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Webcam w;
w.show();
return a.exec();
}

---

Code:

---

// webcam.h
 
#ifndef WEBCAM_H
#define WEBCAM_H
 
#include <QtGui/QMainWindow>
#include "ui_webcam.h"
#include "cv.h"
#include "highgui.h"
 
class Webcam : public QMainWindow
{
	Q_OBJECT
 
public:
	Webcam(QWidget *parent = 0, Qt::WFlags flags = 0);
	~Webcam();
 
private:
	Ui::WebcamClass ui;
 
public slots:
	void showWebcam();
};
 
#endif // WEBCAM_H

---

Code:

---

// webcam.cpp
 
#include "webcam.h"
 
Webcam::Webcam(QWidget *parent, Qt::WFlags flags) : QMainWindow(parent, flags)
{
	ui.setupUi(this);
 
	connect(ui.actionStart, SIGNAL(triggered()), this, SLOT(showWebcam()));
}
 
Webcam::~Webcam() { }
 
void Webcam::showWebcam()
{
	IplImage *image;
	CvCapture *capture;
	capture = cvCaptureFromCAM(0);
 
	int esc = 0;
	while(true)
	{
		image = cvQueryFrame(capture);
 
		cvCvtColor(image, image,CV_BGR2RGB);
		QImage qimg((const uchar *)image->imageData, image->width, image->height, QImage::Format_RGB888);
		ui.label->setPixmap(QPixmap::fromImage(qimg));
		ui.label->repaint();
 
		esc = cvWaitKey(10);
		if (esc == 27) break;
	}
	cvReleaseCapture(&capture);
}

---

In your showWebcam() method, you never give the Qt event loop the chance to process any other events, so that is why your GUI is unresponsive. You need to insert a call to QCoreApplication:: processEvents() somewhere in your loop.

I am guessing that is the reason why you call label->repaint() instead of label->update() - since update() puts a paint event on the event stack and your loop never lets the stack get processed, the label never gets painted.

After adding the QCoreApplication:: processEvents() call, change the label->repaint() to a label->update() and it should all work fine (and make the label a "good player" in the Qt world again).

Hi d_stranz,
thank you very much for your response! I changed my code as you wrote. Hope it is correct now. At least it works and I'am really happy =)
Cheers,
Saul

By the way: the program will not execute the break command if I press ESC.

Code:

---

void Webcam::showWebcam()
{
	IplImage *image;
	CvCapture *capture;
	capture = cvCaptureFromCAM(0);
 
	int esc = 0;
	while(true)
	{
		image = cvQueryFrame(capture);
 
		cvCvtColor(image, image,CV_BGR2RGB);
		QImage qimg((const uchar *)image->imageData, image->width, image->height, QImage::Format_RGB888);
		ui.label->setPixmap(QPixmap::fromImage(qimg));
		ui.label->update();
 
		QCoreApplication::processEvents();
 
		esc = cvWaitKey(10);
		if (esc == 27) break;
	}
	cvReleaseCapture(&capture);
}

---

Hi, its me again.
I just want to ask, if it's conform to use QCoreApplication:: processEvents(); directly in my loop? Or is it unsuitable to use it this way?
I'm a little bit self-conscious in this way, yet. Would be very thankful for a "Yes" or a "NO" =)

Are there any other possibilities to realize a webcam output?
Ok, the first one I tried is the QLabel. Think it could be done with Phonon::VideoPlayer, too.
Are there even more possibilities?
Kind regards,
Saul

It is OK to use QCoreApplication:: processEvents() in a loop.

On the other hand, if you are forced to do that in order to get a responsive application, it is generally a sign that your application design could be improved.

Usually, it is better to try to implement a signal / slot or multithreaded design so that compute-intensive work gets done in a different thread from the GUI, and communicates to the GUI thread using signals (something like "frameReady").

You might be able to use Phonon, but I am not familiar with it so someone else will have to answer that question.

OK, thanks again d_stranz!
I will do my best. Bought a Qt book and found a chapter about threads yesterday. Armed and ready for the fight now :D

But I have just one last Question and Problem. If I close the Program the webcam is still running. My function to break the loop by push the button ESC does not work.

I've posted a simple Qt + OpenCv example here, you may want to check it out. You don't need any separate threads or processEvents() calls to get responsive ui during webcam capture and simple image processing, just use QTimer.