QT - View Webcam on a Qlabel using another Thread?

Question

Hello, I’m doing a C++ project using QT GUI. In the project in question I need to display images of a camera in the window, but in doing so, the performance of the window is very compromised, all other buttons and features that includes in the window are slow, overloaded apparently. Below is the capture code of the device images.

  void mainwindow::on_ButtonShowCamera_clicked(){

ligaCam = true;

if(!this->cap.isOpened()){ // se ja estiver aberto, nao abre denovo //
    this->cap.open(0);
}

QImage img;

while(ligaCam == true){
    Mat frame;
    this->cap.read(frame);

    img = MatToQImage(frame); // converte tipo Mat para QImage //
    ui->labelScreen->setPixmap(QPixmap::fromImage(img));
    ui->labelScreen->setScaledContents(true);

    qApp->processEvents(); 
   //imshow("help", frame);

    if (waitKey(30) >= 0)
        break;
}

}

The link variable is in charge of changing the value when I click another button (Close Camera) to exit the loop.

My question is, is there any way to display camera images on Qlabel using another thread? Or if there is any way to display the images without overloading the window in question? Thanks for your help.

Answer 1

The graphical interface seems locked because you are consuming all the resources of your thread (the main one) when making a loop virtually "infinite".

You can try moving the update code to another thread, but it’s simpler to use a timer to make periodic captures of the webcam image:

1. In the window class where you display the camera image (for example, it may be in another class depending on your organization), please QTimer. Set it a desired refresh interval (for example, every 100 milliseconds), and start. Don’t forget to connect to the slot timeout().

2. In the call of the method connected to the slot timeout() (which will run at each set interval period), read the webcam image and update the QLabel the same way you’re doing.

Code example:

[...]

<Classe>::<Classe>() { // Construtor (por exemplo)
    QTimer *pTimer = new QTimer(this);
    QObject::connect(pTimer, SIGNAL(timeout()), this, SLOT(onTimeout()));
    pTimer->start(100);
}

[...]

<Classe>::onTimeout() {
    Mat frame;
    this->cap.read(frame);

    img = MatToQImage(frame); // converte tipo Mat para QImage //
    ui->labelScreen->setPixmap(QPixmap::fromImage(img));
    ui->labelScreen->setScaledContents(true);
}

A more complete example using this approach can be found in this my test project.