How to Update Qt ui to dispaly opencv mat from a camera;

Question

I want to use QLabel class to display every frame(an OpenCV Mat) from a camera,but update() cannot display images continuously, so I use repain(); but here a problem, my ui cannot move and other button can not click, so if I want to display a video, how can I do; use opencv and qt and vs; Thanks in advance!

Answer 1

Just move your video capturing cycle into another thread and send frames to gui thread with signal/slot system.

Thread

class VideoThread : public QThread
{
    Q_OBJECT
public:
    VideoThread(QObject *parent = nullptr);

protected:
    void run();

signals:
    void frameCaptured(cv::Mat frame);
};

void VideoThread::run()
{
    VideoCapture cap(0);

    if(!cap.isOpened()){
        qDebug() << "Cant capture video";
        return ;
      }

      while(1){

        Mat frame;
        cap >> frame;

        if (frame.empty()) {
            qDebug() << "Empty frame";
            break;
        }

        emit frameCaptured(frame);
      }
}

Using

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    connect(&m_videoThread, &VideoThread::frameCaptured,
            this, &MainWindow::OnFrameCaptured);
}

void MainWindow::OnFrameCaptured(const cv::Mat &frame)
{
    QImage imgIn= QImage((uchar*) frame.data, frame.cols, frame.rows, frame.step, QImage::Format_RGB888);
    ui->lblVideo->setPixmap(QPixmap::fromImage(imgIn));
}

Don't forget register metatype

qRegisterMetaType<cv::Mat>("cv::Mat");

and run thread

m_videoThread.start();