How to register OpenCV Mat data type in the communication between Qt signal and slot－天天向上

First, set up OpenCV building environments

INCLUDEPATH += C:\OpenCV\OpenCV4.2.0DNN\includeLIBS += -LC:\OpenCV\OpenCV4.2.0DNN\lib \-lopencv_world420

Since Qt doesn’t recognise Mat data type that we have to register it

before we start using signal and slot mechanics.

Syntax: qRegisterMetaType<T>("typeName");

qRegisterMetaType helps us to register a new typeName called “cv::Mat&”

and its Type is corresponding to <T>, ie. <cv::Mat>

Besides, followed by a while loop, we notify ui to receive two inputs,

including frame and m_counter. The observer(ui) is notified by a singalFrameGrabbed

whenever there are new incoming frame and counter, both of which are emitted by m_capImg.

mainwindow.h

#ifndef MAINWINDOW_H#define MAINWINDOW_H #include <QMainWindow>#include <captureimage.h>#include <QtConcurrent/QtConcurrent>#include <QCloseEvent>#include <QMessageBox> QT_BEGIN_NAMESPACEnamespace Ui { class MainWindow; }QT_END_NAMESPACE class MainWindow : public QMainWindow{    Q_OBJECT public:    MainWindow(QWidget *parent = nullptr);    ~MainWindow(); private:    Ui::MainWindow *ui;    CaptureImage *m_capImg;    void StartCamera(); public slots:    void slotFrameGrabbed(cv::Mat& frame, int counter); private slots:    void on_pushButton_start_clicked();    void on_pushButton_stop_clicked();    void closeEvent(QCloseEvent *event);};#endif // MAINWINDOW_H

mainwindow.cpp

The above shows connection between signalFrameGrabbed and slotFrameGrabbed is established.

Besides we want to run StartCamera in a sperate thread by using QtConcurrent::run().

#include "mainwindow.h"#include "mainwindow.h"#include "ui_mainwindow.h" //Q_DECLARE_METATYPE(cv::Mat)MainWindow::MainWindow(QWidget *parent)    : QMainWindow(parent)    , ui(new Ui::MainWindow){     ui->setupUi(this); } MainWindow::~MainWindow(){    delete ui;}  void MainWindow::slotFrameGrabbed(cv::Mat& frame, int counter){    auto w = ui->label_image->width();    auto h = ui->label_image->height();    ui->label_image->setPixmap(       QPixmap::fromImage(       QImage(frame.data, frame.cols, frame.rows, frame.step, QImage::Format_BGR888)       ).scaled(w, h, Qt::KeepAspectRatio)    );    ui->label_counter->setText(QString::number(counter));}    void MainWindow::on_pushButton_start_clicked(){     //m_capImg->Start();    m_capImg = new CaptureImage();    connect(m_capImg, SIGNAL(signalFrameGrabbed(cv::Mat&, int)), this, SLOT(slotFrameGrabbed(cv::Mat&, int)));    QtConcurrent::run(this, &MainWindow::StartCamera); }void MainWindow::StartCamera(){     m_capImg->Start();} void MainWindow::on_pushButton_stop_clicked(){    m_capImg->Stop();} void MainWindow::closeEvent (QCloseEvent *event){    QMessageBox::StandardButton resBtn =            QMessageBox::question( this, "CaptureApp",                                   tr("Are you sure?\n"),                                   QMessageBox::Cancel | QMessageBox::No | QMessageBox::Yes, QMessageBox::Yes);       if (resBtn != QMessageBox::Yes) {           event->ignore();       } else {           event->accept();           if(m_capImg!=nullptr)           {               m_capImg->Stop();               delete m_capImg;           }       } } 

You can drag the windows form to any place and the picture keeps refreshing, because the frame acquistion of m_capImg is running in a separate thread.