QT C++ How to return QImage from QThread and display it on QLabel in QMainWindow?

Question

QT C++ How to return QImage from QThread and display it on QLabel in QMainWindow?

I am trying to process some image using OpenCV Library inside a thread, because the process operation takes some time before it's done.

So the problem is QThread always returning a Null QImage to the Slot in QMainWindow.

I get this Exception error:

Exception thrown at 0x00007FFE01962F6D (Qt5Guid.dll) in QtWidgetsApplication1.exe: 0xC0000005: Access violation reading location 0x0000022CAB6EE080.

Error occurs in this file:

qtwidgetsapplication1.cpp this file is for QMainWindow

#include "qtwidgetsapplication1.h"
#include "stdafx.h"


QtWidgetsApplication1::QtWidgetsApplication1(QWidget *parent)
    : QMainWindow(parent)
{
    ui.setupUi(this); 
    
    connect(ui.addItem_btn, SIGNAL(clicked()), this, SLOT(addItem_btn_OnClick())); // Add Item to the List
    
    dftThread = new DetectFaceThread(this);

    connect(dftThread, &DetectFaceThread::detectedFace, this, &QtWidgetsApplication1::onDetectedFace);
    connect(dftThread, &DetectFaceThread::finished, dftThread, &QObject::deleteLater);
}

QtWidgetsApplication1::~QtWidgetsApplication1()
{
    
}

void QtWidgetsApplication1::addItem_btn_OnClick()
{
    dftThread->start();
}

void QtWidgetsApplication1::onDetectedFace(const QImage& face)
{
    
    if (face.isNull())
    {
        QMessageBox::warning(this, QString("Detection Error"), QString("Face not detected!"));
        return;
    }
    ui.imgDisplay_label->setPixmap(QPixmap::fromImage(face));
}

Here is my Code:

DetectFaceThread.h

#pragma once
#include <qthread.h>
#include <QtWidgets/qmessagebox.h>
#include <qmutex.h>
#include <opencv2/opencv.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/objdetect.hpp>


class DetectFaceThread :
    public QThread
{
    Q_OBJECT

public:
    DetectFaceThread(QWidget* parent = nullptr);
    ~DetectFaceThread();
    void run() override;

signals:
    void detectedFace(const QImage &face);
};

DetectFaceThread.cpp

#include "DetectFaceThread.h"

DetectFaceThread::DetectFaceThread(QWidget* parent)
{

}

DetectFaceThread::~DetectFaceThread()
{
    QMessageBox::information(nullptr, QString("Thread Info"), QString("Thread successfully destroyed"));
}

void DetectFaceThread::run()
{
    QMutex mutex;

    mutex.lock();

    std::string img_path = "res/paper.jpg";
    cv::Mat img = cv::imread(img_path);

    if (img.empty())
    {
        QMessageBox::warning(nullptr, QString("Load Error"), QString("Image not found!"));
        return;
    }

    cv::cvtColor(img, img, cv::ColorConversionCodes::COLOR_BGR2RGB);

    float w = 800, h = 1000;

    cv::Point2f src[4] = { {383, 445}, {885, 521}, {89, 1125}, {921, 1270} };
    cv::Point2f dst[4] = { {0.0f, 0.0f}, {w, 0.0f}, {0.0f, h}, {w, h} };

    cv::Mat matrix = getPerspectiveTransform(src, dst);
    cv::Mat img_warp;
    cv::warpPerspective(img, img_warp, matrix, cv::Size(w, h));

    QImage qimage(img_warp.data, img_warp.cols, img_warp.rows, img_warp.step, QImage::Format::Format_RGB888);

    mutex.unlock();

    emit detectedFace(qimage);
}

And in the end the application just terminates, can any one help me please.

Update: I tried your solution but it throws the same Exception error.

connect(dftThread, &DetectFaceThread::detectedFace, this, &QtWidgetsApplication1::onDetectedFace, Qt::QueuedConnection);

c++

visual-studio

qt

asked on Stack Overflow Jan 21, 2021 by

halocedark • edited Jan 21, 2021 by

halocedark

3 Answers

The main problem is that the thread affinity of a QThread is the thread on which it is currently handling events -- usually the thread on which it was created, not the thread it manages and on which its run member function will execute. So, with...

connect(dftThread, &DetectFaceThread::detectedFace, this, &QtWidgetsApplication1::onDetectedFace);

both dftThread and this have the same thread affinity meaning you effectively have a direct connection between code running on two different threads without any synchronization -- that's undefined behaviour. Change the above to..

connect(dftThread, &DetectFaceThread::detectedFace,
        this, &QtWidgetsApplication1::onDetectedFace,
        Qt::QueuedConnection);

Other issues. In DetectFaceThread::run you have...

if (img.empty()) {
    QMessageBox::warning(nullptr, QString("Load Error"), QString("Image not found!"));
    return;
}

This will create a GUI element (a QMessageBox) on a non-GUI thread. That's not supported.

Also, the use of a QMutex scoped locally within DetectFaceThread::run doesn't make any sense. What is it supposed to protect -- it's not visible to any code other than DetectFaceThread::run?

Edit: One further issue is that the QImage constructor you're using doesn't make a deep copy of the data passed. Try changing...

emit detectedFace(qimage);

to...

emit detectedFace(qimage.copy());

That should force a deep copy of the original to be passed to the slot.

answered on Stack Overflow Jan 21, 2021 by

G.M. • edited Jan 21, 2021 by

G.M.

I think the problem is in creating an object image in the DetectFaceThread thread. When the run () function is executed, all data in run () will be destroyed. I suggest creating an object image in the main thread and performing image processing in DetectFace. And use movetothread, but not the run override method.

working example

 ---------------MainWindow.h-------------
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <opencv2/opencv.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/objdetect.hpp>
#include <QThread>
#include <detectface.h>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
    QThread thread;
    DetectFace detect;
private:
    Ui::MainWindow *ui;
public slots:
    void showResult();
private slots:
    void on_pushButton_clicked();
};
#endif // MAINWINDOW_H


 ---------------MainWindow.cpp-------------
#include "mainwindow.h"
#include "ui_mainwindow.h"

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

    connect(&thread,&QThread::started,&detect,&DetectFace::detectFace);
    connect(&detect,&DetectFace::endDetectFace,&thread, &QThread::quit);
    connect(&thread, &QThread::finished, this, &MainWindow::showResult);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::showResult()
{
    cv::Mat *image = detect.getImage();
    QImage qimage = QImage((uchar*) image->data, image->cols, image->rows, image->step, QImage::Format_RGB888);
    ui->label->setPixmap(QPixmap::fromImage(qimage,Qt::AutoColor));
}


void MainWindow::on_pushButton_clicked()
{
    while(thread.isRunning())
    {
        thread.quit();
    }
    detect.moveToThread(&thread); //transfer object img in thread
    thread.start();

}

---------------DetectFace.h--------------
#ifndef DETECTFACE_H
#define DETECTFACE_H

#include <QObject>
#include <opencv2/opencv.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/objdetect.hpp>
#include <QMessageBox>
class DetectFace : public QObject
{
    Q_OBJECT
    cv::Mat img;
    cv::Mat img_warp;
    std::string img_path ;
public:

    DetectFace();
    cv::Mat* getImage();
    void detectFace();
signals:
    void endDetectFace();
};

#endif // DETECTFACE_H

---------------DetectFace.cpp--------------
#include "detectface.h"

DetectFace::DetectFace()
{
    img_path = "res/paper.jpg";
}

cv::Mat *DetectFace::getImage()
{
    return &img_warp;
}

void DetectFace::detectFace()
{

     img = cv::imread(img_path);

     if (img.empty())
     {
         QMessageBox::warning(nullptr, QString("Load Error"), QString("Image not found!"));
         return;
     }

     cv::cvtColor(img, img, cv::ColorConversionCodes::COLOR_BGR2RGB);

     float w = 800, h = 1000;

     cv::Point2f src[4] = { {383, 445}, {885, 521}, {89, 1125}, {921, 1270} };
     cv::Point2f dst[4] = { {0.0f, 0.0f}, {w, 0.0f}, {0.0f, h}, {w, h} };

     cv::Mat matrix = cv::getPerspectiveTransform(src, dst);
     cv::warpPerspective(img, img_warp, matrix, cv::Size(w, h));

     emit endDetectFace();

}

-----------------main cpp------------
#include "mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

answered on Stack Overflow Jan 21, 2021 by

Ruslan • edited Jan 22, 2021 by

Ruslan

Please read what the QImage ctor tells you: The buffer must remain valid throughout the life of the QImage

Your data goes out-of-scope before.

answered on Stack Overflow Jan 21, 2021 by

chehrlic

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