Unable to send signal from mainwindow to working thread

magicstar

I wish to display videos using OpenCV using multithreading. I have followed [this link] and results are as expected. Here is code snippet.

class Worker : public QObject
{
    Q_OBJECT
public:
    Worker(QString path, int id);
    ~Worker();

public slots:
    void readVideo(QString path = "");
    void get_from_main(QString path);

signals:
    // frame and index of label which frame will be displayed
    void frameFinished(cv::Mat frame, int index);

    void finished(int index);

private:
    QString filepath;
    int index;
};

//worker.cpp
#include "worker.h"
#include <QDebug>
#include <QThread>
#include <QTime>
Worker::Worker(QString path, int id) : filepath(path), index(id)
{

}

Worker::~Worker()
{
}

void Worker::get_from_main(QString path)
{
      qDebug() << "updating";
}

void Worker::readVideo(QString path)
{
    if (path.length() > 0)
        filepath = path;

    cv::VideoCapture cap(filepath.toStdString());

    if (! cap.isOpened())
    {
        qDebug() << "Can't open video file " << filepath;
        emit finished(index);
        return;
    }

    cv::Mat frame;
    while (true)
    {
        cap >> frame;
        if (frame.empty())
        {
            frame = cv::Mat(cv::Size(720, 576), CV_8UC3, cv::Scalar(192, 0, 0));
            emit frameFinished(frame, index);
            break;
        }


        emit frameFinished(frame.clone(), index);
        QThread::msleep(30);
    }

    emit finished(index);
}

//mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <opencv2/opencv.hpp>

#include "worker.h"

#define MAX_NUM_CAM 8

namespace Ui {
class MainWindow;
}

class QThread;
class QLabel;

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

    void init();

private slots:
    void displayFrame(cv::Mat frame, int index);
    void file_open_clicked();

signals:
send_to_worker(QString path);


private:
    Ui::MainWindow *ui;

    int numCams;
    QLabel *labels[MAX_NUM_CAM];
    QThread* threads[MAX_NUM_CAM];
    Worker* workers[MAX_NUM_CAM];
};

#endif // MAINWINDOW_H


//mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QDebug>
#include <QThread>
#include <QLabel>
#include <QGridLayout>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    qRegisterMetaType< cv::Mat >("cv::Mat");

    qDebug() << "Main thread " << QThread::currentThreadId();
    init();
}

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

void MainWindow::init()
{
    QGridLayout *grid = new QGridLayout;
    int numCols = 2;

    numCams = 4;

    int row = 0, col = 0;
    for (int i = 0; i < numCams; i++)
    {
        labels[i] = new QLabel;

        row = i / numCols;
        col = i % numCols;
        grid->addWidget(labels[i], row, col);


        threads[i] = new QThread;
        workers[i] = new Worker(QString("/home/shang/Videos/%1.mp4").arg(i+1), i);
        workers[i]->moveToThread(threads[i]);

        connect(workers[i], SIGNAL(frameFinished(cv::Mat, int)), this, SLOT(displayFrame(cv::Mat,int)));
        connect(threads[i], SIGNAL(started()), workers[i], SLOT(readVideo()));

        connect(workers[i], SIGNAL(finished(int)), threads[i], SLOT(quit()));
        connect(workers[i], SIGNAL(finished(int)), workers[i], SLOT(deleteLater()));

        connect(threads[i], SIGNAL(finished()), threads[i], SLOT(deleteLater()));

        threads[i]->start();
    }

    this->centralWidget()->setLayout(grid);

}

void MainWindow::file_open_clicked(){
       QString Path = QFileDialog::getSaveFileName( this,tr("OpenVideo"),"","Video (*.avi)");
    if(Path.isEmpty())
        return;
     view =3; 
    connect(this, SIGNAL(send_to_worker(QString)),workers[view], SLOT(get_from_main(QString)));
    emit this->send_to_worker(recorder_Path);
}

void MainWindow::displayFrame(cv::Mat frame, int index)
{
    QPixmap p = QPixmap::fromImage(QImage(frame.data, frame.cols, frame.rows, frame.step, QImage::Format_RGB888).rgbSwapped());
    p = p.scaled(QSize(frame.cols/2, frame.rows/2));
    labels[index]->setPixmap(p);
}

However, slot get_from_main is never triggered. May I please know how to correctly implement QThreading.

(https://stackoverflow.com/questions/29338250/qt-opencv-play-videos-with-stdthread)

mrjj

Hi
Code does look valid.
Did you check the connect statements that they all return TRUE ?

magicstar

@mrjj
Thanks for the reply.

How do I check if they return true?
I am indeed able to view video. only facing problem with
connect(this, SIGNAL(send_to_worker(QString)),workers[view], SLOT(get_from_main(QString)));

Christian Ehrlicher

The slot can't be called since the thread never reaches it's eventloop. So no chance to retrieve the signal and pass it to your slot.

magicstar

@Christian-Ehrlicher ,
can you explain it in detail. Also, is there any other way to modify private variable "path" of worker class

Christian Ehrlicher

What detail? As I said an eventloop is needed to deliver signals to slots. You don't run an eventloop in your thread and therefore the slot gets no called. See also http://doc.qt.io/qt-5/qthread.html#details
It's not forbidden to call a function of an object in another thread directly. You must only take care that this access is synchronized e.g. with a semaphore. See http://doc.qt.io/qt-5/threads-synchronizing.html for more information.

JKSH

@magicstar said in Unable to send signal from mainwindow to working thread:

while (true)

This is an infinite loop. It causes Worker::readVideo() to block forever and never return. When the function never returns, the event loop never gets the chance to process any more signals. (In other words, a thread that is running an infinite loop cannot run any slots).

A thread can only have run an infinite loop, or it can only run an event loop. It cannot run both.

magicstar

@JKSH , @Christian-Ehrlicher , Thanks