QT OpenCV Play Pause and Stop Video

Stevendragoes

cShowVideo::cShowVideo(Ui::MainWindow& ui, std::string& path, QString& qpath, MainWindow &parent) : MainWindow()
{
    timer = new QTimer(this);
    connect(timer,SIGNAL(timeout()),this,SLOT(getNextFrame()));
    //ui.textEdit->setText(qpath);
    timer->start(1000);
    DisplayVideo(ui,path,parent);

this is my timer code. It starts properly.
when i used qDebug, it shows the text in 1 second which means that the Timer function is working?
However, when I change it to the text edit, as shown below, the text is not appearing as intended
The code printing is as follows:

void MainWindow::getNextFrame()
{
    qDebug("Get Next Frame is working");
    ui->textEdit->append("GetNextFrame");
}

To show its working, the "Get Next Frame is working" is printing every second

Stevendragoes

@jsulm Sorry forgot to tag you

jsulm

@Stevendragoes Do you still have the loop in DisplayVideo?

Stevendragoes

@jsulm Yes I do have the loop, Should I take it out first?

jsulm

@Stevendragoes said in QT OpenCV Play Pause and Stop Video:

Should I take it out first?

Of course. As I said before this loop is blocking the event loop, so UI does not work properly.

Stevendragoes

@jsulm When I commented out the loop, the timer doesn't run anymore.

jsulm

@Stevendragoes Please comment out DisplayVideo call in cShowVideo

Stevendragoes

I have commented it out also.
However, what I want is that when I load the video, the timer to start. So, I initialised the timer in the cShowvideo which is a class inherited from the MainWindow().
Is that an incorrect way of doing so?

jsulm

@Stevendragoes As I said already: in the slot connected to the timer you get next frame and show it. You need to implement this to make it work.

Stevendragoes

Yeap. I think I figured it out.
I shifted the initialisation of the timer in the MainWindow.

MainWindow::MainWindow(QWidget *parent): QMainWindow(parent), ui(new Ui::MainWindow)
{
    timer = new QTimer(this);
    connect(timer,SIGNAL(timeout()),this,SLOT(getNextFrame()));
    ui->setupUi(this);
}

and I used the cShowVideo Class to start the timer when I load the video from the FileDialog

cShowVideo::cShowVideo(Ui::MainWindow& ui, std::string& path, QString& qpath, MainWindow &parent) : MainWindow()
{
    parent.timer->start(1000);
    ui.textEdit->append(qpath);
}

and it works, it is appending every second.

Stevendragoes

@jsulm
From what I saw in the QTimer Class, there is no such thing as pause the timer?

jsulm

@Stevendragoes This is not good design - child should not access parent members directly. And this timer does not belong to MainWindow but to cShowVideo. Move timer again to cShowVideo.
I just noticed that cShowVideo is a local variable in on_actionLoad_Video_triggered(). That means it is destroyed as soon as on_actionLoad_Video_triggered() finishes - that's why your timer is not working. Make sure cShowVideo instance lives as long as it is playing video.
Also another note regarding design: your cShowVideo should not know anything about the MainWindow UI. This issue is called tight coupling and makes the maintenance of your software harder. In cShowVideo you should only get next frame and send it to MainWindow via a slot, MainWindow knows better how to use its UI to show the frames.

Stevendragoes

Hello @jsulm
I have FINALLY made it work already. Thank you for your guidance, it was good to drop the cShowVideo Completely. This is the following codes. However, there are a few issues when I try to close the program

Loading the Video and Starting the Timer so that it gets the next frame.

void MainWindow::on_actionLoad_Video_triggered()
{
    QMessageBox::StandardButton loadBtn = QMessageBox::question( this, "MainWindow",tr("Load Video? \n"), QMessageBox::No | QMessageBox::Yes,QMessageBox::Yes);
    if (loadBtn == QMessageBox::Yes)
    {
        qpath = QFileDialog::getOpenFileName(this, "Load Video File"); //Input code to skip the event when cancel is pressed
        if (qpath.isNull())
        {
            ui->textEdit->append("Cannot get file, user cancelled while choosing file");
        }
        else
        {
            path = qpath.toLocal8Bit().constData();\
            ui->textEdit->append("cShowVideo is called");
            //cShowVideo(*ui, path, qpath, *this);                        //For OPENCV analysis and filtering
            cap.open(path);
            vwidth = ui->display_image->height();
            vheight = ui->display_image->width();
            timer->start(30);                                           //this can set the frame rate
            cap >> frame;
        }
    }
    else
    {
        ui->textEdit->setText("User Cancelled");
    }
}

This is the getNextFrame() function to get the next frame after every timeout() event from the timer

void MainWindow::getNextFrame()
{
    cap.read(frame);
    if(frame.empty()==true) return;
    cv::cvtColor(frame,  frame, cv::COLOR_BGR2RGB);
    cv::resize(frame, frame, cv::Size(vheight,vwidth), 0, 0, cv::INTER_AREA);
    QImage imdisplay((uchar*)frame.data, frame.cols, frame.rows, frame.step, QImage::Format_RGB888);
    ui->display_image->setPixmap(QPixmap::fromImage(imdisplay));
}

I have also implemented 3 pushbuttons to play, pause and stop

void MainWindow::on_Play_clicked()
{
    if(timer->isActive() == true){
        return;
    }
    else {
        timer->start(30);
    }
}

void MainWindow::on_Pause_clicked()
{
    if(timer->isActive() == true)
    {
        timer->stop();
    }
    else{
        return;
    }
}

void MainWindow::on_Stop_clicked()
{
    if(timer->isActive()==true)
    {
        timer->stop();
        cap.release();
    }
}

However, when I try to close it, the program finishes unexpectedly and crashed unexpectedly.
This is the screenshot when i try to use the debugger to trace where it went wrong.

Stevendragoes

maybe @J.Hilk could help me?

jsulm

@Stevendragoes Can you show the destructor of your MainWindow?
Also, when closing your app you should first stop the playback if active.

Stevendragoes

@jsulm this is my destructor

MainWindow::~MainWindow()
{
    cap.release();
    delete ui;
}

Stevendragoes

@jsulm I have also attached the close event captured for reference

void MainWindow::closeEvent(QCloseEvent *event)    //To confirm that the user really wants to quit the application
{
    QMessageBox::StandardButton exitBtn = QMessageBox::question( this, "MainWindow",tr("Are you sure you want to exit?\n"), QMessageBox::Cancel | QMessageBox::No | QMessageBox::Yes,QMessageBox::Yes);
    if (exitBtn == QMessageBox::Yes) {
        event->accept();
    } else {
        event->ignore();
    }
}

jsulm

@Stevendragoes I would stop the timer in closeEvent if it is running and user wants to quit.

Stevendragoes

Hi @jsulm I have implemented timer->stop() but it still gives me the same error. I am not too sure why.

Stevendragoes

Hi @jsulm
I think i have solved the problem

w.setAttribute(Qt::WA_DeleteOnClose, true);

this was inside the main.cpp. I am not sure why this happens.