How to display QPaint Images from other Threads

[vijayQt]

Hi,
I am using 2 threads in my program. 1 is GUI thread and other one is created from main application. I am getting frames in other thread and painting and displaying Images in main thread.

After getting frames from other thread I am calling main thread paint function to paint on the widget. Here I am facing problem. I am getting 3 errors

1. "QPixmap: It is not safe to use pixmaps outside the GUI thread"
2. "QPainter::begin: Paint device returned engine == 0, type: 2"
3. "QPainter::end: Painter not active, aborted"

Here is my sudo code

connect(thr, SIGNAL(sDisplayOnWidget()), this, SLOT(convertImg2Pix()))

void frameThread::run() {
mPlayer.loadVideo("/tmp/stream.bin");
while(1) {
mPlayer.loadVideoGetFrame();
printf("we r in thread func \n");
emit sDisplayOnWidget();
}
}

convertImg2Pix() {
p = QPixmap(img.size());
QPainter painter;
painter.begin(&p);
painter.drawImage(0,0,img);
painter.end();
mLabelWidget->setPixmap(p);
}

please help me to get out of this problem.

Thanks in advance...

[stampede]

QPixmap belongs to QtGui module, it's not safe to use it in threads other than gui. If you need to draw in other thread, use QImage and then send the result for display to gui thread.

[vijayQt]

Thanks for your quick reply... I am doing QPixmap in main thread only.

in my above code "convertImg2Pix()" function is in gui thread.

[stampede]

Are you sure about that ? This warning says different.
Add this debug code in both methods:

qDebug() << /*method name here*/ << QThread::currentThread();

qDebug() << /*method name here*/ << QThread::currentThread();

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in my above code "convertImg2Pix()" function is in gui thread

If it is, then maybe you are using QPixmaps somewhere else in your second thread code.

[vijayQt]

Its not calling SLOT . Its always in the while loop and printing non-gui thread id .

Non GUI thread frameThread(0x9ef78a8)

why its not calling SLOT function?? Please guide me.

[stampede]

It's some kind of video player, so I think you should consider the video file fps, setup a QTimer in order to query frames in regular intervals and send them to display.
Now it looks like you want to play the video as fast as possible, it's definitely not the way to go.
Something like this (a pseudo-code):

void VideoThread::run(){
    player.loadVideo("video file path");
    //...
    if( fps > 0 ){ // video fps
        timer.setInterval( 1000.0 / fps ); //
        connect(timer, SIGNAL(timeout()), this, SLOT(grabFrame()));
        timer.start();
    } else{
    // some kind of error handling
    }
}
 
void VideoThread::grabFrame(){
    QImage img = player.getFrame();
    emit newFrame(img); // signal connected to some object from the gui thread, with Qt::QueuedConnection
}
 
//...
 
void MainGui::frameCallback( const QImage& img ){
    qDebug() << "got frame";
    QPixmap p = /*img to pixmap*/;
    displayWidget->setPixmap(p);
}

void VideoThread::run(){
    player.loadVideo("video file path");
    //...
    if( fps > 0 ){ // video fps
        timer.setInterval( 1000.0 / fps ); //
        connect(timer, SIGNAL(timeout()), this, SLOT(grabFrame()));
        timer.start();
    } else{
    // some kind of error handling
    }
}

void VideoThread::grabFrame(){
    QImage img = player.getFrame();
    emit newFrame(img); // signal connected to some object from the gui thread, with Qt::QueuedConnection
}

//...

void MainGui::frameCallback( const QImage& img ){
    qDebug() << "got frame";
    QPixmap p = /*img to pixmap*/;
    displayWidget->setPixmap(p);
}

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I think you can work on the details yourself.

[joyer83]

Do your main thread have event loop running? It will not call the convertImg2Pix() slot if it is not running.

[vijayQt]

Your guess is correct... I am working on video frames displaying on widget.

void VideoThread::grabFrame(){
QImage img = player.getFrame();
emit newFrame(img); // signal connected to some object from the gui thread, with Qt::QueuedConnection
}

I can not emit from here because this peace of code is belong to MainGUI. What I am doing here is

while(1) {
gPlayer->nextFrame();
QImage img = gPlayer->displayFrame();
emit sDisplayOnWidget(img);
qDebug() << "Non GUI thread" << QThread::currentThread();
msleep(400);
}

but it's not emiting signal. what I am doing is wrong way or am I missing some thing??

what is event loop??? I am using connect call to emit signal from other thread and slot to render the image.
I don't know what it is??... Please can you give sudo code to run event loop.

[joyer83]

The event loop is what calls the slots. Read Qt's documentation to find out more.
The main event loop is started with QCoreApplication::exec();

Try running your application inside a debugger, pause execution and check if main thread's call stack contains the exec() call.

EDIT: Also do check that the object which contains the convertImg2Pix()-slot is living in main thread. You can get the object's thread with method QObject::thread().

You should also check that the connect() call successes, like this:

bool ok = connect( ... );
Q_ASSERT( ok );

bool ok = connect( ... );
Q_ASSERT( ok );

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[stampede]

If you can use gui, you can be sure there is an event loop running.

I can not emit from here because this peace of code is belong to MainGUI

This piece of code does not belong to anything, it's just a pseudo-code, you've asked about advice and here it is - I suggest you to setup a timer to grab frames. Don't forget to call "exec()" in your thread's run() method as well.

[vijayQt]

Hi Stampede,
I am talking about my code, not your pseudo-code. In comment#8 I pasted my code and that is from MainGUI.

void thread::run() {
while(1) {
gPlayer->nextFrame(); // This function belong to MainGUI thread
QImage img = gPlayer->displayFrame(); // This function belong to Main GUI thread
emit sDisplayOnWidget(img); // from here I am calling SLOT function which is there in again MainGUI thread
qDebug() << "Non GUI thread" << QThread::currentThread();
msleep(400);
}
}

Please let me know whether this is correct way of doing or not???

If it is correct it's not uploading frame on the Label Widget...