QCameraImageCapture moveToThread error

nefarious

I am sort of new to Qt and am trying to sort out how to work with the QCamera and QCameraImageCapture technology.

I have the QCamera and QCameraImageCapture in a thread with no ui. The invocation of the thread is:

QCameraImageCapture theImageCapture = new ImageCapture();
QThread* theImageThread = new QThread();
theImageCapture->moveToThread(theImageThread);

connect(theImageThread, SIGNAL(started()), theImageCapture, SLOT(process()));
connect(theImageCapture, SIGNAL(finished()), theImageThread, SLOT(quit()));

connect(theImageThread, SIGNAL(finished()), theImageThread, SLOT(deleteLater()));
theImageThread->start();

The imageCapture instance instantiates a QCamera and a QCameraImageCapture. All of this code is taken from the camera demo.

the captureImage process method is:

void ImageCapture::process()
{
    inProcess = true;
    isRunning = true;

    while (inProcess)
    {
        sync.lock();
        inProcess = isRunning;
        sync.unlock();

        if (isReadyForCapture){
            toggleLock();
            takeImage();
            toggleLock();
        }
    }
    emit finished();
}

The image is sent to a widget for processing with:

void ImageCapture::processCapturedImage(int requestId, const QImage& img)
{
    Q_UNUSED(requestId);
    QImage scaledImage = img.scaled(QSize(400, 300), //ui->viewfinder->size(),
                                    Qt::KeepAspectRatio,
                                    Qt::SmoothTransformation);

    thePixmap = QPixmap::fromImage(scaledImage);
    emit ProcessPixmap(thePixmap);
}

The process works for a few images before I get the message

QObject::moveToThread: Current thread (0x2775377aa10) is not the object's thread (0x2775377c540).
Cannot move to target thread (0x277536ad1f0)

And after the message the images continue to be sent to the Widget.

If I comment out the takeImage() line in the thread process line the message goes away. I do not have a view finder connected to the camera.

What did I do wrong that causes the message?

Is there an alternative approach to the situation. What I want to do is take an image periodically, get the QPixmap of the image so I can draw on it, and then write it to a Widget.

kshegunov

@nefarious said in QCameraImageCapture moveToThread error:

QObject::moveToThread: Current thread (0x2775377aa10) is not the object's thread (0x2775377c540).
Cannot move to target thread (0x277536ad1f0)

It appears you're trying to "pull" an object from a thread, so that's why Qt's complaining. QObject::moveToThread is NOT thread safe and can be used only from the thread the object currently resides to "push" into another.

On a related note you shouldn't use pixmaps from worker threads QPixmap::fromImage (assuming processCapturedImage is run in the worker). The QPixmap class isn't reentrant.

Kind regards.

nefarious

It is interesting. I changed the implementation to use a timer in the widget so I have no QThreads of my own and I still get the message. Is there a way to catch the message, I get the error when I run from the ide, but when I run as a real application I can't know that something is wrong, and handle the situation or write a status.

If even anything is wrong.

I can show all of the capture error messages and the camera error messages and none are shown.

Like I said, I am new the qt and trying to figure this out. The message quotes a thread address, it looks like an address, is there a way to find what thread is causing the message.

kshegunov

The "easy" way is to put a breakpoint in Qt's source where the message is emitted and just walk through the stack to see what call causes it.