How to send a Qt signal containing a cv::Mat?

Question

In short, I get following error:

QObject::connect: Cannot queue arguments of type 'cv::Mat'
(Make sure 'cv::Mat' is registered using qRegisterMetaType().)

What I'm trying to do is send a signal containing two cv::Mat images from a QThread to the main thread, so that I can display the output. There's no compile time error, but when I run the program, it gets stuck at a breakpoint in qglobal.h (inline void qt_noop() {}).

I've tried to add Q_DECLARE_METATYPE(cv::Mat) to the code, to no avail. I'm quite suck with what to do now.

code

In a QThread class:

signals:
void sndFlow(cv::Mat &leftEye, cv::Mat &rightEye);

void eyesDriver::run()
{
    forever
    {
        flow->draw(leftEye, rightEye);
        sndFlow(leftEye, rightEye);
    }
}

Capturing in a QObject class:

public slots:
void recFlow(cv::Mat &leftEye, cv::Mat &rightEye);

void myClass::recFlow(cv::Mat &leftEye, cv::Mat &rightEye)
{
    cv::imshow("left", leftEye);
    cv::imshow("rigth", rightEye);
    cv::waitKey(40);
}

In main:

Q_DECLARE_METATYPE(cv::Mat)
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    qRegisterMetaType< cv::Mat >("cv::Mat");
    // create objects from QThread and QObject class
    QObject::connect(&qthread, SIGNAL(sndFlow(cv::Mat&,cv::Mat&)),
                     &qobject, SLOT(recFlow(cv::Mat&,cv::Mat&)));
    qthread.start();
    return a.exec();
}

Changing the signal-slot variables to QSharedPointer< cv::Mat > does not work either. Gives the same error:

QObject::connect: Cannot queue arguments of type 'QSharedPointer<cv::Mat>'
(Make sure 'QSharedPointer<cv::Mat>' is registered using qRegisterMetaType().)

WORKS

All right, it seems to work. I've move qRegisterMetaType< cv::Mat >("cv::Mat"); right before the QObject::connect call. However I still have to 'F5' past the breakpoint in qglobal.h, it works afterwards.

I might be wrong, but it seems that the location of qRegisterMetaType is not trivial.

Answer 1

You need to call qRegisterMetaType in addition to the macro (or instead of it, depending on your needs). This is necessary for the signals to be able to marshal your data across threads. However, it might be a wiser idea to pass by reference or smart pointer, or raw pointer if you are using the QObject hierarchy to manage the object lifetime.