Qt 5.2.1 + OpenCV 2.4.9 in Visual Studio 2012

Alexander Linne

Hi!

I'm currently trying to capture camera using cv::VideoCapture.
Basically I subclassed QQuickPaintedItem, starting the camera and installing a QTimer, that calls update() every 50ms, in componentCompleted(). In paint() I then grab a frame and convert it to a QImage like this:
@cv::Mat frame;
cap >> frame;

QImage qFrame(frame.data, frame.cols, frame.rows, frame.step, QImage::Format_RGB888);
qFrame = qFrame.rgbSwapped();@

My problem now is, that as soon as I start the program the output in Visual Studio looks like this:
@[...]
The thread 0xec4 has exited with code 0 (0x0).
The thread 0x1b0c has exited with code 0 (0x0).
The thread 0x4f8 has exited with code 0 (0x0).
The thread 0x1b14 has exited with code 0 (0x0).
The thread 0x19e4 has exited with code 0 (0x0).
The thread 0x1a4c has exited with code 0 (0x0).
The thread 0x10c8 has exited with code 0 (0x0).
The thread 0x58c has exited with code 0 (0x0).
The thread 0xe10 has exited with code 0 (0x0).
The thread 0x1ac4 has exited with code 0 (0x0).
The thread 0x1578 has exited with code 0 (0x0).
The thread 0x128c has exited with code 0 (0x0).
The thread 0x1a64 has exited with code 0 (0x0).
The thread 0x1a6c has exited with code 0 (0x0).
The thread 0x1844 has exited with code 0 (0x0).
The thread 0x1a60 has exited with code 0 (0x0).
The thread 0x1924 has exited with code 0 (0x0).
The thread 0x10d0 has exited with code 0 (0x0).
The thread 0x1af0 has exited with code 0 (0x0).
The thread 0x10e4 has exited with code 0 (0x0).
[...]
@
And that's just a small part of it!
I wrote another program with OpenCV only:
@int main( int argc, char** argv ) {
cv::VideoCapture cap(0);
if (!cap.isOpened())
return -1;

cap.set(CV_CAP_PROP_FRAME_WIDTH, 1024);
cap.set(CV_CAP_PROP_FRAME_HEIGHT, 768);

cv::namedWindow("frame", 1);
while (true) {
    cv::Mat frame;
    cap >> frame;

    imshow("frame", frame);

    if(cv::waitKey(30) >= 0) break;
}

return 0;

}@
This works fine. No threads exiting all the time.

I really don't know why this happens. I only found out that this starts as soon as I open the camera and I also tried to grab all frames in an extra thread. Nothing works...
At least I see the output works correctly and everything is fine on the front-end, but this output is not only a bit strange but also annoying...

Any ideas?

Regards

Alexander Linne

I now also tried a Widget-based Qt app (as cv::namedWindow uses Widgets) with the following source code:
@int main(int argc, char *argv[])
{
QApplication a(argc, argv);

cv::VideoCapture cap(0);
if(!cap.isOpened())
    return -1;

cv::namedWindow("frame", 1);

cv::Mat frame;
while (true) {
    cap >> frame;

    imshow("frame", frame);

    if(cv::waitKey(30) >= 0) break;
}

}@

Same result...

SGaist

Hi,

Can you show the code you used for your qml element ?

Alexander Linne

Yeah, of course!
@class SHARED_EXPORT CaptureItem : public QQuickPaintedItem {
Q_OBJECT

cv::VideoCapture cap;
QTimer timedUpdate;

public:
CaptureItem(QQuickItem *parent = 0);

public:
void paint(QPainter *painter);

protected:
void componentComplete();
};@
@CaptureItem::CaptureItem(QQuickItem parent / = 0 */)
: QQuickPaintedItem(parent)
{
}

void CaptureItem::paint(QPainter *painter) {
QRectF boundingRect = contentsBoundingRect();
boundingRect.setWidth(boundingRect.width() - 1);
boundingRect.setHeight(boundingRect.height() - 1);

cv::Mat frame;
if (cap.isOpened()) {
    cap >> frame;

    QImage qFrame(frame.data, frame.cols, frame.rows, frame.step, QImage::Format_RGB888);
    qFrame = qFrame.rgbSwapped();

    float aspectRatio = qFrame.width() / (float)qFrame.height();
    float itemAspectRatio = boundingRect.width() / boundingRect.height();

    QRectF paintRect;
    if (aspectRatio >= itemAspectRatio) {
        paintRect.setWidth(boundingRect.width());

        int height = boundingRect.width() / aspectRatio;
        paintRect.setY((boundingRect.height() - height) / 2.f);
        paintRect.setHeight(height);
    } else {
        paintRect.setHeight(boundingRect.height());

        int width = boundingRect.height() * aspectRatio;
        paintRect.setX((boundingRect.width() - width) / 2.f);
        paintRect.setWidth(width);
    }

    painter->drawImage(paintRect, qFrame);
}

}

void CaptureItem::componentComplete() {
QQuickPaintedItem::componentComplete();

cap.open(0);

timedUpdate.setInterval(100);
connect(&timedUpdate, SIGNAL(timeout()), this, SLOT(update()));
timedUpdate.start();

}@

Then within registerTypes(const char *):
@qmlRegisterType<CaptureItem>(uri, 1, 0, "CaptureImpl");@

And the qml file:
@CaptureImpl {
implicitWidth: 300
implicitHeight: 300
}@

Is that enough? ^^

Regards

SGaist

This:

@timedUpdate.setInterval(100);
connect(&timedUpdate, SIGNAL(timeout()), this, SLOT(update()));@

Why not put it in the constructor ?

Alexander Linne

Of course I could put this in the contructor, but in the end that doesn't make any difference!

SGaist

It might since it looks like there's some threading strange things going on.

Alexander Linne

Yeah, could habe been...
I tried both... not changing anything... :/

I'll try to get another webcam and will have a look if that is the reason!