Using QVideoProbe from QML

[Scorp2us]

I am trying to do the following: Using a camera (Well, a video for testing purposes, but will eventually be a QML Camera) decode a bar code. I have the barcode working for static images (VinScanner. C++, exposed to QML, works). Work want me to use the live camera feed so no capture is needed. It will just use the live feed. VideoProbe is my C++ class which wraps QVideoProbe. What I expect to see, at least, are several "frame" messages logged to the console. Nothing happens.

Here is the QML:

Video {
		id: video
		anchors.fill: parent
		autoLoad: true
		autoPlay: true
		source: "vin1.mp4"
		onPlaying: {
			probe.source = video
			console.log("source="+video.source)
			console.log("has video=" + video.hasVideo + " status="+video.status+ " playstate="+video.playbackState)
			console.log("videoCodec="+ video.metaData.videoCodec)
		}
	}
VideoProbe {
		id: probe
		source: video
		onFrame: {
			console.log("frame\n")
			scanner.decode(videoFrame)
		}
	}
VinScanner {
		id: scanner
		onDecoded: vinText.text = vin
	}

Video {
		id: video
		anchors.fill: parent
		autoLoad: true
		autoPlay: true
		source: "vin1.mp4"
		onPlaying: {
			probe.source = video
			console.log("source="+video.source)
			console.log("has video=" + video.hasVideo + " status="+video.status+ " playstate="+video.playbackState)
			console.log("videoCodec="+ video.metaData.videoCodec)
		}
	}
VideoProbe {
		id: probe
		source: video
		onFrame: {
			console.log("frame\n")
			scanner.decode(videoFrame)
		}
	}
VinScanner {
		id: scanner
		onDecoded: vinText.text = vin
	}

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Now, what happens is a variety of things.
First application output:
VideoProbe::setSource() not a QMediaObject, got a: Video_QMLTYPE_0
source=file:///home/xxx/Projects/build-vinscan-Desktop_Qt_5_2_1_GCC_64bit-Debug/qml/vinscan/vin1.mp4
has video=false status=2 playstate=1
videoCodec=undefined

Now, what happens on the screen (QML window) is fine. I see the video I collected of me hovering over a bar code. However the hasVideo property is false.
When I take this Video object and pass it to C++, I get a "Video_QMLTYPE_0" classname from obj->metaObject()->className() or, when it is a Camera, I get a QDeclarativeCamera (not part of QT API!) . Neither of which I expected to get. I expected to get a QObject* that I could promote to a QMediaObject*. But the qobject_cast<>() fails. Both the QCamera an QMediaPlayer inherit QMediaObject. I based my approach on that.

I think the rest of what I have will work once I get the setSource(QObject*) function squared away.

--VIDEOPROBE.H--

class VideoProbe : public QVideoProbe
{
	Q_OBJECT
 
	QObject *m_source; Q_PROPERTY(QObject* source READ source WRITE setSource)
	//QObject *m_dest;		Q_PROPERTY(QObject* dest READ dest WRITE setDest)
	bool m_active;			Q_PROPERTY(bool active READ isActive WRITE setActive)
 
public:
	explicit VideoProbe(QObject *parent = 0);
	bool isActive();		void setActive(bool);
	QObject* source();		bool setSource(QObject *);
 
signals:
	void frame(QVideoFrame videoFrame);
 
public slots:
	void emitFrame(QVideoFrame videoFrame);
 
};

class VideoProbe : public QVideoProbe
{
	Q_OBJECT

	QObject *m_source; Q_PROPERTY(QObject* source READ source WRITE setSource)
	//QObject *m_dest;		Q_PROPERTY(QObject* dest READ dest WRITE setDest)
	bool m_active;			Q_PROPERTY(bool active READ isActive WRITE setActive)

public:
	explicit VideoProbe(QObject *parent = 0);
	bool isActive();		void setActive(bool);
	QObject* source();		bool setSource(QObject *);

signals:
	void frame(QVideoFrame videoFrame);

public slots:
	void emitFrame(QVideoFrame videoFrame);

};

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----- SET SOURCE----

bool VideoProbe::setSource(QObject* sourceObj)
{
	m_source = sourceObj;
	bool ret =false;
	if (qobject_cast<QMediaObject*>(sourceObj))
	{
		ret = QVideoProbe::setSource((QMediaObject*)sourceObj);
		qDebug() << "VideoProbe::setSource()" << sourceObj << ret;
	} else {
		qDebug() << "VideoProbe::setSource() not a QMediaObject, got a:" << sourceObj->metaObject()->className();
	}
	return ret;
 
}

bool VideoProbe::setSource(QObject* sourceObj)
{
	m_source = sourceObj;
	bool ret =false;
	if (qobject_cast<QMediaObject*>(sourceObj))
	{
		ret = QVideoProbe::setSource((QMediaObject*)sourceObj);
		qDebug() << "VideoProbe::setSource()" << sourceObj << ret;
	} else {
		qDebug() << "VideoProbe::setSource() not a QMediaObject, got a:" << sourceObj->metaObject()->className();
	}
	return ret;

}

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[anda_skoa]

But the qobject_cast<>() fails. Both the QCamera an QMediaPlayer inherit QMediaObject. I based my approach on that.

But you have neither a QCamera nor a QMediaPlayer.

The objects you have are subclasses of QDeclarativeItem which in turn is a QGraphicsObject which in turn derives from QObject.
So you can qobject_cast to QGraphicsObject or QDeclarativeItem or to the objects' final classes. qobject_cast cannot transform an object into another, unrelated, class.

Cheers,
_

[Scorp2us]

Thanks, but why wouldn't these QML element classes be just subclasses of their Q* variants, along with being a subclass of QDeclartiveItem? Where does their magic happen? Are they proxies to a d pointer that does the magic?

[anda_skoa]

A class can not be a subclass of to QObject derived classes.
My guess is that those two are visual QtQuick components, i.e. they appear visually in the QML scene. In which case they need to derive from QDeclarativeItem.

I don't know any details about their implementation but it is likely that they hold pointers to the non-UI classes, very likely indirectly through a d-pointer.

Cheers,
_

[Scorp2us]

I got it working like this

bool VideoProbe::setSource(QObject* sourceObj)
{
	m_source = sourceObj;
	bool ret =false;
	QMediaPlayer *player = qvariant_cast<QMediaPlayer*>(sourceObj->property("mediaObject"));
	qDebug() << "VideoProbe::setSource() player"<< player;
	ret = QVideoProbe::setSource((QMediaObject*)player);
	return ret;
}

bool VideoProbe::setSource(QObject* sourceObj)
{
	m_source = sourceObj;
	bool ret =false;
	QMediaPlayer *player = qvariant_cast<QMediaPlayer*>(sourceObj->property("mediaObject"));
	qDebug() << "VideoProbe::setSource() player"<< player;
	ret = QVideoProbe::setSource((QMediaObject*)player);
	return ret;
}

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[anda_skoa]

Ah, good to know!

Btw, you don't have to do another cast in line 7, the variable player is already of type QMediaPlayer

Cheers,
_