> > 4. On the removal of QAbstractVideoFilter AND QVideoProbe: Disappointed to > > hear this. I previously used this for read-only frames for analysis, i.e. > > Barcode reading and object detection. How do we do that now? > > You can get full access to the video data from a QVideoSink through the > newVideoFrame() signal and mapping the video frame (or converting it to a > QImage). With that you should have the access to the data that you need. (A > small detail that’s still missing right now but that I hope to have in place > for 6.2 is the ability to connect several sinks to a media player or capture > session, but it’s not really required as you can connect several slots to the > signal). If you feel like to need more than that, let me know what it is.
So the most common case I need supported is getting the video frame for processing while maintaining the full-speed live preview. Is this the multi-sink scenario? Typically, With Camera connected o a VideoOutput, I use QVideoProbe to throw the frame (pixel data as QByteArray, because the library doesn't care) to a thread for multicore async processing. A typical 1 megapixel image on RaspberryPi4 takes ~150ms using ZBar or ZXing (I find ZXing is more like 100ms), so this gets about 6 processed frames a second, which seems responsive enough to the user because they are looking at the live display. Since you asked for actual code, attached is the code I use to do this. It may not be perfect code (long story made short, I just rewrote this from memory) but it is what I whipped up, and works reasonably well for now. I've used this approach for barcodes and OpenCV. If it matters: I disclaim any copyright for the attached files.
barcodevideoprobe.cpp
Description: Binary data
#ifndef BARCODEVIDEOPROBE_H
#define BARCODEVIDEOPROBE_H
#include <QObject>
#include <QThread>
#include <QtQml>
#ifndef Q_OS_WASM
#include <QVideoProbe>
#include "zbar.h"
#include <QVideoFrame>
#include <QCamera>
#endif
class BarcodeVideoProbe : public QObject
{
Q_OBJECT
Q_PROPERTY(QObject* qmlCamera READ qmlCamera WRITE setQmlCamera NOTIFY
qmlCameraChanged)
Q_PROPERTY(QString barcodeFormat READ barcodeFormat WRITE
setBarcodeFormat NOTIFY barcodeFormatChanged)
Q_PROPERTY(QString barcode READ barcode WRITE setBarcode NOTIFY
barcodeChanged)
Q_PROPERTY(bool workerProcessing READ workerProcessing WRITE
setWorkerProcessing NOTIFY workerProcessingChanged)
Q_PROPERTY(bool active READ active WRITE setActive NOTIFY activeChanged)
QThread m_workerThread;
QObject *m_qmlCamera = nullptr;
bool m_workerProcessing = false;
bool m_active = true;
QString m_barcode, m_barcodeFormat;
#ifndef Q_OS_WASM
QCamera *m_camera = nullptr;
QVideoProbe m_probe;
QByteArray extractYChannel(const QVideoFrame &frame);
#endif
QML_ELEMENT
public:
explicit BarcodeVideoProbe(QObject * parent=nullptr);
~BarcodeVideoProbe();
QString barcode() const;
QString barcodeFormat() const;
bool workerProcessing() const;
bool active() const;
signals:
void process(const QByteArray &pixelData, const QSize& size);
void barcodeFound(QString barcode, QString format);
void barcodeChanged(QString barcode);
void barcodeFormatChanged(QString barcodeFormat);
void workerProcessingChanged(bool workerProcessing);
void qmlCameraChanged();
void activeChanged(bool active);
public slots:
QObject* qmlCamera() const;
void setQmlCamera(QObject*qmlCamera);
#ifndef Q_OS_WASM
void processFrame(QVideoFrame);
#endif
void setBarcode(QString barcode);
void setBarcodeFormat(QString barcodeFormat);
void setWorkerProcessing(bool workerProcessing);
// handle worker signals
void handleBarcode(QString barcode, QString format);
void handleProcessingComplete();
void setActive(bool active);
};
class BarcodeDecodeWorker : public QObject
{
Q_OBJECT
#ifndef Q_OS_WASM
zbar::ImageScanner scanner;
#endif
public slots:
void process(const QByteArray &pixelData, const QSize& size);
signals:
void barcode(const QString &result, const QString &format); // this may
be emitted multiple times
void processingComplete();
};
#endif // BARCODEVIDEOPROBE_H
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