QVideoSink: process each frame of QCamera and Paint to QVideoWidget

gween

Overview

I have followed and tried to implement similar code to what is in https://forum.qt.io/topic/133893/painting-a-qvideoframe-on-a-qvideowidget-with-qt6 but continue to get errors of QWidget::paintEngine: Should no longer be called, QPainter::worldTransform: Painter not active, and QPainter::setWorldTransform: Painter not active. After using solutions from https://forum.qt.io/topic/64693/unable-to-paint-on-qt-widget-shows-error-paintengine-should-no-longer-be-called I still get the same errors and my QVideoWidget is black with no output.

Details

I'm using Qt 6.4.0 and testing both on android emulator as well as on a Google Pixel 6.

The main differences in my application vs the post's example are:

I've created ui/h/cpp files, and the class inherits from QWidget and not the QMainWindow
My custom class is put inside a scroll area widget.
Again, trying on android (I think the post was on desktop?)

I use the same code from that discussion:

#scannerpage.h
    QVideoSink m_videoSink;
    QMediaCaptureSession m_captureSession;
    QScopedPointer<QCamera> m_camer;

# scannerpage.cpp
    m_camera.reset(new QCamera(QMediaDevices::defaultVideoInput()));
    m_captureSession.setCamera(m_camera.data());

    connect(&m_videoSink, &QVideoSink::videoFrameChanged, this, &ScannerPage::processVideoFrame);

    m_captureSession.setVideoSink(&m_videoSink);
    m_captureSession.setVideoOutput(m_videoWidget);
    m_camera->start();

    m_videoWidget = new QVideoWidget(this);
    ui->gridLayout->addWidget(m_videoWidget);

I can see via debug that my code is reaching processVideoFrame(), but the errors that I hit are:

QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::worldTransform: Painter not active
QPainter::setWorldTransform: Painter not active
QPainter::setWorldTransform: Painter not active

So, after seeing those errors I found https://forum.qt.io/topic/64693/unable-to-paint-on-qt-widget-shows-error-paintengine-should-no-longer-be-called and https://stackoverflow.com/questions/25781353/qt-qwidgetpaintengine-should-no-longer-be-called. Note in the stackoverflow one, I'm not using QML/Quick, I'm trying to just put my QVideoWidget into the scroll area, process each frame, and still have a preview in the QVideoWidget.

So I changed the processVideoFrame() function to the following:

void ScannerPage::processVideoFrame()
{
    m_tmpVideoFrame = m_videoSink.videoFrame();

    if (m_tmpVideoFrame.map(QVideoFrame::ReadOnly))
    {
        #videoframe.paint(new QPainter(m_videoWidget), QRectF(0.0f,0.0f,100.0f,100.0f), QVideoFrame::PaintOptions());
        #videoframe.unmap();
        m_videoWidget->update();
    }
}

And added the following to my .h file and .cpp file:

#scannerpage.h
private slots:
     void paintEvent(QPaintEvent *event) override;
private:
     QVideoFrame m_tmpVideoFrame;

#scannerpage.cpp
void ScannerPage::paintEvent(QPaintEvent *event)
{
    m_tmpVideoFrame.paint(new QPainter(m_videoWidget), QRectF(0.0f,0.0f,100.0f,100.0f), QVideoFrame::PaintOptions());
    m_tmpVideoFrame.unmap();
}

But I still get those errors I mentioned earlier of paintEngine should no longer be called and the QVideoWidget is still black.

What am I missing here?