#include <QApplication>
#include <QCamera>
#include <QMediaDevices>
#include <QMediaCaptureSession>
#include <QVideoWidget>
#include <QPainter>

class myVideoWidget : public QVideoWidget
{
public:
    explicit myVideoWidget(QWidget* parent = nullptr) : QVideoWidget(parent) {
    }

protected:
    void paintEvent(QPaintEvent* event)  override {
        QVideoWidget::paintEvent(event);

        QPainter painter(this);
        QPen pen;
        pen.setWidth(20);
        pen.setColor(Qt::black);
        painter.setPen(pen);
        painter.drawRect(10, 10, 500, 500);
    }

};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    myVideoWidget videoWidget;
    videoWidget.resize(640, 480);
    videoWidget.show();

    QCamera camera(QMediaDevices::defaultVideoInput());
    camera.start();

    QMediaCaptureSession mediaCaptureSession;
    mediaCaptureSession.setCamera(&camera);
    mediaCaptureSession.setVideoOutput(&videoWidget);

    return a.exec();
}