Blur QWidget window content(with any 3D on top)

main.cpp

namespace
{
    QImage applyEffectToImage(QImage src, QGraphicsEffect *effect)
    {
        if(src.isNull()) return QImage();   // No need to do anything else
        if(!effect) return src;             // No need to do anything else
        QGraphicsScene scene;
        QGraphicsPixmapItem item;
        item.setPixmap(QPixmap::fromImage(src));
        item.setGraphicsEffect(effect);
        scene.addItem(&item);
        QImage res(src.size(), QImage::Format_ARGB32);
        res.fill(Qt::transparent);
        QPainter ptr(&res);
        scene.render(&ptr, QRectF(), QRectF(0, 0, src.width(), src.height()));
        return res;
    }

    QPixmap prepareDialogBackgroundImage(QWidget* parent)
    {
        QScreen *screen =  QGuiApplication::primaryScreen();
        auto pixmap = screen->grabWindow(parent->winId());

        const auto effect = new QGraphicsBlurEffect();
        effect->setBlurHints(QGraphicsBlurEffect::PerformanceHint);
        effect->setBlurRadius(2);

        auto image = applyEffectToImage(pixmap.toImage(), effect);

        return QPixmap::fromImage(image);
    }

}