如何用背景色填充QCameraViewFinder？

Question

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    m_QPushButton_calibration = new QPushButton("Calibrate", this);
    connect(m_QPushButton_calibration, SIGNAL (released()),this, SLOT (handleButton()));

    QList<QCameraInfo> l_QListQCameraInfo_available_cameras = QCameraInfo::availableCameras();

    m_QCameraViewfinder_viewfinder = new QCameraViewfinder(this);

    if (l_QListQCameraInfo_available_cameras.length() >= 2)
    {
        m_QCamera_required_camera = new QCamera (l_QListQCameraInfo_available_cameras[1]);
        m_QCamera_required_camera->setViewfinder(m_QCameraViewfinder_viewfinder);
        m_QCamera_required_camera->start ();
    }

    m_QWidget_central         = new QWidget;
    m_QGridLayout_main_screen = new QGridLayout;

    m_QWidget_central->setStyleSheet ("background-color: green");
    m_QWidget_central->setLayout (m_QGridLayout_main_screen);

    m_QPushButton_calibration->setStyleSheet ("background-color: pink");
    m_QGridLayout_main_screen->addWidget (m_QPushButton_calibration, 0, 0, 1, 1);

    m_QCameraViewfinder_viewfinder->setStyleSheet ("background-color: blue");
    m_QGridLayout_main_screen->addWidget (m_QCameraViewfinder_viewfinder, 1, 1, 2, 1);

    this->setCentralWidget (m_QWidget_central);

    m_QCameraViewfinder_viewfinder->show();
}

我试过这个：m_QCameraViewfinder_viewfinder->setStyleSheet ("background-color: blue");

没有帮助。

截至目前尚未安装相机。如何用背景颜色填充QCameraViewFinder？

输出在这里：