如何在QPainter原生绘画之间获得默认FBO？

Question

我想在QPainter的原生绘画区域之间获得默认的FBO。如：

void QGraphicsScene::drawBackground(QPainter *painter, const QRectF &rect)
{
    painter->beginNativePainting();

    unsign int defaultFBO = getDefaultFBO(); // how ?

    glBindFramebuffer(GL_FRAMEBUFFER, defaultFBO );
    glClearColor(1, 0, 0, 1);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glBindFramebuffer(GL_FRAMEBUFFER, defaultFBO );

    painter->endNativePainting();
}

有人可以帮我吗？