How to use QGLWidget as the viewport for QGraphicsView?

[tonnot]

I'm trying to use a QGLWIDGET to test if my app. runs faster.
I see how the svgviewer example is faster when using it.
I have used a similar code :

glwidget =  new QGLWidget(QGLFormat(QGL::SampleBuffers));
  G_view->setViewport(glwidget);

glwidget =  new QGLWidget(QGLFormat(QGL::SampleBuffers));
  G_view->setViewport(glwidget);

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This is my paintEvent code for a QGraphicsView based widget, where I apply the transform and finally call to the internal paintevent

void A_Gview2D::paintEvent(QPaintEvent *event) {  
     QPainter painter  (this->viewport());
..... 
      this->QGraphicsView::paintEvent(event);  
}

void A_Gview2D::paintEvent(QPaintEvent *event) {  
     QPainter painter  (this->viewport());
..... 
      this->QGraphicsView::paintEvent(event);  
}

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I see a 'white' empty space and I have the errors:
A paint device can only be painted by one painter at a time.
QPainter::setRenderHint: Painter must be active to set rendering hints
QPainter::setWorldTransform: Painter not active
etc.

It seems that it is not so easy to apply. Any help ?
Thanks.