!http://www.bilderload.com/daten/screenshotSY80M.png(OK)!
!http://www.bilderload.com/daten/screenshot1KQZWZ.png(Artifacts)!

Code:

dialog.ui
@<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
<class>Dialog</class>
<widget class="QDialog" name="Dialog">
<property name="geometry">
<rect>
<x>0</x>
<y>0</y>
<width>400</width>
<height>300</height>
</rect>
</property>
<property name="windowTitle">
<string>Dialog</string>
</property>
<layout class="QHBoxLayout" name="horizontalLayout">
<item>
<widget class="QGraphicsView" name="graphicsView"/>
</item>
</layout>
</widget>
<layoutdefault spacing="6" margin="11"/>
<resources/>
<connections/>
</ui>@

dialog.h
@#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>

namespace Ui {
class Dialog;
}

class QGraphicsView;
class QGraphicsScene;
class VipuGuiCanvasItem;
class MyRectItem;

class Dialog : public QDialog
{
Q_OBJECT

public:
explicit Dialog(QWidget *parent = 0);
~Dialog();

protected:
bool eventFilter(QObject *pObject, QEvent *pEvent);

private:
Ui::Dialog *ui;

public:
QGraphicsView *m_pView;
QGraphicsScene *m_pScene;
VipuGuiCanvasItem *m_pCanvasItem;
MyRectItem *m_pMyRectItem;
};

#endif // DIALOG_H@

dialog.cpp
@#include "dialog.h"
#include "ui_dialog.h"
#include <QGLWidget>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsRectItem>
#include <QGraphicsSceneWheelEvent>
#include <QGraphicsObject>

class MyRectItem : public QGraphicsObject
{
public:
MyRectItem(QGraphicsItem *pParent = 0) : QGraphicsObject(pParent)
{}
QRectF boundingRect() const
{
return m_oBoundingRect;
}
void paint(QPainter *pPainter, const QStyleOptionGraphicsItem *, QWidget *)
{
QRectF oRect(QPointF(0,0), boundingRect().size());
oRect.moveCenter(boundingRect().center());
oRect.adjust(5, 5, -5, -5);
#if 1 // --> PROBLEM!
QPen oPen(Qt::DashLine);
#else
QPen oPen(Qt::SolidLine);
#endif
oPen.setColor(QColor(255,0,0,255)); // RED
pPainter->setPen(oPen);
pPainter->drawRect(oRect);

    oPen.setColor(QColor(0,255,0,255)); // GREEN
    pPainter->setPen(oPen);
    pPainter->drawEllipse(oRect);
}
void setRect(const QRectF& oRect)
{
    m_oBoundingRect = oRect;
    update();
}

private:
QRectF m_oBoundingRect;
};

class VipuGuiCanvasItem : public QGraphicsRectItem
{
public:
VipuGuiCanvasItem(Dialog *pDialog) : m_pDialog(pDialog)
{}
protected:
void wheelEvent(QGraphicsSceneWheelEvent *pEvent)
{
{
double fScale = pEvent->delta();
QRectF oRect = m_pDialog->m_pMyRectItem->boundingRect();
m_pDialog->m_pMyRectItem->setRect(
oRect.adjusted(fScale >= 0. ? -1. : 1.,
fScale >= 0. ? -1. : 1.,
fScale >= 0. ? 1. : -1.,
fScale >= 0. ? 1. : -1.));
}
}
private:
Dialog *m_pDialog;
};

Dialog::Dialog(QWidget *parent) :
QDialog(parent),
ui(new Ui::Dialog)
{
ui->setupUi(this);

m_pView = ui->graphicsView;

#if 1 // --> PROBLEM!
m_pView->setViewport(new QGLWidget(QGLFormat(QGL::SampleBuffers)));
#endif
m_pView->viewport()->installEventFilter(this);

m_pScene = new QGraphicsScene(this);
m_pView->setScene(m_pScene);

m_pCanvasItem = new VipuGuiCanvasItem(this);
m_pCanvasItem->setVisible(true);
m_pCanvasItem->setPen(Qt::NoPen);
m_pCanvasItem->setBrush(Qt::NoBrush);
m_pCanvasItem->setFlag(QGraphicsItem::ItemHasNoContents);
m_pScene->addItem(m_pCanvasItem);

m_pMyRectItem = new MyRectItem(m_pCanvasItem);

}

Dialog::~Dialog()
{
delete ui;
}

bool Dialog::eventFilter(QObject *pObject, QEvent *pEvent)
{
if (pObject == m_pView->viewport())
{
if (pEvent->type() == QEvent::Resize)
{
m_pScene->setSceneRect(m_pView->viewport()->rect());
m_pCanvasItem->setRect(m_pView->viewport()->rect());
m_pMyRectItem->setRect(m_pCanvasItem->rect());
}
}
return QWidget::eventFilter(pObject, pEvent);
}@

main.cpp
@#include <QtGui/QApplication>
#include "dialog.h"

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Dialog w;
w.show();

return a.exec();

}@