QGraphicsItem и ItemIgnoresTransformations

    class GraphicsNode :
         public QObject,
         public QGraphicsItemGroup
      {
         Q_OBJECT

      public:
         GraphicsNode(..., QGraphicsItem * pParent = 0);

      private:
         ...
         QGraphicsRectItem* m_pStateRect;
         QGraphicsSimpleTextItem* m_pNodeLabel;
    };

GraphicsNode::GraphicsNode(..., QGraphicsItem* pParent) :
      QObject(),
      QGraphicsItemGroup(pParent),
      m_pStateRect(new QGraphicsRectItem(pParent)),
      m_pNodeLabel(new QGraphicsSimpleTextItem(pParent))
   {
      setZValue(-1);
      setAcceptsHoverEvents(true);
      setFlag(QGraphicsItem::ItemIsSelectable);

      m_pStateRect->setRect(0, 0, 0.4, 0.4);

      m_pNodeLabel->setText(QString::number(m_nodeId));
      m_pNodeLabel->setScale(0.6);

      addToGroup(m_pStateRect);
      addToGroup(m_pNodeLabel);

      // Set the item to be fixed size
      setFlag(QGraphicsItem::ItemIgnoresTransformations);
      setScale(20.0);
   }

   void MapWidget::zoomIn()
   {
      if (m_currentScale < m_scaleMax)
      {
         scale(m_scaleFactor, m_scaleFactor);
      }
   }

   void MapWidget::zoomOut()
   {
      if (m_currentScale > m_scaleMin)
      {
         scale(1 / m_scaleFactor, 1 / m_scaleFactor);
      }
   }