With the mouse I create a polyline. That's the first QGraphicsItem. For every click with the mouse I create a child Item which draws an ellipse on every position I clicked the Mouse. The ellipse is movable and the purpose is to modify the polyline. When I move the ellipse the polyline is modified indeed but not following the ellipse. It has a big offset from the ellipse. What am I missing here?

Parent item:
Qt Code: 
  1. itemSpline::itemSpline(QGraphicsItem *parent)
  3. {
  4.   // TODO Auto-generated constructor stub
  5.   //setAcceptHoverEvents(true);
  6.   //  setFlags(ItemIsMovable | ItemIsFocusable | QGraphicsItem::ItemIsSelectable);
  7.   setFlags(QGraphicsItem::ItemIsSelectable);
  8.   setZValue(20.);
  9.   myColor = Qt::black;
  10.   isFinished = false;
  11. }
  12.  
  13. QVariant itemSpline::itemChange(GraphicsItemChange change, const QVariant &value)
  14. {
  15.     if(ItemPositionHasChanged)
  16.     {
  17.         //Should I do something here?
  18.     }
  19.     return QGraphicsPolygonItem::itemChange(change, value);
  20. }
  21.  
  22. void itemSpline::alterPolygon(QPointF pos, int index)
  23. {
  24. 	qDebug("alterPolygon");
  25.  
  26. 	//Calculate offset and change boundingRectangle?
  27.         prepareGeometryChange();
  28.         vVertices[index] = pos;
  29. }
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Child item: (Ellipse)
Qt Code: 
  1. Node::Node(QPointF nodePos, itemSpline *parent, int ind)
  2. 		: QGraphicsItem(parent), splineItem(parent)
  3. {
  4. 	// TODO Auto-generated constructor stub
  5.         newPos = mapToParent(nodePos);
  6.  
  7. 	index = ind;
  8. 	setAcceptsHoverEvents(true);
  9. 	setFlag(ItemIsMovable);
  10. 	setFlag(ItemSendsGeometryChanges);
  11. 	setCacheMode(DeviceCoordinateCache);
  12. 	setZValue(-1);
  13. }
  14.  
  15. QRectF Node::boundingRect() const
  16. {
  17.         return QRectF(newPos.x()-4, newPos.y()-4, 8,8);
  18. }
  19.  
  20. //This does nothing?
  21. bool Node::advance()
  22. {
  23. 	qDebug("Node: Advance");
  24.  
  25. 	if (newPos == pos())
  26. 		return false;
  27.  
  28.         setPos(newPos);
  29.  
  30. 	return true;
  31. }
  32.  
  33. QVariant Node::itemChange(GraphicsItemChange change, const QVariant &value)
  34. {
  35.     //only if polyline is finished!
  36.     if(splineItem->isFinished)
  37.     {
  38.         if(ItemPositionHasChanged)
  39.         {
  40.             QPointF valuePos(value.toPointF());
  41.  
  42.             splineItem->alterPolygon(mapToParent(valuePos), index);
  43.  
  44.         }
  45.     }
  46.  
  47.     return QGraphicsItem::itemChange(change, value);
  48. }
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