Mam pewne trudnościQGraphicsView iQGraphicsScene. Kiedy powiększam / unzoom w scenie i tworzę elementy za pomocą mousePressEvent, mam przesunięcie w tej pozycji. Jak można tego uniknąć?

event.pos() wydaje się być problemem ..

from PyQt4 import QtCore, QtGui

class graphicsItem (QtGui.QGraphicsItem):
    def __init__ (self):
        super(graphicsItem, self).__init__()
        self.rectF = QtCore.QRectF(0,0,10,10)
    def boundingRect (self):
        return self.rectF
    def paint (self, painter=None, style=None, widget=None):
        painter.fillRect(self.rectF, QtCore.Qt.red)

class graphicsScene (QtGui.QGraphicsScene):
    def __init__ (self, parent=None):
        super (graphicsScene, self).__init__ (parent)

class graphicsView (QtGui.QGraphicsView):
    def __init__ (self, parent = None):
        super (graphicsView, self).__init__ (parent)
        self.parent = parent
    def mousePressEvent(self, event):
        super (graphicsView, self).mousePressEvent(event)
        item = graphicsItem()
        position = QtCore.QPointF(event.pos()) - item.rectF.center()
        item.setPos(position.x() , position.y())
        self.parent.scene.addItem(item)
    def wheelEvent (self, event):
        super (graphicsView, self).wheelEvent(event)
        factor = 1.2
        if event.delta() < 0 :
            factor = 1.0 / factor
        self.scale(factor, factor)

class window (QtGui.QMainWindow):
    def __init__ (self, parent = None ) :
        super (window, self).__init__(parent)
        self.width = 800
        self.height = 600

        self.resize(self.width,self.height)
        self.mainLayout = QtGui.QVBoxLayout(self)

        self.view = graphicsView(self)
        self.scene = graphicsScene(self)
        self.view.setScene (self.scene)

        factor = 1
        self.scene.setSceneRect(0, 0, self.width * factor, self.height * factor)
        self.view.setMinimumSize(self.width, self.height)

        self.mainLayout.addWidget(self.view)

    def show (self):
        super (window, self).show()