what is the qtransform in QGraphicsScene::itemAt()

I create a custom QGraphicsItem. And overwrite the boundingRect() and paint().

QRectF myTile::boundingRect() const
{
  return QRectF(xPos*10, yPos*10, 10, 10);
}

void myTile::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
  QRectF rec = boundingRect();
  int gvi = value * 255;
  QColor gv(gvi, gvi, gvi, 255);
  QBrush brush(gv);
  painter->fillRect(rec, brush);
  painter->drawRect(rec);
}

Then I use addItem() to add a item to a scene. Now I want to get it from the scene by its position. I find the itemAt function. But the problem is I don't know what is the const QTransform & deviceTransform. What should I use for the QTransform?.

Because I didn't implement any transform in the QGraphicsItem. This confuses me.