public void UpdateViewPortRect()
{
// calc the view port window
viewPort = new Rect(0, 0, ActualWidth, ActualHeight);
viewPort = RenderTransform.TransformBounds(viewPort);
Point topLeft = RenderTransform.Inverse.Transform(new Point(0, 0));
Point bottomRight = RenderTransform.Inverse.Transform(new Point(ActualWidth, ActualHeight));
Rect r = new Rect(topLeft, bottomRight);
viewPort = r;
}