question: public Point2D convertWindowToReference ( Point2D apt ) { if ( scrollBackReferenceRect == null ) return null ; else return new Point2D . Double ( ( apt . getX ( ) - scrollBackReferenceRect . getX ( ) ) / scrollBackReferenceRect . getWidth ( ) , ( apt . getY ( ) - scrollBackReferenceRect . getY ( ) ) / scrollBackReferenceRect . getHeight ( ) ) ; }
target:  converts a point from window coordinates to reference coordinates