Sensing the viewer
Using visibility and proximity sensors
 
- VisibilitySensor and
ProximitySensor nodes sense a
box-shaped region
	- center - region center
	- size - region dimensions
 
 
- Both nodes have similar outputs:
	- enterTime - sends time on visible
		or region entry
	- exitTime - sends time on not visible
		or region exit
	- isActive - sends true on entry,
		false on exit