The distance between the position of the shadow and a reference position in the two dimensional image is monitored, and {HEAD ~ this distance} is used to derive {TAIL ~ information} relating to the relative positions of the first object and the second object  
is used to derive
The distance between the position of the shadow and a reference position in the two dimensional image is monitored , and { HEAD ~ this distance } is used to derive { TAIL ~ information } relating to the relative positions of the first object and the second object
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH