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