{HEAD ~ The distance} between {TAIL ~ 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 the relative positions of the first object and the second object  
{ HEAD ~ The distance } between { TAIL ~ 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 the relative positions of the first object and the second object
HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL 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 OTH OTH OTH OTH OTH OTH OTH