// trace function tests for intersection with a specified object

#declare Obj = sphere { 0, 1 }
#declare Norm = <0, 0, 0>;
#declare Start = <0.5, 0.5, 1>;
#declare Pos = trace (
                  Obj,             // object to test
                  Start,           // starting point
                  -z,              // direction
                  Norm );          // normal

// if intersection is found, normal differs from 0
#if (Norm.x != 0 | Norm.y != 0 | Norm.z != 0)
 //...
#end