trace
// 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