o.k., at IDL IC = 100, I can reproduce bill's shadow boundary artifact in the corner reflector geometry group.  the coder for this is aware of it AFAIK, but somebody may wish to remind him. these artifacts are occurring at unwelded seams with no beveling or irregularities of any kind, or at seams which represent the intersection of two solid bodies, even tho real cubes have slightly beveled irregular edges, and two real solid cubes can't occupy the same space.   however, one would still like to see these artifacts eliminated, if poss.