// bounding box test
fun int bboxhit( vec o, vec d, vec c1, vec c2 ) {
  vec p1 = ( c1 - o ) / d;
  vec p2 = ( c2 - o ) / d;
  vec mins = p1 <? p2;
  vec maxs = p1 >? p2;
  float t1 = mins\x >? mins\y >? mins\z;
  float t2 = maxs\x <? maxs\y <? maxs\z;
  if ( t1 < t2 && t2 > 0.0 )
    return 1;
  return 0;
}