// A fast ray-bounding box intersection routine
//    Requires:  a ray structure that stores (a) the origin, (b) the inverse direction
//               (i.e., (1/dir_x, 1/dir_y, 1/dir_z) where dir_xyz is the ray's direction)
//               as well as (c) a quantity related to the sign of the inverse direction.
//               (i.e., invDirSign[0] is 1 if 1/dir_x < 0 and is 0 if 1/dir_x >= 0)              
//    Also requires an array of points representing the bounding box.  In this case,
//    bounds[0] is the minimum point of the bounding box and bounds[1] is the maximum.

bool IntersectBBox( Ray &ray, Point *bounds )
{
  float tmin  = (bounds[ray.invDirSign[0]].X()   - ray.origin.X()) * ray.invDir.X();
  float tymax = (bounds[1-ray.invDirSign[1]].Y() - ray.origin.Y()) * ray.invDir.Y();
  if ( tmin > tymax ) return false;
  float tmax  = (bounds[1-ray.invDirSign[0]].X() - ray.origin.X()) * ray.invDir.X();
  float tymin = (bounds[ray.invDirSign[1]].Y()   - ray.origin.Y()) * ray.invDir.Y();
  if ( tymin > tmax ) return false;
  if ( tymax < tmax ) tmax = tymax;
  float tzmin = (bounds[ray.invDirSign[2]].Z()   - ray.origin.Z()) * ray.invDir.Z();
  if ( tzmin > tmax ) return false;
  if ( tymin > tmin ) tmin = tymin;
  float tzmax = (bounds[1-ray.invDirSign[2]].Z() - ray.origin.Z()) * ray.invDir.Z();
  if ( tmin > tzmax ) return false;
  if ( tzmin > tmin ) tmin = tzmin;
  if ( tzmax < tmax ) tmax = tzmax;
  return ( (tmin < ray.hitDist) && (tmax > EPSILON) );  // EPSILON is a small value > 0
}