public override Intersection Intersect(Ray ray)
        {
            Intersection result = new Intersection();

            result.Thing = this;
            result.Ray = ray;

            // Find t.
            double t = - (Vector3.Dot(Norm,ray.Start) + Offset) / (Vector3.Dot(Norm, ray.Dir));
            result.Dist = t;

            // Get a point on the plane.
            Vector3 p = ray.Start + t * ray.Dir;

            // Does the ray intersect the plane inside or outside?
            Vector3 planeToRayStart = ray.Start - p;
            double dot = Vector3.Dot (planeToRayStart, Norm);
            if (dot > 0) {
                result.Inside = false;
            } else {
                result.Inside = true;
            }

            return result;
        }