int Triangle :: Intersect (  Ray & r , double & t )
{
 double vd = n & r . Dir ;

 if ( vd > - EPS && vd < EPS ) return 0 ;
 if (( t = ( ( Loc - r . Org ) & n ) / vd ) < GeomThreshold ) return 0 ;

 Vector p = r . Point ( t ) ;
 double u = u0 + ( p & ku ) ;
 double v = v0 + ( p & kv ) ;

 return u > 0 && v > 0 && u + v < 1;
}