   side of plane
Plane::Side Plane::side (const D3DXVECTOR3& point) const{    float fDistance = distanceToPoint(point);    if ( fDistance < 0.0 )   return Plane::NEGATIVE_SIDE;    if ( fDistance > 0.0 )   return Plane::POSITIVE_SIDE;    return Plane::NO_SIDE;}it judge which side of plane is point I think it should use something likestd::numeric_limits::epison() .is it necessary?   
  I think it would make no difference - epsilon() will return very tiny number. Sticking with checks with 0 should be ok. However you may need to introduce some epsilon which you define, if you for example know that the input point may "contain" some error. But that depends on how you use the function...   ;  Whether you need the epsilon (and what it should be) depends on the circumstances. For example, for clipping a polygon to a plane, an epsilon is often required to avoid various robustness problems.   