OSL_HOSTDEVICE inline void operator() (Dual2<float> &result, const Dual2<Vec3> &p,
                                        const Dual2<float> &t, int seed=0) const { result.set (0.5f, 0.5f, 0.5f); }
 OSL_HOSTDEVICE inline void operator() (Dual2<Vec3> &result, const Dual2<Vec3> &p, const Dual2<float> &t) const { result.set (v(), v(), v()); }
 OSL_HOSTDEVICE inline void operator() (Dual2<float> &result, const Dual2<float> &x,
                                        const Dual2<float> &y, int seed=0) const { result.set (0.0f, 0.0f, 0.0f); }