scalar_t r = 1.0 / (tc1.u * tc2.v - tc2.u * tc1.v);
Vector3 udir( (tc2.v * vec1.x - tc1.v * vec2.x) * r,
(tc2.v * vec1.y - tc1.v * vec2.y) * r,
scalar_t r = 1.0 / (tc1.u * tc2.v - tc2.u * tc1.v);
Vector3 udir( (tc2.v * vec1.x - tc1.v * vec2.x) * r,
(tc2.v * vec1.y - tc1.v * vec2.y) * r,