LOL i dont consider myself as a guru :) but thanx. (if it was to Ajax, i agree :) Ok, back from the math papers, if you do add dNdv and dNdu AND do the calculation afterward you will not have the right result. The reason is pretty ease, if you add those vectors, its like you transform one to the other, hence the lenght will be smaller (or equal), then the original |dNdu|+|dNdv|. Furthermore if dNdu and dNdv points absolutely in the opposite direction (and thats not uncommon), the proposed addition will show a small value, which will not reflect the change of the surface. If only you could do a "normals forward" on_that_node, or call an abs function of the vector's axis', would you get 'similar' values, but even then there would be a difference (and would lose its extra speed).