hi,

could you please help me?

i'm at the stage of writing a surface interrogation program where everything

is in place and it should all work.

but, for some reason my unit normal calculator is giving me incorrect

results.

I think the problem lies in my derivatives of the B-Spline basis function

(i'm using NURBS).

could anyone point me in the direction of a correct, working version of the

b-spline derivative?

thanks

STU

p.s. just to make sure i'm not being stupid:

to calculat the normal at a point on a NURBS surface, do the cross product

of the two tangent vectors.

to get these tangent vectors (one for u and one for v(the paameters)) you

apply the same NURBS formula as to find a point but replace either the u or

v basis functions with its derivative.

correct????