Thread Subject: NURBS - second derrivative

Has anyone got any experience with the NURBS toolbox on the file exchange?

I am trying to find the second derivative of aNURBS surface wtr to u:

I tried:
%%%%%%%%%%%%%%%%
srf = nrbtestsrf;%nurb test surf
dsrf = nrbderiv(srf);%first derrivative of srf
dsrf_du = dsrf{1};%first derivative wrt to u of srf

d_dsrf_du = nrbderiv(dsrf_du);%derivate of dsrf/du
%this give 2 surfaces -> {d^srf/dudu d^2srf/dudv}

nrbdeval(dsrf_du,d_dsrf_du,{0.1 0.2})%eval @ u=0.1 v=0.2
%%%%%%%%%%%%%%%%%%%%%%%

But this return Inf or NaN. I thought the derrivative of a NURBS surface would also be a NURBS surface but it appears not. Any suggestions on how to calculate the second derivative would be appreciated. I am thinking I will try do it numerically if there is no work around with the toolbox.


Regards Jake