[Octave-bug-tracker] [bug #45295] Division by Zero in 2nd derivative of NURBS surface

[Carlo de Falco]

Follow-up Comment #2, bug #45295 (project octave):

Hi,

It seems that the example geometry you provided has only C0 regularity, so it
is to be expected that the 2nd order derivatives be undefined.

using the knots and control points along the first direction
given in your example you can try the following univariate example:


>> knts =
[1,1,1,1,1,1,1,2,2,2,2,2,2,3,3,3,3,3,3,4,4,4,4,4,4,5,5,5,5,5,5,6,6,6,6,6,6,7,7,7,7,7,7,8,8,8,8,8,8,9,9,9,9,9,9,9];
>> pts = [sin([0:48]/48*2*pi); cos([0:48]/48*2*pi)];
>> nrb = nrbmak (pts, knts);
>> u = linspace (1, 9, 1000);
>> [d1, d2] = nrbderiv (nrb);
>> [fu, fdu, fd2u] = nrbdeval (nrb, d1, d2, u);
>> plot (u, fdu(1,:))


notice the discontinuities in the first derivative are clearly visible in the
plot,
we are considering whether it makes sense to add a check and an error message
in cases like this.

Do you have an example where computing derivatives fails with a more regular
geometry?


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