[vtkusers] Finding the velocity vector of a curve

[Ron Inbar]

I have a curve defined by three vtkSplines (or subclasses thereof).  I need
to find the derivative (dx/dt, dy/dt, dz/dt) at a given t.  I was thinking
of taking [(x(t + dt) - x(t - dt)) / 2dt] as an approximation, but I believe
it can be done much more accurately inside the vtkSpline subclass, using the
spline formulas.  As far as I saw, the vtkSpline subclasses don't support
this calculation.  Is there an accurate way of doing it, or is my
approximation as good as it gets?

Ron