How to differentiate a cubic spline

Hi all.

I'm plotting the position of a particle vs time using a cubic spline as follows:

xx = csapi(t, pos);

fnplt(xx);

Here, t and pos are both vectors.

I now need to plot the velocity vs time for this particle. My question is, how do I differentiate a cubic spline? Any help would be appreciated.

Answer 1

[John D'Errico]

Simple enough to do by differentiating each polynomial segment.

That is, if we have

pp = csapi(t,pos);

Then for a cubic spline in pp form, this will suffice:

ppd = pp;
ppd.coefs = ppd.coefs*diag([3 2 1],1);

[MeEngr]

Thanks, John. I'm sorry, but I'm still confused.

What exactly are we achieving using these two commands and how do I plot the velocity? I'm trying to look this up online and in Matlab's documentation but I'm not getting anything.