There is the standard cubic spline:
Provide 'n' through-points <xi, yi>, and it gives 'n-1' cubic polynomials,
one for each range[<xi,yi>-<x(i+1),y(i+1)>], with position, first, and second
derivative continuity.
The method might not be amenable to a POV solution, though, because you solve
a system of linear equations to get the coefficients for the cubic
polynomials, which is something that might be inefficient in POV script.
