SplineEvaluate.txt

Given an x,  xi <= x < xi+1
and the coefficients ai, bi, ci, di

h := x-xi
y := ai h^3 + bi h^2 + ci h + di (in theory)

in practice, Horner's rule is used to evaluate the polynomial

y := di+(ci+(bi+ai*h)*h)*h

Note that di is just yi and the formula is a simple
interpolation between (xi, yi) and (xi+1, yi+1)