When you pass in a third parameter, the "spline" function automatically evaluates the spline for those points using "ppval". If you only pass in two parameters, "spline" returns a struct representing the spline created. The coefficients for the piecewise polynomials can be extracted using the function "unmkpp".
>> x = [1,2,3,4,5];
>> y = [6,7,8,9,10];
>> pp = spline(x, y);
>> [~, coeffs] = unmkpp(pp);
Each row of "coeffs" has the coefficients for one of the piecewise polynomials at each break.
For example:
>> coeffs(1, :)
is
[0, 0, 1, 6]
representing the function "x + 6".
