cubic spline interpolation for surface

Hi,

I'm a Japanese enginner. I'd linke to know how to interpolate with cubic spline for surface. The polynominal function for curve is easy. Thera ara many references. It's a 4th order equation of 1 variable. But I dont' know How the function for surface is.

(a3*x^3 + a2*x^2 + a1*x + a0)*(b3*y^3 + b2*y^2 + b1*y + b0) ???

It seems that applying partial differential conditions to the above function get the solution.

MATLAB has this interpolation m-file, but it's not easy to understand it.

So please let me know.

Hiroki