How do I calculate the rate of change of surface normals?

Hi all,

Suppose I have a surfave defines by

M = peaks(32);

then the normal veoctors on that surface can be calculated by

[x,y,z] = surfnorm(M);

You can see the vecots surf(M) hold all surfnorm(M)

How can I calculate the rate at which the surface normals change in both the X and Y directions.

Thanx

Petri