3-D surface normals.
[Nx,Ny,Nz] = SURFNORM(X,Y,Z) returns the components of the 3-D
surface normal for the surface with components (X,Y,Z). The
normal is unnormalized.
[Nx,Ny,Nz] = SURFNORM(Z) returns the surface normal components
for the surface Z.
Without lefthand arguments, SURFNORM(X,Y,Z) or SURFNORM(Z)
plots the surface with the normals emanating from it. Normals
are not shown for surface elements that face away from the viewer.
The surface normals returned are based on a bicubic fit of