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

  the data.