Without going into details of the solution (its similar to curves), one
example is given. It is constructed in such a way, that multiple knots are
inserted in the given surface from figure 10 and the surface is
disturbed with help of random numbers.
Figure: The isophotes of the disturbed Bézier-spline surface and after
fairing with the extended functional (max_error=, sec.).
Figure: The isophotes of the faired Bézier-spline surface with previous
knot removal (max_error=, sec.).