NRBEXTRACT: construct NURBS curves by extracting the boundaries of a NURBS surface, or NURBS surfaces by extracting the boundary of a NURBS volume.
 It only works for geometries constructed with open knot vectors. For a NURBS curve, 
 it returns two structures with the the boundary knots and control points.
 
 Calling Sequence:
 
   crvs = nrbextract(surf, [sides]);
 
 INPUT:
 
   surf        : NURBS surface or volume, see nrbmak.
   sides       : the list of boundary sides to be extracted
 
 OUTPUT: 
 
   crvs        : array of NURBS curves or NURBS surfaces extracted.
 
 Description:
 
  Constructs either an array of four NURBS curves, by extracting the boundaries
  of a NURBS surface, or an array of six surfaces, by extracting the boundaries
  of a NURBS volume. The new entities are ordered in the following way

    1: U = 0
    2: U = 1
    3: V = 0
    4: V = 1
    5: W = 0 (only for volumes)
    6: W = 1 (only for volumes)

    Copyright (C) 2010,2014,2015 Rafael Vazquez

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

Package: nurbs