NRBEXTRUDE: Construct a NURBS surface by extruding a NURBS curve, or 
  construct a NURBS volume by extruding a NURBS surface.
 
 Calling Sequence:
 
   srf = nrbextrude(crv,vec);
 
 INPUT:
 
   crv		: NURBS curve or surface to extrude, see nrbmak.
 
   vec		: Vector along which the entity is extruded.

 OUTPUT: 
 
   srf		: NURBS surface or volume constructed.
 
 Description:
 
   Constructs either a NURBS surface by extruding a NURBS curve along a  
   defined vector, or a NURBS volume by extruding a NURBS surface. In the 
   first case, the NURBS curve forms the U direction of the surface edge, and
   is extruded along the vector in the V direction. In the second case, the 
   original surface forms the U and V direction of the volume, and is extruded
   along the W direction.

 Examples:
 
   Form a hollow cylinder by extruding a circle along the z-axis.

   srf = nrbextrude(nrbcirc, [0,0,1]);

    Copyright (C) 2000 Mark Spink
    Copyright (C) 2010 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.

Demonstration 1

The following code

 crv = nrbtestcrv;
 srf = nrbextrude(crv,[0 0 5]);
 nrbplot(srf,[40 10]);
 title('Extrusion of a test curve along the z-axis');
 hold off

Produces the following figure

Figure 1

Demonstration 2

The following code

 crv1 = nrbcirc (1, [0 0], 0, pi/2);
 crv2 = nrbcirc (2, [0 0], 0, pi/2);
 srf  = nrbruled (crv1, crv2);
 vol  = nrbextrude (srf, [0 0 1]);
 nrbplot (vol, [30 10 10])
 title ('Extrusion of the quarter of a ring')

Produces the following figure

Figure 1

Demonstration 3

The following code

 srf = nrbtestsrf;
 vol = nrbextrude(srf, [0 0 10]);
 nrbplot(vol,[20 20 20]);
 title('Extrusion of a test surface along the z-axis');
 hold off

Produces the following figure

Figure 1

Package: nurbs