The polyhedron can be parameterized by a \italic{traits} class in
order to extend the vertex, halfedge and facet primitives. In this
tutorial all primitives (facets, halfedges and vertices) are
extended. The facet is extended with a normal and with a
general-purpose integer tag:

{ \scriptsize
\begin{verbatim}

template <class	Refs, class T, class P, class Norm>
class Enriched_facet : 
  public CGAL::HalfedgeDS_face_base<Refs, T>
{
  // tag
  int m_tag;

  // normal
  Norm m_normal;

public:

  // no  constructors to  repeat,  since  only
  // default constructor mandatory
  Enriched_facet()
  {
  }

  // tag
  const int&  tag()  {  return m_tag;  }
  void tag(const int& t)  {  m_tag  =  t; }

  // normal
  typedef  Norm Normal_3;
  Normal_3&  normal() { return  m_normal;  }
  const  Normal_3&  normal() const { return  m_normal;  }
};
\end{verbatim}}

The halfedge is extended with a general-purpose tag and a binary tag
to indicate wether it belongs to the control mesh or not. The latter
tag is used to superimpose the control mesh as shown in
Fig.\ref{fig:teaser}.

{ \scriptsize
\begin{verbatim}

template <class  Refs, class Tprev, class Tvertex, 
          class Tface, class Norm>
class Enriched_halfedge : public 
  CGAL::HalfedgeDS_halfedge_base<Refs,Tprev,Tvertex,Tface>
{
private:

  // tag
  int  m_tag; 

  // option  for control edge superimposing
  bool m_control_edge; 

public:

  // life  cycle
  Enriched_halfedge()
  {
    m_control_edge = true;
  }

  // tag
  const int& tag() const { return m_tag;  }
  int& tag() { return m_tag;  }
  void tag(const int& t)  {  m_tag  =  t; }

  // control edge  
  bool& control_edge()  { return m_control_edge; }
  const bool& control_edge()  const { return m_control_edge; }
  void control_edge(const bool& flag) { m_control_edge  =  flag;  }
};

\end{verbatim}}

The vertex is extended with a normal and a general-purpose integer
tag:

{ \scriptsize
\begin{verbatim}


template <class  Refs, class T, class P, class Norm>
class Enriched_vertex :  
  public CGAL::HalfedgeDS_vertex_base<Refs, T, P>
{
  // tag
  int  m_tag; 

  // normal
  Norm m_normal;

public:
  // life  cycle
  Enriched_vertex() {}
  // repeat  mandatory  constructors
  Enriched_vertex(const  P& pt)
    :  CGAL::HalfedgeDS_vertex_base<Refs, T,  P>(pt)
  {
  }

  // normal
  typedef  Norm Normal_3;
  Normal_3&  normal() { return  m_normal;  }
  const  Normal_3&  normal() const { return  m_normal;  }

  // tag
  int& tag() {  return m_tag;  }
  const int& tag() const {  return m_tag;  }
  void tag(const int& t)  {  m_tag  =  t; }
};

\end{verbatim}}



A redefined items class for the polyhedron uses the class wrapper
mechanism to embedd all three extended primitives within one unique
class.

{ \scriptsize
\begin{verbatim}

struct Enriched_items : public CGAL::Polyhedron_items_3
{
    // wrap  vertex
    template <class  Refs,  class  Traits>
    struct Vertex_wrapper
    {
        typedef  typename Traits::Point_3  Point;
        typedef  typename Traits::Vector_3  Normal;
        typedef  Enriched_vertex<Refs,
                          CGAL::Tag_true,
                          Point,
                          Normal>  Vertex;
    };

    // wrap  face
    template <class  Refs,  class  Traits>
    struct Face_wrapper
    {
        typedef  typename Traits::Point_3  Point;
        typedef  typename Traits::Vector_3  Normal;
        typedef  Enriched_facet<Refs,
                         CGAL::Tag_true,
                         Point,
                         Normal> Face;
    };

    // wrap  halfedge
    template <class  Refs,  class  Traits>
    struct Halfedge_wrapper
    {
        typedef  typename Traits::Vector_3  Normal;
        typedef  Enriched_halfedge<Refs,
                            CGAL::Tag_true,
                            CGAL::Tag_true,
                            CGAL::Tag_true,
                            Normal>  Halfedge;
    };
};
\end{verbatim}}

The trait class is then used for templating a polyhedron
\italic{Enriched\_polyhedron}:

{ \scriptsize
\begin{verbatim}

template <class  kernel,  class  items>
class  Enriched_polyhedron :
  public CGAL::Polyhedron_3<kernel,items>
{
  //...
};
\end{verbatim}}

The corresponding instanciation of an enriched polyhedron follows:

{ \scriptsize
\begin{verbatim}

#include <CGAL/Simple_cartesian.h>
#include "enriched_polyhedron.h"

typedef double number_type;
typedef CGAL::Simple_cartesian<number_type> kernel;

Enriched_polyhedron<kernel,Enriched_items> polyhedron;

\end{verbatim}}