CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

A writable property map, as describe in the concept of the boost web
site is a mecanism that allow to generically associate a property
(the value type) to a given object (the key).

If you want to associate a normal to a facet of a Polyhedron, here is a 
simple example (not tested):


struct Poly_normal_map{

  typedef Polyhedron::Facet_handle key_type;
  typedef Kernel::Vector_3 value_type;
  typedef boost::writable_property_map_tag category;

  std::map<Polyhedron::Facet_handle, Kernel::Vector_3>* normals;

  Poly_normal_map() : normals(NULL)
  {}

  Poly_normal_map(std::map<Polyhedron::Facet_handle,
                           Kernel::Vector_3>& n)
    : normals(*n)
  {}

  friend void put(Poly_normal_map pnm,
                  Polyhedron::Facet_handle fh,
                  const Kernel::Vector_3& n
  {
    CGAL_assertion(pnm.normals!=0);
    pnm.normals.insert( std::make_pair(fh, n) );
  }
};

std::map<Polyhedron::Facet_handle, Kernel::Vector_3> normals;
Poly_normal_map vpm(normals); //vpm is you writable property map


If you want to make the property map read-write, you can add
a get function:

  friend const value_type& get(Poly_normal_map pnm,
                               Polyhedron::Facet_handle fh)
  {
    CGAL_assertion(pnm.normals!=0);
    return pnm.normals[fh];
  }

and update the category tag.

Note that the code I've written is roughly equivalent to using a
boost::associative_property_map 
(http://www.boost.org/doc/libs/1_62_0/libs/property_map/doc/associative_property_map.html) 
:

std::map<Polyhedron::Facet_handle, Kernel::Vector_3> normals;
boost::associative_property_map<
         std::map<Polyhedron::Facet_handle,
                  Kernel::Vector_3> > vpm(normals);

If you're not stuck with Polyhedron_3, you can use class
Surface_mesh that comes with its own property mechanism
as illustrated in this example:

http://doc.cgal.org/latest/Polygon_mesh_processing/Polygon_mesh_processing_2compute_normals_example_8cpp-example.html

Sebastien.



On 10/31/2016 12:46 PM, Iason Manolas wrote:
> Hi,
>
> I am new to CGAL. I have constructed a CGAL::Polyhedron_3 and now I want
> to compute the normal vector for each of its vertices. So I found the
> Polygon Mesh Processing package which happens to have a function
> <http://doc.cgal.org/latest/Polygon_mesh_processing/group__PMP__normal__grp.html#gad46aaf003999b2939f3f9e4f6fa58d0e>
> doing exactly what I want . In the above link I found out that the
> second argument was of type  VertexNormalMap which is a model of
> WritablePropertyMap. There was a link
> <http://www.boost.org/doc/libs/1_62_0/libs/property_map/doc/WritablePropertyMap.html>
> to boost libraries explaining(?) what writable property map is .
>
> Unfortunately this is my first encounter with property maps and
> consequently I am a bit confused..
> I also had a look at cgal's explanation of property maps page
> <http://doc.cgal.org/latest/Property_map/index.html >, but I did not
> understand much..
>
> Could anyone suggest a way of creating the property map I need?
>
> Best Regards,
> Iason