Extending the mesh using traits

This examples shows:

• How to extend the behaviour of entities using traits.

In the previous tutorial we used attributes and changed the type of the data types Point, Normal, TexCoord, and Color. But we can do even more with traits. We can change the behaviour of the mesh entities Vertex, Face, Edge, and Halfedge.

One goal in the design was a highly customizable data structure. Using the traits technique makes it possible. We pick up the smoother again and show an alternative way to implement it. Now we place the necessary data and the functions in the vertex itself

struct MyTraits : public OpenMesh::DefaultTraits
{
  // store barycenter of neighbors in this member
  VertexTraits
  {
  private:
    Point  cog_;
  public:
 
    VertexT() : cog_( Point(0.0f, 0.0f, 0.0f ) ) { }
 
    const Point& cog() const { return cog_; }
    void set_cog(const Point& _p) { cog_ = _p; }
  };

Note the definition of the vertex entity. We use the supplied define VertexTraits (which resolves in a rather inconvenient template definition). Similary we can use the defines FaceTraits, EdgeTraits, and HalfedgeTraits to extend these entities. Now we enhanced the vertex, with the additional member variable cog_, and the get/set-method pair to access the new member.

As before we compute in a first loop the barycenters for all vertices and store the information at the vertices

      mesh.data(*v_it).set_cog(cog / valence);

In the second pass we set the new position of each vertex

        mesh.set_point( *v_it, mesh.data(*v_it).cog());

It looks neat, but on the other hand we can't remove the data anymore as we could do with properties! By using traits one creates a 'static' configuration, which can't be changed during runtime.

The complete source looks like this:

#include <iostream>
#include <vector>
// -------------------- OpenMesh
#include <OpenMesh/Core/IO/MeshIO.hh>
#include <OpenMesh/Core/Mesh/TriMesh_ArrayKernelT.hh>
#include <OpenMesh/Core/Mesh/Traits.hh>
 
struct MyTraits : public OpenMesh::DefaultTraits
{
  // store barycenter of neighbors in this member
  VertexTraits
  {
  private:
    Point  cog_;
  public:
 
    VertexT() : cog_( Point(0.0f, 0.0f, 0.0f ) ) { }
 
    const Point& cog() const { return cog_; }
    void set_cog(const Point& _p) { cog_ = _p; }
  };
};
 
typedef OpenMesh::TriMesh_ArrayKernelT<MyTraits>  MyMesh;
typedef OpenMesh::TriMesh_ArrayKernelT<>          MyMesh2;
 
// ---------------------------------------------------------------------------
#define SIZEOF( entity,b ) \
  std::cout << _prefix << "size of " << #entity << ": " \
            << sizeof( entity ) << std::endl;          \
  b += sizeof( entity )
 
template <typename Mesh> 
void print_size(const std::string& _prefix = "")
{
  size_t total=0;
  SIZEOF(typename Mesh::Vertex, total);
  SIZEOF(typename Mesh::Halfedge, total);
  SIZEOF(typename Mesh::Edge, total);
  SIZEOF(typename Mesh::Face, total);
  std::cout << _prefix << "total: " << total << std::endl;
}
 
#undef SIZEOF
// ---------------------------------------------------------------------------
 
 
int main(int argc, char **argv)
{
  MyMesh  mesh;
 
  // check command line options
  if (argc < 4 || argc > 5) 
  {
    std::cerr << "Usage:  " << argv[0] << " [-s] #iterations infile outfile\n";
    exit(1);
  }
 
  int idx=2;
 
  // display size of entities of the enhanced and the default mesh type
  // when commandline option '-s' has been used.
  if (argc == 5)
  {
    if (std::string("-s")==argv[idx-1])
    {
      std::cout << "Enhanced mesh size statistics\n";
      print_size<MyMesh>("  ");
      
      std::cout << "Default mesh size statistics\n";
      print_size<MyMesh2>("  ");
    }
    // else ignore!
    ++idx;
  }
 
 
  // read mesh from stdin
  std::cout<< " Input mesh: " << argv[idx] << std::endl;
  if ( ! OpenMesh::IO::read_mesh(mesh, argv[idx]) )
  {
    std::cerr << "Error: Cannot read mesh from " << argv[idx] << std::endl;
    return 0;
  }
 
 
 
  // smoothing mesh argv[1] times
  MyMesh::VertexIter          v_it, v_end(mesh.vertices_end());
  MyMesh::VertexVertexIter    vv_it;
  MyMesh::Point               cog;
  MyMesh::Scalar              valence;
  unsigned int                i, N(atoi(argv[idx-1]));
 
  std::cout<< "Smooth mesh " << N << " times\n";
 
  for (i=0; i < N; ++i)
  {
    for (v_it=mesh.vertices_begin(); v_it!=v_end; ++v_it)
    {
      cog[0] = cog[1] = cog[2] = valence = 0.0;
      
      for (vv_it=mesh.vv_iter(*v_it); vv_it.is_valid(); ++vv_it)
      {
        cog += mesh.point( *vv_it );
        ++valence;
      }
 
      mesh.data(*v_it).set_cog(cog / valence);
    }
    
    for (v_it=mesh.vertices_begin(); v_it!=v_end; ++v_it)
      if (!mesh.is_boundary(*v_it))
        mesh.set_point( *v_it, mesh.data(*v_it).cog());
  }
 
 
  // write mesh to stdout
  std::cout<< "Output mesh: " << argv[idx+1] << std::endl;
 
  if ( ! OpenMesh::IO::write_mesh(mesh, argv[idx+1]) )
  {
      std::cerr << "Error: cannot write mesh to " << argv[idx+1] << std::endl;
      return 0;
  }
  return 1;
}