Deleting geometry elements

This small example shows how to remove faces and vertices from a mesh.

We basically use the geometry created in First Steps - Building a cube.

If we want our mesh class to be able to remove vertices, faces or edges we have to extend the default traits for our mesh class. Vertices, faces and (half-)edges need the OpenMesh::Attributes::Status attribute which is used to hold the flag "deleted" if an element is deleted.

Instead of defining the required attributes via traits on compile-time, it is possible to request attributes dynamically on run-time by requesting them. In this example, we want to delete faces, edges and vertices, therefore requesting the status attribute is required.

Note

You have to request attributes before you use them, if you don't enable them via traits.

  // the request has to be called before a vertex/face/edge can be deleted. it grants access to the status attribute
  mesh.request_face_status();
  mesh.request_edge_status();
  mesh.request_vertex_status();

After having created the geometry of the cube one can delete faces and vertices by simply calling delete_vertices() (delete_faces() or delete_edges() respectively).

Note that there is actually no way to directly delete halfedges since they are automatically affected when the parent edge is marked as deleted!

The status whether an element is marked as deleted can be requested by

mesh.status(handle).deleted();  // true if element handle is marked as deleted
                                // false otherwise

where handle is either a vertex-, edge- or face-handle.

In this example we delete all faces except one and the corresponding vertices. The code looks like this

  // Delete face 0
  mesh.delete_face(fhandle[0], false);
  // ... face 2
  mesh.delete_face(fhandle[2], false);
  // ... face 3
  mesh.delete_face(fhandle[3], false);
  // ... face 4
  mesh.delete_face(fhandle[4], false);
  // ... face 5
  mesh.delete_face(fhandle[5], false);
  

  // If isolated vertices result in a face deletion
  // they have to be deleted manually. If you want this
  // to happen automatically, change the second parameter
  // to true.

  // Now delete the isolated vertices 0, 1, 2 and 3
  mesh.delete_vertex(vhandle[0], false);
  mesh.delete_vertex(vhandle[1], false);
  mesh.delete_vertex(vhandle[2], false);
  mesh.delete_vertex(vhandle[3], false);

Now the deleted faces and vertices are marked as "deleted" internally. Call garbage_collection() to definitely remove them from memory.

  // Delete all elements that are marked as deleted
  // from memory.
  mesh.garbage_collection();

The full source code of the example:

/* ========================================================================= *
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/*===========================================================================*\
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 *   $Revision: 736 $                                                         *
 *   $Date: 2012-10-08 09:30:49 +0200 (Mo, 08. Okt 2012) $                   *
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\*===========================================================================*/


#include <iostream>
// -------------------- OpenMesh
#include <OpenMesh/Core/IO/MeshIO.hh>
#include <OpenMesh/Core/Mesh/PolyMesh_ArrayKernelT.hh>
#include <OpenMesh/Core/System/config.h>
#include <OpenMesh/Core/Mesh/Status.hh>

// ----------------------------------------------------------------------------

struct MyTraits : public OpenMesh::DefaultTraits
{
};


typedef OpenMesh::PolyMesh_ArrayKernelT<MyTraits>  MyMesh;


// ----------------------------------------------------------------------------
// Build a simple cube and delete it except one face
  
int main()
{
  MyMesh mesh;

  // the request has to be called before a vertex/face/edge can be deleted. it grants access to the status attribute
  mesh.request_face_status();
  mesh.request_edge_status();
  mesh.request_vertex_status();

  // generate vertices

  MyMesh::VertexHandle vhandle[8];
  MyMesh::FaceHandle   fhandle[6];

  vhandle[0] = mesh.add_vertex(MyMesh::Point(-1, -1,  1));
  vhandle[1] = mesh.add_vertex(MyMesh::Point( 1, -1,  1));
  vhandle[2] = mesh.add_vertex(MyMesh::Point( 1,  1,  1));
  vhandle[3] = mesh.add_vertex(MyMesh::Point(-1,  1,  1));
  vhandle[4] = mesh.add_vertex(MyMesh::Point(-1, -1, -1));
  vhandle[5] = mesh.add_vertex(MyMesh::Point( 1, -1, -1));
  vhandle[6] = mesh.add_vertex(MyMesh::Point( 1,  1, -1));
  vhandle[7] = mesh.add_vertex(MyMesh::Point(-1,  1, -1));


  // generate (quadrilateral) faces

  std::vector<MyMesh::VertexHandle>  tmp_face_vhandles;

  tmp_face_vhandles.clear();
  tmp_face_vhandles.push_back(vhandle[0]);
  tmp_face_vhandles.push_back(vhandle[1]);
  tmp_face_vhandles.push_back(vhandle[2]);
  tmp_face_vhandles.push_back(vhandle[3]);
  fhandle[0] = mesh.add_face(tmp_face_vhandles);
 
  tmp_face_vhandles.clear();
  tmp_face_vhandles.push_back(vhandle[7]);
  tmp_face_vhandles.push_back(vhandle[6]);
  tmp_face_vhandles.push_back(vhandle[5]);
  tmp_face_vhandles.push_back(vhandle[4]);
  fhandle[1] = mesh.add_face(tmp_face_vhandles);
 
  tmp_face_vhandles.clear();
  tmp_face_vhandles.push_back(vhandle[1]);
  tmp_face_vhandles.push_back(vhandle[0]);
  tmp_face_vhandles.push_back(vhandle[4]);
  tmp_face_vhandles.push_back(vhandle[5]);
  fhandle[2] = mesh.add_face(tmp_face_vhandles);
 

  tmp_face_vhandles.clear();
  tmp_face_vhandles.push_back(vhandle[2]);
  tmp_face_vhandles.push_back(vhandle[1]);
  tmp_face_vhandles.push_back(vhandle[5]);
  tmp_face_vhandles.push_back(vhandle[6]);
  fhandle[3] = mesh.add_face(tmp_face_vhandles);


  tmp_face_vhandles.clear();
  tmp_face_vhandles.push_back(vhandle[3]);
  tmp_face_vhandles.push_back(vhandle[2]);
  tmp_face_vhandles.push_back(vhandle[6]);
  tmp_face_vhandles.push_back(vhandle[7]);
  fhandle[4] = mesh.add_face(tmp_face_vhandles);
 

  tmp_face_vhandles.clear();
  tmp_face_vhandles.push_back(vhandle[0]);
  tmp_face_vhandles.push_back(vhandle[3]);
  tmp_face_vhandles.push_back(vhandle[7]);
  tmp_face_vhandles.push_back(vhandle[4]);
  fhandle[5] = mesh.add_face(tmp_face_vhandles);

  // And now delete all faces and vertices
  // except face (vh[7], vh[6], vh[5], vh[4])
  // whose handle resides in fhandle[1]

   
  // Delete face 0
  mesh.delete_face(fhandle[0], false);
  // ... face 2
  mesh.delete_face(fhandle[2], false);
  // ... face 3
  mesh.delete_face(fhandle[3], false);
  // ... face 4
  mesh.delete_face(fhandle[4], false);
  // ... face 5
  mesh.delete_face(fhandle[5], false);
  

  // If isolated vertices result in a face deletion
  // they have to be deleted manually. If you want this
  // to happen automatically, change the second parameter
  // to true.

  // Now delete the isolated vertices 0, 1, 2 and 3
  mesh.delete_vertex(vhandle[0], false);
  mesh.delete_vertex(vhandle[1], false);
  mesh.delete_vertex(vhandle[2], false);
  mesh.delete_vertex(vhandle[3], false);

  // Delete all elements that are marked as deleted
  // from memory.
  mesh.garbage_collection();

  // write mesh to output.obj
  try {
        if ( !OpenMesh::IO::write_mesh(mesh, "output.off") ) {
          std::cerr << "Cannot write mesh to file 'output.off'" << std::endl;
          return 1;
        }
  }
  catch( std::exception& x )
  {
    std::cerr << x.what() << std::endl;
    return 1;
  }
 
  return 0;
}