【OpenMesh】删除几何元素

这个小例子演示如何移除网格中的面和顶点。
我们使用第一个例子中的正方体。
如果我们希望我们的网格能够移除顶点，面或者边，我们必须为网格类扩展默认特性。顶点，面和Halfedge需要OpenMesh::Attributes::Status属性，这个属性用于当一个元素被删除的时候保持其已删除标记。
定义网格特性的结构如下：

struct MyTraits : public OpenMesh::DefaultTraits
{
VertexAttributes(OpenMesh::Attributes::Status);
FaceAttributes(OpenMesh::Attributes::Status);
EdgeAttributes(OpenMesh::Attributes::Status);
};

当我们创建立方体图形后，我们可以简单的调用delete_vertices()删除面和顶点(delete_face()或者delete_edge())。
注意，没有方法能够直接删除Halfedge，因为当边被删除的时候Halfedge会自动的删除。
一个元素是否会被标记为删除可以通过下面的函数来请求

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

handle可以是顶点的边的或者面的。
在这个例子中我们删除删除所有的面只留下一个面以及这个面对应的顶点。代码如下：

// 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);

删除掉的顶点和面被标记为"deleted"。调用garbage_collection()来将删除数据移除内存。
// Delete all elements that are marked as deleted
// from memory.
mesh.garbage_collection();

完整代码：

//=============================================================================
// 
// OpenMesh 
// Copyright (C) 2001-2005 by Computer Graphics Group, RWTH Aachen 
// www.openmesh.org 
// 
//-----------------------------------------------------------------------------
// 
// License 
// 
// This library is free software; you can redistribute it and/or modify it 
// under the terms of the GNU Library General Public License as published 
// by the Free Software Foundation, version 2. 
// 
// This library is distributed in the hope that it will be useful, but 
// WITHOUT ANY WARRANTY; without even the implied warranty of 
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 
// Library General Public License for more details. 
// 
// You should have received a copy of the GNU Library General Public 
// License along with this library; if not, write to the Free Software 
// Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. 
// 
//-----------------------------------------------------------------------------
// 
// $Revision: 83 $
// $Date: 2009-02-27 17:31:45 +0100 (Fri, 27 Feb 2009) $
// 
//=============================================================================
#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
{
VertexAttributes(OpenMesh::Attributes::Status);
FaceAttributes(OpenMesh::Attributes::Status);
EdgeAttributes(OpenMesh::Attributes::Status);
};
typedef OpenMesh::PolyMesh_ArrayKernelT<MyTraits> MyMesh;
// ----------------------------------------------------------------------------
// Build a simple cube and delete it except one face

int main()
{
MyMesh mesh;
// 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;
}