OpenMesh 教程：删除网格元素

删除网格元素

翻译，原文链接

本文将说明如何从网格中删除面和顶点。

这里使用的网格为第一篇教程 (创建立方体) . 如果我们想要从网格中删除顶点、面或者边，就必须先打开网格的状态属性 OpenMesh::Attributes::Status 用来保存元素是否删除。一般用动态方式定义状态变量，代码如下

mesh.request_face_status();
mesh.request_edge_status();
mesh.request_vertex_status();

创建立方体后，我们就可以用 delete_vertices(), delete_faces(), delete_edges() 分别删除顶点、面和边。需要注意的是，我们不能够直接删除半边，因为它们都是根据边的删除与否来确定是否删除。

要想知道一个元素是否被标记为删除，可以用下面的代码

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

在下面的代码中，出去唯一的面外，其它的面连同顶点全部删除

// 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);

在最后一步，我们需要调用 garbage_collection() 将前面标记为待删除的元素从内存中删除。

完整代码如下

#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;
}