两个基本操作-边的翻转和折叠

原文出处：http://www.openmesh.org/media/Documentations/OpenMesh-Doc-Latest/a00034.html
  本节介绍OpenMesh中提供的两种基本操作，及对边的翻转和折叠操作。其中翻转操作是针对三角网格的，而折叠操作就是折叠网格中的相邻顶点。

三角网格中的边的翻转

  首先考虑两个相邻的三角形，那么它们的公共可以有两种不同的方向。调用函数OpenMesh::TriConnectivity::flip(EdgeHandle _eh)可以将指定的边翻转为另一个方向，图示如下：

  下面为一段例子代码：

TriMesh mesh;
// Add some vertices
TriMesh::VertexHandle vhandle[4];
vhandle[0] = mesh.add_vertex(MyMesh::Point(0, 0, 0));
vhandle[1] = mesh.add_vertex(MyMesh::Point(0, 1, 0));
vhandle[2] = mesh.add_vertex(MyMesh::Point(1, 1, 0));
vhandle[3] = mesh.add_vertex(MyMesh::Point(1, 0, 0));
// Add two faces
std::vector<TriMesh::VertexHandle> face_vhandles;
face_vhandles.push_back(vhandle[2]);
face_vhandles.push_back(vhandle[1]);
face_vhandles.push_back(vhandle[0]);
mesh.add_face(face_vhandles);
face_vhandles.clear();
face_vhandles.push_back(vhandle[2]);
face_vhandles.push_back(vhandle[0]);
face_vhandles.push_back(vhandle[3]);
mesh.add_face(face_vhandles);
// Now the edge adjacent to the two faces connects
// vertex vhandle[0] and vhandle[2].
// Find this edge and then flip it
for(TriMesh::EdgeIter it = mesh.edges_begin(); it != mesh.edges_end(); ++it) {
        if(!mesh.is_boundary(*it)) {
                // Flip edge
                mesh.flip(*it);
        }
}
// The edge now connects vertex vhandle[1] and vhandle[3].

边的折叠

  折叠边主要是让相邻的顶点合并（其实指定的边和其相对的半边都被删除了）。OpenMesh中用于折叠边的函数为OpenMesh::PolyConnectivity::collapse(HalfedgeHandle _heh)。对于指定的半边，半边的起点将被折叠到其终点上（就是起点和半边都被删除了），示例如下图所示，需要注意的是，在折叠边操作之后，有可能会导致网格拓扑的不一致，可以调用函数OpenMesh::PolyConnectivity::is_collapse_ok()进行验证。

注意
为了使用折叠和删除操作，需要请求基本元的状态属性（后续介绍）。

查看下面的例子代码：

PolyMesh mesh;
// Request required status flags
mesh.request_vertex_status();
mesh.request_edge_status();
mesh.request_face_status();
// Add some vertices as in the illustration above
PolyMesh::VertexHandle vhandle[7];
vhandle[0] = mesh.add_vertex(MyMesh::Point(-1, 1, 0));
vhandle[1] = mesh.add_vertex(MyMesh::Point(-1, 3, 0));
vhandle[2] = mesh.add_vertex(MyMesh::Point(0, 0, 0));
vhandle[3] = mesh.add_vertex(MyMesh::Point(0, 2, 0));
vhandle[4] = mesh.add_vertex(MyMesh::Point(0, 4, 0));
vhandle[5] = mesh.add_vertex(MyMesh::Point(1, 1, 0));
vhandle[6] = mesh.add_vertex(MyMesh::Point(1, 3, 0));
// Add three quad faces
std::vector<PolyMesh::VertexHandle> face_vhandles;
face_vhandles.push_back(vhandle[1]);
face_vhandles.push_back(vhandle[0]);
face_vhandles.push_back(vhandle[2]);
face_vhandles.push_back(vhandle[3]);
mesh.add_face(face_vhandles);
face_vhandles.clear();
face_vhandles.push_back(vhandle[1]);
face_vhandles.push_back(vhandle[3]);
face_vhandles.push_back(vhandle[5]);
face_vhandles.push_back(vhandle[4]);
mesh.add_face(face_vhandles);
face_vhandles.clear();
face_vhandles.push_back(vhandle[3]);
face_vhandles.push_back(vhandle[2]);
face_vhandles.push_back(vhandle[6]);
face_vhandles.push_back(vhandle[5]);
mesh.add_face(face_vhandles);
// Now find the edge between vertex vhandle[2]
// and vhandle[3]
for(PolyMesh::HalfedgeIter it = mesh.halfedges_begin(); it != mesh.halfedges_end(); ++it) {
  if( mesh.to_vertex_handle(*it) == vhandle[3] &&
      mesh.from_vertex_handle(*it) == vhandle[2])
  {
    // Collapse edge
    mesh.collapse(*it);
    break;
  }
}
// Our mesh now looks like in the illustration above after the collapsing.