【OpenMesh】创建一个正方体

原文出处： http://openmesh.org/Documentation/OpenMesh-Doc-Latest/tutorial.html
这个例程演示了：

• 如何声明MyMesh
• 如何添加顶点和面到Mesh中
• 如何使用IO函数写入网格数据

每一个程序第一步都是定义MyMesh类型。OpenMesh支持常见的多边形网格（面由可变顶点数的多边形组成）以及特殊的三角网格（所有面都是三角形的）。在这个例子中我们希望构建一个六面的正方体，因此我们选择多边形网格。

OpenMesh同样支持不同的网格内核，指定顶点、边和面是如何存储的（参考Mesh Kernels）。然而，存储器必须提供一个阵列接口。在这篇指南中我们使用提供的ArrayKernel。预先定义的TriMesh/PolyMesh和内核包含在OpenMesh/src/OpenMesh/Core/Mesh中，我们使用PolyMesh_ArrayKernelT。

#include <OpenMesh/Core/Mesh/PolyMesh_ArrayKernelT.hh>
typedef OpenMesh::PolyMesh_ArrayKernelT<> MyMesh;

限制我们定义了MyMesh类型，我们只用添加8个顶点和6个四边形来构建一个正方体。使用add_vertex方法添加顶点。这个函数获取坐标值返回添加顶点的handle。我们保存所有的顶点到array中，我们需要这些顶点指定面。

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));

为了添加面到网格中，我们必须构建一个vector保存面的顶点的handle。这个vector当做add_face的参数被传入。下列的代码使用四个点创建一个面：

std::vector<MyMesh::VertexHandle> face_vhandles;
face_vhandles.clear();
face_vhandles.push_back(vhandle[0]);
face_vhandles.push_back(vhandle[1]);
face_vhandles.push_back(vhandle[2]);
face_vhandles.push_back(vhandle[3]);
mesh.add_face(face_vhandles);

面的方向由提供的点的方向来定义：如果你朝着多边形的前面看，这些点的方向是逆时针的。
当创建完所有的面的时候，我们希望将网格结果写入标准输出中。OpenMesh提供基本的输入输出方法，定义在名字空间OpenMesh::IO：

if ( !OpenMesh::IO::write_mesh(mesh, "output.off") )

使用IO的函数首先应包含MeshIO.hh

#include <OpenMesh/Core/IO/MeshIO.hh>
#include <OpenMesh/Core/Mesh/PolyMesh_ArrayKernelT.hh>

完整的代码像这样：

//=============================================================================
//
// 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>
// ----------------------------------------------------------------------------
typedef OpenMesh::PolyMesh_ArrayKernelT<> MyMesh;
// ----------------------------------------------------------------------------
// Build a simple cube and write it to std::cout

int main()
{
MyMesh mesh;
// generate vertices
MyMesh::VertexHandle vhandle[8];
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> face_vhandles;
face_vhandles.clear();
face_vhandles.push_back(vhandle[0]);
face_vhandles.push_back(vhandle[1]);
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[7]);
face_vhandles.push_back(vhandle[6]);
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[1]);
face_vhandles.push_back(vhandle[0]);
face_vhandles.push_back(vhandle[4]);
face_vhandles.push_back(vhandle[5]);
mesh.add_face(face_vhandles);
face_vhandles.clear();
face_vhandles.push_back(vhandle[2]);
face_vhandles.push_back(vhandle[1]);
face_vhandles.push_back(vhandle[5]);
face_vhandles.push_back(vhandle[6]);
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[7]);
mesh.add_face(face_vhandles);
face_vhandles.clear();
face_vhandles.push_back(vhandle[0]);
face_vhandles.push_back(vhandle[3]);
face_vhandles.push_back(vhandle[7]);
face_vhandles.push_back(vhandle[4]);
mesh.add_face(face_vhandles);
// 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;
}