c++ - 在 CGAL 中创建 3D Alpha 形状和可视化

Question

我是具有基本 C++ 经验的 CGAL 新用户，试图找到一组点的 3D Alpha 形状。我正在使用 ex_alpha_shapes_3示例代码，然后使用 saving CGAL alpha shape surface mesh 中的说明用于结果的可视化。一切似乎都正常工作，但是当我尝试通过替换

来更改 alpha 的值时

    Alpha_shape_3 as(lp.begin(),lp.end());

与

    Alpha_shape_3 as(lp.begin(),lp.end(),1, Alpha_shape_3::GENERAL);

假设第三个变量是alpha的值(=1)，每次都改变这个值，得到的结果没有变化。

更具体地说，在我的一组粒子中，一些粒子与大多数粒子分离，我想使用凹包或 alpha 形状用单独的体积(类似于 Figure 41.1 )来表示它们。目前我得到的(使用 Tecplot 进行可视化)是: 如您所见，分离的粒子与其他粒子相连。最后，我还附上了我的代码。对于此事，我将不胜感激。

    #include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
    #include <CGAL/Delaunay_triangulation_3.h>
    #include <CGAL/Alpha_shape_3.h>

    #include <iostream>
    #include <fstream>
    #include <list>
    #include <cassert>

    typedef CGAL::Exact_predicates_inexact_constructions_kernel Gt;

    typedef CGAL::Alpha_shape_vertex_base_3<Gt>          Vb;
    typedef CGAL::Alpha_shape_cell_base_3<Gt>            Fb;
    typedef CGAL::Triangulation_data_structure_3<Vb,Fb>  Tds;
    typedef CGAL::Delaunay_triangulation_3<Gt,Tds>       Triangulation_3;
    typedef CGAL::Alpha_shape_3<Triangulation_3>         Alpha_shape_3;

    typedef Gt::Point_3                                  Point;
    typedef Alpha_shape_3::Alpha_iterator               Alpha_iterator;

    using namespace std;

    int main()
    {
      std::list<Point> lp;

      //read input
      std::ifstream is("./data/finalwater4.dat");
      int n;
      is >> n;
      std::cout << "Reading " << n << " points " << std::endl;
      Point p;
      for( ; n>0 ; n--)    {
        is >> p;
        lp.push_back(p);
      }

      // compute alpha shape
    //  Alpha_shape_3 as(lp.begin(),lp.end());
      Alpha_shape_3 as(lp.begin(),lp.end(),0.001, Alpha_shape_3::GENERAL);

      // find optimal alpha value
      Alpha_iterator opt = as.find_optimal_alpha(1);
      std::cout << "Optimal alpha value to get one connected component is "
            <<  *opt    << std::endl;
      as.set_alpha(*opt);
      assert(as.number_of_solid_components() == 1);

      /// the rest of the code, prepares the output to be written into a file

      /// collect all regular facets (fetch regular facets from as and inserts in facets)
      std::vector<Alpha_shape_3::Facet> facets;
      as.get_alpha_shape_facets(std::back_inserter(facets), Alpha_shape_3::REGULAR);

      std::stringstream pts;
      std::stringstream ind;

      std::size_t nbf=facets.size();
      for (std::size_t i=0;i<nbf;++i)
      { 
        //To have a consistent orientation of the facet, always consider an exterior cell
        if ( as.classify( facets[i].first )!=Alpha_shape_3::EXTERIOR )
          facets[i]=as.mirror_facet( facets[i] );
        CGAL_assertion(  as.classify( facets[i].first )==Alpha_shape_3::EXTERIOR  );

        int indices[3]={
          (facets[i].second+1)%4,
          (facets[i].second+2)%4,
          (facets[i].second+3)%4,
        };

        /// according to the encoding of vertex indices, this is needed to get
        /// a consistent orienation
        if ( facets[i].second%2==0 ) std::swap(indices[0], indices[1]);


        pts << 
        facets[i].first->vertex(indices[0])->point() << "\n" <<
        facets[i].first->vertex(indices[1])->point() << "\n" <<
        facets[i].first->vertex(indices[2])->point() << "\n"; 
        ind << 3*i+1 << " " << 3*i+2 << " " << 3*i+3 << "\n";
      }

      ofstream myfile;
      myfile.open ("output.dat");
      myfile << "variables = x, y, z\n";
      myfile << "zone n="<< 3*nbf << " , e=" << nbf << " , f=fepoint, et=triangle\n";
      myfile << pts.str();
      myfile << ind.str();
      myfile.close();

      return 0;
    }

Answer 1

如果你想过滤掉东西，你需要取一个 alpha 形状，其值小于 as.find_optimal_alpha(1) 返回的值。我建议运行 alpha-shape 演示，它有一个 slider ，根据 alpha 的值显示 alpha-shape 的样子。输入文件的扩展名应为 .pts，并且必须包含点数，后跟点的坐标。