在CGAL和可視化中創建3D Alpha形狀

Question

我是一個具有基本C++經驗的新CGAL用戶，試圖找到一組點的3D Alpha形狀。我正在使用ex_alpha_shapes_3示例代碼，然後使用saving CGAL alpha shape surface mesh中的說明來顯示結果。一切似乎正常工作，但是當我嘗試通過與

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

假設第三可變替換

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

改變的α的值是阿爾法的值（= 1）和改變該值每次都沒有獲得結果的變化。

更具體地說，在我的一組粒子中，有些與大多數是分離的，我想用分開的體積（類似於Figure 41.1）使用凹形或alpha形狀來表示它們。目前我得到的結果（使用Tecplot進行可視化）是： 正如您所看到的，分離的粒子與其他粒子相連。最後，我也附上我的代碼。我希望在這個問題上有任何幫助。

#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

如果你想過濾的東西出來，你需要採取阿爾法形狀值小於as.find_optimal_alpha（1）返回的一個。我建議運行阿爾法形狀的演示，其中包含一個滑塊，該滑塊將根據alpha的值顯示阿爾法形狀。輸入文件應該帶有.pts擴展名，並且必須包含點數，然後是點的座標。