CGAL users discussion list

[Renato Silveira <>]

Try to tune your alpha parameter to better fit your problem:

"A.set_alpha(value);"

On Sun, Jul 29, 2018 at 12:09 PM bingxin pu <> wrote:

Hi everyone,

I use Basic Alpha Shapes to get boundary points from some 2d points. As shown in attachments, boundary points like a  rectangle is what I want to get. But I failed. Through seeing demo about Alpha_shapes_2, I think that I maybe set a wrong Alpha. I hope that you can give me some suggestions so that I can get correct result.

The wrong boundary points is result of the following code.

1. typedef CGAL::Exact_predicates_inexact_constructions_kernel  K;  
2.   
3. typedef K::FT                                                FT;  
4. typedef K::Point_2                                           Point;  
5. typedef K::Segment_2                                         Segment;   
6.   
7. typedef CGAL::Alpha_shape_vertex_base_2<K>                   Vb;  
8. typedef CGAL::Alpha_shape_face_base_2<K>                     Fb;  
9. typedef CGAL::Triangulation_data_structure_2<Vb,Fb>          Tds;   
10. typedef CGAL::Delaunay_triangulation_2<K,Tds>                Triangulation_2;   
11. typedef CGAL::Alpha_shape_2<Triangulation_2>                 Alpha_shape_2;    
12.   
13. typedef Alpha_shape_2::Alpha_shape_edges_iterator            Alpha_shape_edges_iterator;  
14.   
15.   
16. template <class OutputIterator>  
17. void alpha_edges( const Alpha_shape_2& A, OutputIterator out)  
18. {  
19.   Alpha_shape_edges_iterator it = A.alpha_shape_edges_begin(),  
20.                              end = A.alpha_shape_edges_end();  
21.     
22.   for (; it != end; ++it)  
23.     out++ = A.segment(*it);  
24.     
25. }  
26.   
27. bool file_input(std::list<Point> &points)             
28. {  
29.     std::ifstream is("J:\\General_Random_points2D.pts", std::ios::in);  
30.   if(is.fail())  
31.   {  
32.     std::cerr << "unable to open file for input" << std::endl;  
33.     return false;  
34.   }  
35.   Point p;  
36.   for (int i = 0; i < 2000; i++)  
37.   {  
38.       is >> p;  
39.       points.push_back(p);  
40.   }  
41.   CGAL::set_pretty_mode(std::cout);  
42.   std::cout << p << std::endl;  
43.   
44.   return true;  
45. }  
46.   
47. // Reads a list of points and returns a list of segments  
48. // corresponding to the Alpha shape.  
49. int main()  
50. {  
51.   std::list<Point> points;  
52.   if(! file_input(points))  
53.     return -1;  
54.   
55.   Alpha_shape_2 A(points.begin(), points.end(),  
56.                   FT(10000),  
57.                   Alpha_shape_2::GENERAL);  
58.   std::vector<Segment> segments;  
59.   alpha_edges(A, std::back_inserter(segments));  
60.   
61.   
62.   std::ofstream of("vertix.pts");  
63.   CGAL::set_ascii_mode(of);  
64.   
65.   CGAL::set_pretty_mode(std::cout);  
66.   for (int i = 0; i < segments.size(); i++)  
67.   {  
68.       of << segments[i].vertex(1) << std::endl << segments[i].vertex(2) << std::endl;  
69.   }  
70.   std::cout << "Alpha Shape computed" << std::endl;  
71.   std::cout << segments.size() << " alpha shape edges" << std::endl;  
72.   std::cout << "Optimal alpha: " << *A.find_optimal_alpha(1)<<std::endl;  
73.   system("pause");  
74.   return 0;  
75. }  

Thank you, in advance. I will be honored to receive your help.

bingxin

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--

Renato Silveira

     Ph.D. in Computer Science
     NTU / UFRGS | Núcleo de Tecnologia Urbana
     Porto Alegre - RS - Brazil