how can I store collapsed edges using CGAL

[n-m-ya]

_

Issue Details

I'm creating an application on QT-creator and using CGAL to read .off file as Linear_cell_complex_for_bgl_combinatorial_map_helper and simplify it using edge_collapse method . I want to store the list of collapsed edges,incident vertices, position of points , and other needed information to re-insert the removed edges again .

Source Code

namespace SMS = CGAL::Surface_mesh_simplification ; typedef CGAL::Simple_cartesian Kernel; typedef CGAL::Linear_cell_complex_traits<3, Kernel> MyTraits; typedef CGAL::Linear_cell_complex_for_bgl_combinatorial_map_helper<2, 3, MyTraits>::type LCC;

typedef boost::graph_traits::vertex_descriptor vertex_descriptor; typedef SMS::Edge_profile Profile ; struct Stats { Stats() : collapsed(0) {} std::size_t collapsed ; } ;

struct My_visitor : SMS::Edge_collapse_visitor_base {

My_visitor( Stats* s) : stats(s){} void OnCollapsed( Profile const&, vertex_descriptor ) { ++ stats->collapsed; }

Stats* stats ; };

namespace SMS = CGAL::Surface_mesh_simplification ;

SMS::Count_stop_predicate<LCC> stop(1000);
Stats stats ;

My_visitor vis(&stats) ;

int r = SMS::edge_collapse (lcc ,stop ,CGAL::parameters::halfedge_index_map(get(CGAL::halfedge_index, lcc)) .vertex_index_map(get(boost::vertex_index, lcc)) .get_cost(SMS::Edge_length_cost()) .get_placement(SMS::Midpoint_placement()).visitor(vis) );

std::cout << "\nEdges collapsed: " << stats.collapsed << std::endl;

Environment

• Operating system (fedora):
• CGAL version:4.13

I tried to use Edge_collapse_visitor_base to get no of collapsed edges , but I don't know to to get the information related to the collapsed edges .

I appreciate any help .

[lrineau]

The same question was already posted on StackOverflow, and I have already made an answer:

https://stackoverflow.com/questions/54840943/how-can-i-store-collapsed-edges-using-cgal

Do you prefer to discuss on Github or on StackOverflow?

[afabri]

Hello, I have a branch afabri/SMS-undo_example-GF where I use the visitor to record edge collapses. And I added further callbacks to the visitor so that one can undo the edge collapses. I made it work for CGAL::Surface_mesh as well as for OpenMesh.

[n-m-ya]

@afabri thanks a lot , it's very helpful for me but when I tried the following method :

void OnCollapsing(Profile const& profile ,boost::optional<Point_3>placement  )
{
  std::cout << "My_visitor::OnCollapsing()" << std::endl;
std::cout <<  profile.v0() << " will collapse into "<< profile.v1()<<std::endl;
}

it's shows the following error : no match for ‘operator<<’ (operand types are ‘std::ostream’ {aka ‘std::basic_ostream’} and ‘const vertex_descriptor’

[n-m-ya]

The same question was already posted on StackOverflow, and I have already made an answer:

https://stackoverflow.com/questions/54840943/how-can-i-store-collapsed-edges-using-cgal

Do you prefer to discuss on Github or on StackOverflow?

any of them , I need a quick help

[afabri]

@n-m-ya What exactly did you try? You should check out the particular branch, and first reproduce that the example compiles and runs.

[n-m-ya]

I checked undo_edge_collapse_surface_mesh I tried to get no of collapsed edges and the edges in collapsing process , my code :

struct Stats
{
  Stats() :  collapsed(0) {}
  std::size_t collapsed ;
} ;

struct My_visitor : SMS::Edge_collapse_visitor_base<LCC>
{

My_visitor( Stats* s) : stats(s){}
void OnCollapsed( Profile const& , vertex_descriptor )
  {
    ++ stats->collapsed;

Profile::halfedge_descriptor();
  }

Stats* stats ;

void OnCollapsing(Profile const&  profile,boost::optional<Point> placement  )
{
  std::cout << "My_visitor::OnCollapsing()" << std::endl;
 std::cout <<( profile.v0()) << " will collapse into "<< profile.v1() << std::endl;
}
};
void MainWindow ::simplify()
{
My_visitor vis(&stats) ;

 int r = SMS::edge_collapse
   (lcc
    ,stop
    ,CGAL::parameters::halfedge_index_map(get(CGAL::halfedge_index, lcc))
             .vertex_index_map(get(boost::vertex_index, lcc))
             .get_cost(SMS::Edge_length_cost<LCC>())
   .get_placement(SMS::Midpoint_placement<LCC>()).visitor(vis)
    );

 std::cout << "\nEdges collapsed: "  << stats.collapsed
            << std::endl;
}

[afabri]

As I wrote earlier, I made it work for Surface_mesh. It needs more effort to make it work with Polyhedron or LCC.

[n-m-ya]

As I wrote earlier, I made it work for Surface_mesh. It needs more effort to make it work with Polyhedron or LCC.

ok , thanks a lot