[deal.II] confusing result when re-read a mesh in gmsh format

[zeng]

hi all, I tried to create a mesh as follws:
1. Generate the initial mesh using gmsh. Use 1, 2, 3 as boundary_indicator 
to denote three diffenrent boundary. Then I got the initial square.msh 
file. It looks like this:

<https://lh3.googleusercontent.com/-B9nFe31_nm0/WucM0SD0buI/AAAAAAAAAEU/W-6SNDv7WhUf1atPyc_A_Ar1k226Wi5-QCLcBGAs/s1600/square.png>




2. Read the mesh into dealii. Refine those cells with boundary indicator 3 
and output to square_refined0.msh. When output this refined mesh, I used 
flags to keep the boundary indicators. so the boundary faces still have 
correct boundary_indicators after refinement.

<https://lh3.googleusercontent.com/-bL-WfDv6tBs/WucO5rF0STI/AAAAAAAAAEg/kLIBz66R6ZY_UKtv3rbcgQCogDjMVs14QCLcBGAs/s1600/square_refined0.msh.png>

My problem is:

When I use the following code to test the resulting mesh square_refined0.msh. 
The result shows that there are 24 faces whose boundary_id is 0 in the 
triangulation. Since from the initial mesh, I only set boundary_ids to 
1,2,3. Why the resulting mesh have boundary_id 0? Is this a bug?



My code is like:

void test_square(){

   Triangulation<2> triangulation;

   GridIn<2> gridin;

   gridin.attach_triangulation(triangulation);

   std::ifstream f("square_good_refined0.msh");

   gridin.read_msh(f);

   int i1=0;

   typename Triangulation<2>::active_cell_iterator 

            cell = triangulation.begin_active(),

            endc = triangulation.end();

   for(; cell!=endc; cell++)

      for(unsigned int face = 0; face<GeometryInfo<2>::faces_per_cell; 
face++){

         if(cell->face(face)->boundary_id()==0)

            i1 ++;

      }

   std::cout<<"number of boundary_id=0 faces "<<i1<<std::endl;

} 


 

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square_refined0.msh
Description: Mesh model

#include<deal.II/grid/tria.h>
#include<deal.II/grid/tria_accessor.h>
#include<deal.II/grid/tria_iterator.h>
#include<deal.II/grid/grid_generator.h>
#include<deal.II/grid/grid_tools.h>
#include<deal.II/grid/manifold_lib.h>
#include<deal.II/grid/grid_in.h>
#include<deal.II/grid/grid_out.h>

#include<iostream>
#include<fstream>

#include<map>

using namespace dealii;

void test_square(){
   void test_square(){
   Triangulation<2> triangulation;
   GridIn<2> gridin;
   gridin.attach_triangulation(triangulation);
   std::ifstream f("square_refined0.msh");
   gridin.read_msh(f);
   int i1=0;
   typename Triangulation<2>::active_cell_iterator 
            cell = triangulation.begin_active(),
            endc = triangulation.end();
   for(; cell!=endc; cell++)
      for(unsigned int face = 0; face<GeometryInfo<2>::faces_per_cell; face++){
         if(cell->face(face)->boundary_id()==0)
            i1 ++;
      }
   std::cout<<"number of boundary_id=0 faces "<<i1<<std::endl;
} 

int main(){
	try{
		   test_square();
	}
	catch(std::exception &exc){
		std::cerr<<std::endl<<std::endl<<"--------------"<<std::endl;
		std::cerr<<"exception on processing:"<<std::endl<<exc.what()<<std::endl;
		return 1;
	}
	catch(...){
		std::cerr<<std::endl<<"---------------"<<std::endl;
		std::cerr<<"unknown exception"<<std::endl;
		return 1;
	}
}