quads_to_tris.cpp

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// Brandon Smith
// January 20, 2009

/*
   loop over all surface meshsets
     get all quads of surface meshset
     loop over all quads
       make tris from quad
       add tris to the surface meshset
       remove quad from surface meshset
       delete quad
     end loop
   end loop
*/

#include "quads_to_tris.hpp"
#include "moab/CartVect.hpp"
using namespace moab;

// Generic function to create two tris from a quad. This can be improved later.
ErrorCode make_tris_from_quad( Interface *MBI,
                                 EntityHandle quad,  /* intput */
                                 EntityHandle &tri0, /* output */
                 EntityHandle &tri1  /* output */) {
  
  // get connectivity (ordered counterclockwise for 2D elements in MOAB)
  ErrorCode result;    
  const EntityHandle *quad_conn;
  int n_verts=0;
  result = MBI->get_connectivity( quad, quad_conn, n_verts );
  assert( 4 == n_verts );
  assert( MB_SUCCESS == result);
   
  // find length of diagonals
  std::vector<CartVect> coords(n_verts);
  result = MBI->get_coords( quad_conn, n_verts, coords[0].array() );
  if(MB_SUCCESS != result) return result;
  CartVect diagA = coords[0] - coords[2];
  double lenA_sqr= diagA.length_squared();
  CartVect diagB = coords[1] - coords[3];
  double lenB_sqr= diagB.length_squared();

  // choose the shortest diagonal
  EntityHandle tri0_conn[3], tri1_conn[3];
  if(lenA_sqr < lenB_sqr) {
    tri0_conn[0] = quad_conn[0];
    tri0_conn[1] = quad_conn[1];
    tri0_conn[2] = quad_conn[2];
    tri1_conn[0] = quad_conn[0];
    tri1_conn[1] = quad_conn[2];
    tri1_conn[2] = quad_conn[3];
  } else {
    tri0_conn[0] = quad_conn[0];
    tri0_conn[1] = quad_conn[1];
    tri0_conn[2] = quad_conn[3];
    tri1_conn[0] = quad_conn[1];
    tri1_conn[1] = quad_conn[2];
    tri1_conn[2] = quad_conn[3];
  }

  // make tris from quad
  result = MBI->create_element(MBTRI, tri0_conn, 3, tri0);
    assert( MB_SUCCESS == result);
  result = MBI->create_element(MBTRI, tri1_conn, 3, tri1);
    assert( MB_SUCCESS == result);

  return MB_SUCCESS;
}

ErrorCode make_tris_from_quads( Interface *MBI,
                                  const Range quads,
                                  Range &tris ) {
  tris.clear();
  for(Range::const_iterator i=quads.begin(); i!=quads.end(); ++i) {
    EntityHandle tri0, tri1;
    ErrorCode result = make_tris_from_quad( MBI, *i, tri0, tri1 );
    assert(MB_SUCCESS == result);
    if (MB_SUCCESS != result) return result;
    tris.insert( tri0 );
    tris.insert( tri1 );
  }
  return MB_SUCCESS;
}  

ErrorCode quads_to_tris( Interface *MBI, EntityHandle input_meshset ) {

  // create a geometry tag to find the surfaces with
  ErrorCode result;
  Tag geom_tag, id_tag;
  result = MBI->tag_get_handle( GEOM_DIMENSION_TAG_NAME, 1, MB_TYPE_INTEGER,
                                geom_tag, MB_TAG_DENSE|MB_TAG_CREAT );
  if (MB_SUCCESS != result)
    return result;

  // create an id tag to find the surface id with
  result = MBI->tag_get_handle( GLOBAL_ID_TAG_NAME, 1, MB_TYPE_INTEGER, id_tag,
                                MB_TAG_DENSE|MB_TAG_CREAT );
  if (MB_SUCCESS != result)
    return result;

  // get all surface meshsets
  Range surface_meshsets;
  int dim = 2;
  void* input_dim[] = {&dim};
  result = MBI->get_entities_by_type_and_tag( input_meshset, MBENTITYSET, &geom_tag,
                                              input_dim, 1, surface_meshsets);
    assert( MB_SUCCESS == result );
  std::cout << surface_meshsets.size() << " surfaces found." << std::endl;

  // ******************************************************************
  // Loop over every surface meshset and grab each surface's quads.
  // ******************************************************************
  for( Range::iterator i=surface_meshsets.begin(); i!=surface_meshsets.end(); i++ ) {

    // get the surface id of the surface meshset
    int surf_id=0;
    result = MBI->tag_get_data( id_tag, &(*i), 1, &surf_id );
      assert(MB_SUCCESS == result);
    std::cout << "  Surface " << surf_id << " has ";

    // get all quads of the surface
    Range quads;
    result = MBI->get_entities_by_type( *i, MBQUAD, quads );
      assert( MB_SUCCESS == result );
    std::cout << quads.size() << " quads." << std::endl;

    // ******************************************************************
    // For each quad, make two triangles then delete the quad.
    // ******************************************************************
    for(Range::iterator j=quads.begin(); j!=quads.end(); j++ ) {
    
      // make the tris
      EntityHandle tri0 = 0, tri1 = 0;
      result = make_tris_from_quad( MBI, *j, tri0, tri1 );
        assert( MB_SUCCESS == result );
      
      // add all the tris to the same surface meshset as the quads were inside.
      result = MBI->add_entities( *i, &tri0, 1 );
        if ( MB_SUCCESS != result ) std::cout << "result=" << result << std::endl;
    assert( MB_SUCCESS == result);
      result = MBI->add_entities( *i, &tri1, 1 );
        assert( MB_SUCCESS == result);

      // remove the quad from the surface meshset
      result = MBI->remove_entities( *i, &(*j), 1 );
        assert( MB_SUCCESS == result);

      // delete the quad
      result = MBI->delete_entities( &(*j), 1 );
        assert( MB_SUCCESS == result);

    } // end quad loop
  }   // end surface meshset loop
  return MB_SUCCESS; 
}