QuadTriConv.cpp

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/**
 * @file QuadTriConv.cpp
 * @brief Example demonstrating conversion of repeated-vertex cells to triangles/quads/polygons
 *
 * This example shows how to:
 * - Load a mesh file with repeated vertices in 2D cells (e.g., at poles)
 * - Merge repeated vertices in cell connectivity
 * - Convert cells to triangles, quads, or polygons as appropriate
 * - Preserve global ID tags during conversion
 * - Write the modified mesh to a new file
 *
 * This is useful for cleaning up rll meshes or other grids with repeated
 * vertices at the poles or other singularities.
 *
 * @author MOAB Development Team
 * @date 2024
 *
 
 * \brief Merge vertices in 2d cell, if they are repeated \n
 * <b>To run</b>: QuadTriConv  input_file output_file \n
 *
 * In this example, a mesh that has vertices repeated due to south or north pole, for
 *  example in an rll mesh, need to be converted to triangles, preserving GLOBAL_ID tag
 *
 * @param argc Number of command line arguments
 * @param argv Command line arguments array
 * @return 0 on success, 1 on failure
 */
 
#include <iostream>
#include <vector>
//#include <string>
 
// Include header for MOAB instance and tag conventions for
#include "moab/Core.hpp"
#include "MBTagConventions.hpp"
 
using namespace moab;
using namespace std;
 
int main( int argc, char** argv )
{
    // Get MOAB instance
    Interface* mb = new( std::nothrow ) Core;
    if( NULL == mb ) return 1;
 
    std::string filename, outfile;
    outfile = string( "out.h5m" );
    if( argc == 1 ) return 0;
    if( argc > 1 ) filename = string( argv[1] );
    if( argc > 2 ) outfile = string( argv[2] );
 
    // This file is in the mesh files directory
    MB_CHK_SET_ERR( mb->load_file( filename.c_str() ), "Failed to read" );
 
    // get all cells of dimension 2;
    Range cells;
    MB_CHK_SET_ERR( mb->get_entities_by_dimension( 0, 2, cells ), "Failed to get cells" );
 
    cout << " number of cells : " << cells.size() << "\n";
 
    Tag gid;  // global id tag
    MB_CHK_SET_ERR( mb->tag_get_handle( "GLOBAL_ID", gid ), "Failed to get Global ID tag" );
 
    Range modifiedCells;  // will be deleted at the end; keep the gid
 
    for( Range::iterator cit = cells.begin(); cit != cells.end(); cit++ )
    {
        EntityHandle cell          = *cit;
        const EntityHandle* connec = NULL;
        int num_verts              = 0;
        MB_CHK_SET_ERR( mb->get_connectivity( cell, connec, num_verts ), "Failed to get connectivity" );
 
        vector< EntityHandle > newConnec;
        newConnec.push_back( connec[0] );  // at least one vertex
        int index    = 0;
        int new_size = 1;
        while( index < num_verts - 2 )
        {
            int next_index = ( index + 1 );
            if( connec[next_index] != newConnec[new_size - 1] )
            {
                newConnec.push_back( connec[next_index] );
                new_size++;
            }
            index++;
        }
        // add the last one only if different from previous and first node
        if( ( connec[num_verts - 1] != connec[num_verts - 2] ) && ( connec[num_verts - 1] != connec[0] ) )
        {
            newConnec.push_back( connec[num_verts - 1] );
            new_size++;
        }
        if( new_size < num_verts )
        {
            // cout << "new cell from " << cell << " has only " << new_size << " vertices \n";
            modifiedCells.insert( cell );
            // create a new cell with type triangle, quad or polygon
            EntityType type = MBTRI;
            if( new_size == 3 )
                type = MBTRI;
            else if( new_size == 4 )
                type = MBQUAD;
            else if( new_size > 4 )
                type = MBPOLYGON;
 
            // create new cell
            EntityHandle newCell;
            MB_CHK_SET_ERR( mb->create_element( type, &newConnec[0], new_size, newCell ), "Failed to create new cell" );
            // set the old id to the new element
            int id;
            MB_CHK_SET_ERR( mb->tag_get_data( gid, &cell, 1, &id ), "Failed to get global id" );
            MB_CHK_SET_ERR( mb->tag_set_data( gid, &newCell, 1, &id ), "Failed to set global id on new cell" );
        }
    }
 
    mb->delete_entities( modifiedCells );
    // in case global ids are not set for vertices, set them, in order;
    // they are needed for migrating later on
    //
    Range verts;
    MB_CHK_SET_ERR( mb->get_entities_by_dimension( 0, 0, verts ), "Failed to get vertices" );
 
    vector< int > gids;
    gids.resize( verts.size() );
    MB_CHK_SET_ERR( mb->tag_get_data( gid, verts, &gids[0] ), "Failed to get gids on vertices" );
    if( gids[0] <= 0 )
    {
        // gids were never set, assign them
        for( size_t i = 1; i <= verts.size(); i++ )
            gids[i - 1] = (int)i;
        MB_CHK_SET_ERR( mb->tag_set_data( gid, verts, &gids[0] ), "Failed to set gids on vertices" );
    }
    MB_CHK_SET_ERR( mb->write_file( outfile.c_str() ), "Failed to write new file" );
 
    cout << " wrote file " << outfile << " with " << modifiedCells.size() << " modified cells\n";
 
    delete mb;
    return 0;
}