mbex1.cpp

beginner tutorial, example 1: Demonstrates constructing/saving a simple 2x2x2 hex meshThis example creates a 2x2x2 mesh (uniform grid) and writes it out to a VTK file and an H5M file. Each cell is of size 1x1x1 and is axis aligned. This example demonstrates how to manually create a mesh using the unstructured mesh interface. The mesh is then written out to file.

Author

Milad Fatenejad

/** @example mbex1.cpp
 * \brief beginner tutorial, example 1: Demonstrates
 *        constructing/saving a simple 2x2x2 hex mesh
 *
 * This example creates a 2x2x2 mesh (uniform grid) and writes it out
 * to a VTK file and an H5M file. Each cell is of size 1x1x1 and is
 * axis aligned. This example demonstrates how to manually create a
 * mesh using the unstructured mesh interface. The mesh is then
 * written out to file.
 *
 * \author Milad Fatenejad
 */
 
// The moab/Core.hpp header file is needed for all MOAB work...
#include "moab/Core.hpp"
 
// C++ includes
#include <iostream>
 
// ****************
// *              *
// *     main     *
// *              *
// ****************
int main()
{
    moab::ErrorCode rval;
    // MOAB functionality is accessed through an instance of the
    // moab::Interface class:
    moab::Core mbcore;
 
    // ***************************
    // *   Create the vertexes   *
    // ***************************
 
    // We are going to create 27 vertexes which will be used to define 8
    // hexahedron cells. First, we have to create an array to store the
    // coordinates of each vertex.
 
    const unsigned NUMVTX = 27;  // The number of vertexes
    const unsigned NUMHEX = 8;   // The number of hexahedrons
 
    // This double array stores the x, y, z coordinate of each vertex.
    const double vertex_coords[3 * NUMVTX] = {
        0, 0, 0, 1, 0, 0, 2, 0, 0, 0, 1, 0, 1, 1, 0, 2, 1, 0, 0, 2, 0, 1, 2, 0, 2, 2, 0,
 
        0, 0, 1, 1, 0, 1, 2, 0, 1, 0, 1, 1, 1, 1, 1, 2, 1, 1, 0, 2, 1, 1, 2, 1, 2, 2, 1,
 
        0, 0, 2, 1, 0, 2, 2, 0, 2, 0, 1, 2, 1, 1, 2, 2, 1, 2, 0, 2, 2, 1, 2, 2, 2, 2, 2 };
 
    // Create the vertexes and store their entity handles in the
    // vertex_handles range. In MOAB, entities are defined using Entity
    // Handles (type moab::EntityHandle). An entity handle is a unique
    // integer that is used to identify/refer to specific entities (such
    // as vertexes, edges, hexahedrons, etc...) on the mesh. MOAB
    // guarantees that when multiple entities are created at the same
    // time, as in the create_vertices call below, that those entities
    // get adjacent handles. Thus, the second of the 27 vertexes we are
    // creating will have an entity handle that is 1 greater than the
    // handle of the first vertex. A range (type moab::Range) is a
    // container which stores entity handles. Of course, sets of handles
    // can also be stored in vectors or arrays, but ranges are much more
    // memory efficient, so use them when possible!
    moab::Range vertex_handles;
    MB_CHK_SET_ERR( mbcore.create_vertices( vertex_coords, NUMVTX, vertex_handles ), "create_vertices failed" );
 
    // You can print out a range to see what elements it contains:
    std::cout << "Created 27 vertex entities:" << vertex_handles;
 
    // ******************************
    // *   Create the Hexahedrons   *
    // ******************************
 
    // The conn array stores the connectivity for each hex. It defines
    // which 8 vertexes connect to define a single hexahedron. The loop
    // below adds the handle of the first vertex (stored in
    // first_vertex_handle) to conn thereby ensuring that the entries of
    // conn are actual entity handles. This only works because MOAB
    // guarantees that entities (such as our vertexes) created at once
    // have adjacent entity handles.
    moab::EntityHandle conn[NUMHEX][8] = { { 0, 1, 4, 3, 9, 10, 13, 12 },      { 1, 2, 5, 4, 10, 11, 14, 13 },
                                           { 3, 4, 7, 6, 12, 13, 16, 15 },     { 4, 5, 8, 7, 13, 14, 17, 16 },
                                           { 9, 10, 13, 12, 18, 19, 22, 21 },  { 10, 11, 14, 13, 19, 20, 23, 22 },
                                           { 12, 13, 16, 15, 21, 22, 25, 24 }, { 13, 14, 17, 16, 22, 23, 26, 25 } };
 
    // Lets get the handle for the first vertex. Note that we can use
    // the square brackets operator on ranges just like vectors or
    // arrays:
    moab::EntityHandle first_vertex_handle = vertex_handles[0];
 
    for( unsigned i = 0; i < NUMHEX; ++i )
    {
        for( unsigned j = 0; j < 8; ++j )
        {
            // Add first_vertex_handle to each element of conn. This ensures
            // that the handles are specified properly (i.e. when
            // first_vertex_handle > 0)
            conn[i][j] = conn[i][j] + first_vertex_handle;
        }
    }
 
    // Now that the connectivity of each hex has been defined, we can
    // create each hex using a call to the create_element method which
    // gives back an entity handle for the hex that was created. We'll
    // then insert that entity into a range:
    moab::Range hexahedron_handles;
    moab::EntityHandle element;
    for( unsigned i = 0; i < NUMHEX; ++i )
    {
        MB_CHK_SET_ERR( mbcore.create_element( moab::MBHEX, conn[i], 8, element ), "create_element failed" );
 
        hexahedron_handles.insert( element );
    }
 
    // Let's see what entities we just created:
    std::cout << "Created HEX8 entities: " << hexahedron_handles;
 
    // ***************************
    // *   Write Mesh to Files   *
    // ***************************
 
    // Now that we've created this amazing mesh, we can write it out to
    // a file. Since MOAB can write out to a variety of standard file
    // formats, you can quickly visualize and manipulate your mesh using
    // standard tools, such as VisIt.
 
    // In these examples, we will stick to using the VTK file format
    // because it is text based and will work whether or not you've got
    // HDF5, NETCDF, etc... installed and is a fairly standard file
    // format so a lot of tools work with it out of the box.
    MB_CHK_SET_ERR( mbcore.write_file( "mbex1.vtk" ), "write_file failed" );
 
    return 0;
}