MOAB: Mesh Oriented datABase  (version 5.3.0)
mbex4.cpp File Reference

beginner tutorial, example 4: Create a 2D structured mesh and set some tag data on it More...

#include "moab/Core.hpp"
#include "moab/ScdInterface.hpp"
#include <iostream>
#include <cmath>
+ Include dependency graph for mbex4.cpp:

Go to the source code of this file.


int main ()

Detailed Description

beginner tutorial, example 4: Create a 2D structured mesh and set some tag data on it

Milad Fatenejad

In this example, we create a 2D structured mesh (actually a 3D mesh made of quads) and then we will actually set data on the mesh. Tags represent data that is attached to entities. In this example, we will create two tags:

  1. A "temperature" tag that is a single double precision number attached to each quad.
  2. A "velocity" tag that is an array of 2 double precision numbers attached to each vertex.

We will write the mesh out to a file, and you can visualize the data using your favorite tool.

In this example, I am demonstrating these operations in the clearest possible way - not using the most efficient method. MOAB has been designed so that users do not need to sacrifice performance - future examples will demonstrate how to access/manipulate the mesh using the fastest methods possible.

Definition in file mbex4.cpp.

Function Documentation

int main ( )

Definition at line 42 of file mbex4.cpp.

References moab::ScdInterface::construct_box(), ErrorCode, moab::ScdBox::get_element(), moab::ScdBox::get_vertex(), MB_CHK_SET_ERR, MB_TAG_CREAT, MB_TAG_DENSE, MB_TYPE_DOUBLE, mbint, NI, NJ, moab::Interface::query_interface(), moab::Core::set_coords(), moab::Core::tag_get_handle(), moab::Core::tag_set_data(), velocity, and moab::Core::write_file().

    moab::ErrorCode rval;
    moab::Core mbint;

    // ***********************
    // *   Create the Mesh   *
    // ***********************

    // First, lets make the mesh. It will be a 100 by 100 uniform grid
    // (there will be 100x100 quads, 101x101 vertexes) with dx = dy =
    // 0.1. Unlike the previous example, we will first make the mesh,
    // then set the coordinates one at a time.

    const unsigned NI = 100;
    const unsigned NJ = 100;

    // moab::ScdInterface is the structured mesh interface class for
    // MOAB.
    moab::ScdInterface* scdint;

    // Tell MOAB that our mesh is structured:
    rval = mbint.query_interface( scdint );MB_CHK_SET_ERR( rval, "mbint.query_interface failed" );

    // Create the mesh:
    moab::ScdBox* scdbox = NULL;
    rval = scdint->construct_box( moab::HomCoord( 0, 0, 0 ), moab::HomCoord( NI, NJ, 0 ), NULL, 0, scdbox );MB_CHK_SET_ERR( rval, "scdint->construct_box failed" );

    // MOAB knows to make quads instead of hexes because the last start
    // and end indexes are the same (0). Note that it is still a "3D"
    // mesh because each vertex coordinate is still defined using three
    // numbers although every element in the mesh is a quadrilateral.

    // ******************************
    // *   Set Vertex Coordinates   *
    // ******************************

    // The "NULL" and "0" arguments in the call to construct_box are
    // where we could specify the vertex coordinates. Since we didn't
    // give any coordinates, every vertex is given a position of 0,0,0
    // by default. Now we will set the vertex coordinates...

    const double DX = 0.1;
    const double DY = 0.1;

    for( unsigned i = 0; i < NI + 1; i++ )
        for( unsigned j = 0; j < NJ + 1; j++ )
            // First, get the entity handle:
            moab::EntityHandle handle = scdbox->get_vertex( i, j );

            // Compute the coordinate:
            double coord[3] = { DX * i, DY * j, 0.0 };

            // Change the coordinate of the vertex:
            mbint.set_coords( &handle, 1, coord );

    // *******************
    // *   Attach Tags   *
    // *******************

    // The vertex coordinates have been defined, now let's attach some
    // data to the mesh. In MOAB this is done using "tags". Tags are
    // little bits of information that can be attached to any mesh
    // entity. In our example, we want to create two tags. The
    // "temperature" tag will be attached to each quad and will be 1
    // double. The "velocity" tag will be attached to each vertex and
    // will be an array of two doubles.

    // zero and twozeros represent the initial tag

    // Create the tags:
    moab::Tag temp_tag;
    double temp_default_value = 0.0;
    rval                      = mbint.tag_get_handle( "temperature", 1, moab::MB_TYPE_DOUBLE, temp_tag,
                                 moab::MB_TAG_DENSE | moab::MB_TAG_CREAT, &temp_default_value );MB_CHK_SET_ERR( rval, "mbint.tag_get_handle(temperature) failed" );

    moab::Tag vel_tag;
    double vel_default_value[2] = { 0.0, 0.0 };
    rval = mbint.tag_get_handle( "velocity", 2, moab::MB_TYPE_DOUBLE, vel_tag, moab::MB_TAG_DENSE | moab::MB_TAG_CREAT,
                                 vel_default_value );MB_CHK_SET_ERR( rval, "mbint.tag_get_handle(velocity) failed" );

    // Note that when we created each tag, we specified two flags:
    // The moab::MB_TAG_DENSE flag tells MOAB that this is a dense
    // tag. Dense tags will get automatically assigned to entities which
    // have continuous handles. For this example, this means that once
    // we set a tag on one vertex, memory will be allocated for
    // assigning the tag to all vertexes. The same is true of
    // quads. Dense tags are much more efficient when assigning tags to
    // lots of entities. If you only want to assign a tag to a few
    // entities, it is more efficient to use sparse tags
    // (moab::MB_TAG_SPARSE).
    // The moab::MB_TAG_CREAT flag tells MOAB to create the tag if it
    // doesn't already exist.

    // The tags have now been created, now we have to attach them to
    // entities and set their values. NOTE: I am going to do this in a
    // manner which emphasizes clarity - this is not the most efficient
    // approach - that will be saved for later tutorial examples.

    // Loop through each quad and set the temperature:
    for( unsigned i = 0; i < NI; i++ )
        for( unsigned j = 0; j < NJ; j++ )
            // Get the handle for this quad:
            moab::EntityHandle handle = scdbox->get_element( i, j );

            // Compute the temperature...
            double xc          = DX * ( i + 0.5 );
            double yc          = DY * ( j + 0.5 );
            double r           = std::sqrt( xc * xc + yc * yc );
            double temperature = std::exp( -0.5 * r );

            // Set the temperature on a single quad:
            rval = mbint.tag_set_data( temp_tag, &handle, 1, &temperature );MB_CHK_SET_ERR( rval, "mbint.tag_set_data(temp_tag) failed" );

    // Loop through each vertex and set the velocity:
    for( unsigned i = 0; i < NI + 1; i++ )
        for( unsigned j = 0; j < NJ + 1; j++ )
            // Get the handle for this vertex:
            moab::EntityHandle handle = scdbox->get_vertex( i, j );
            double velocity[2]        = { i, j };

            // Set the velocity on a vertex:
            rval = mbint.tag_set_data( vel_tag, &handle, 1, velocity );MB_CHK_SET_ERR( rval, "mbint.tag_set_data(vel_tag) failed" );

    // ***************************
    // *   Write Mesh to Files   *
    // ***************************

    // NOTE: Some visualization software (such as VisIt) may not
    // interpret the velocity tag as a vector and you may not be able to
    // plot it. But you should be able to plot the temperature on top of
    // the mesh.

    rval = mbint.write_file( "mbex4.vtk" );MB_CHK_SET_ERR( rval, "write_file(mbex4.vtk) failed" );

    return 0;
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