MOAB: Mesh Oriented datABase  (version 5.3.1)
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

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.

{
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

// 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;
}