mbex4.cpp

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/// \file mbex4.cpp
///
/// \author Milad Fatenejad
///
/// \brief 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:
///
/// -# A "temperature" tag that is a single double precision number
/// attached to each quad.
/// -# 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.
 
// The moab/Core.hpp header file is needed for all MOAB work...
#include "moab/Core.hpp"
 
// The moab/ScdInterface.hpp contains code which defines the moab
// structured mesh interface
#include "moab/ScdInterface.hpp"
 
#include <iostream>
#include <cmath>
 
// ****************
// * *
// * main *
// * *
// ****************
int main()
{
 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;
}