Reading in a mesh

Applications commonly need to read a mesh file from disk. Quest provides the STLReader class, which can read binary or ASCII STL files, as well as the PSTLReader class for use in parallel codes. STL (stereolithography) is a common file format for triangle surface meshes. The STL reader classes will read the file from disk and build a mint::Mesh object. Quest also provides the ProEReader class, for ASCII Pro/E files containing tetrahedra, and the PProEReader class for use in parallel codes. PTC Creo is a modeling application formerly known as Pro/ENGINEER, and its file format is in use among Axom’s users.

Reading an STL file

The code examples are excerpts from the file <axom>/src/tools/mesh_tester.cpp.

We include the STL reader header

#include "axom/quest/readers/STLReader.hpp"

and also the mint Mesh and UnstructuredMesh headers.

#include "axom/mint/mesh/Mesh.hpp"
#include "axom/mint/mesh/UnstructuredMesh.hpp"

For convenience, we use typedefs in the axom namespace.

using namespace axom;

using UMesh = mint::UnstructuredMesh<mint::SINGLE_SHAPE>;

The following example shows usage of the STLReader class:

  // Read file
  SLIC_INFO("Reading file: '" << params.stlInput << "'...\n");
  quest::STLReader* reader = new quest::STLReader();
  reader->setFileName(params.stlInput);
  reader->read();

  // Get surface mesh
  UMesh* surface_mesh = new UMesh(3, mint::TRIANGLE);
  reader->getMesh(surface_mesh);

  // Delete the reader
  delete reader;
  reader = nullptr;

  SLIC_INFO("Mesh has " << surface_mesh->getNumberOfNodes() << " vertices and "
                        << surface_mesh->getNumberOfCells() << " triangles.");

After reading the STL file, the STLReader::getMesh method gives access to the underlying mesh data. The reader may then be deleted.

Reading a Pro/E file

As read by Axom, an ASCII Pro/E tet file contains:

• Zero or more comment lines starting with a # character

• One line with two integers: the number of nodes n and the number of tetrahedra t

• n lines, one for each node; each line contains a contiguous integer ID starting at 1 and three floating-point numbers specifying the node location

• t lines, one for each tetrahedron; each line contains a contiguous integer ID starting at 1 and four integers specifying the tet’s nodes

Reading an ASCII Pro/E tet file is similar to reading an STL file. The code examples are excerpts from the file <axom>/src/axom/quest/examples/quest_proe_bbox.cpp. The Pro/E reader has the ability to read a subset of the mesh in the file, defined by a user-supplied predicate function. The example code shows how to use a convenience function to specify a predicate that keeps only tets fully included in a user-supplied bounding box.

We include the ProEReader header

#include "axom/quest/readers/ProEReader.hpp"

and also the mint Mesh and UnstructuredMesh headers.

#include "axom/mint/mesh/Mesh.hpp"
#include "axom/mint/mesh/UnstructuredMesh.hpp"
#include "axom/mint/utils/vtk_utils.hpp"  // for write_vtk

For convenience, we specify some type aliases.

namespace mint = axom::mint;
namespace primal = axom::primal;
namespace slic = axom::slic;

using IndexType = axom::IndexType;
using UMesh = mint::UnstructuredMesh<mint::SINGLE_SHAPE>;

The following example shows how to use the ProEReader class. Calling reader.setTetPredFromBoundingBox(bbox, false), as shown in the code, makes a tetrahedron predicate that accepts tets with all four nodes falling in bbox and rejects others. Alternately, the user can specify an arbitrary predicate function with setTetPred(). If the user specifies no tetrahedron predicate, the reader reads all tets in the file.

  // Read file
  axom::quest::ProEReader reader;
  reader.setFileName(args.file_name);

  // Set up a bounding box to keep only certain tets.
  // bbox_min and bbox_max are pointers to double.
  axom::quest::ProEReader::BBox3D bbox;
  bbox.addPoint(axom::quest::ProEReader::Point3D {bbox_min, 3});
  bbox.addPoint(axom::quest::ProEReader::Point3D {bbox_max, 3});
  // Keep only tets with all four nodes inside the bounding box.
  reader.setTetPredFromBoundingBox(bbox, false);
  // Pass true as the second argument of setTetPredFromBoundingBox() to
  // keep tets with at least one node inside the bounding box.
  // To keep all tets, do not set a TetPred.

  // Read in the file.
  reader.read();

  // Get surface mesh
  UMesh mesh(3, axom::mint::TET);
  reader.getMesh(&mesh);

After reading the Pro/E file, the ProEReader::getMesh method gives access to the underlying mesh data. The reader may then be deleted.