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// -----------------------------------------------------------------------------
//
// Gmsh C++ extended tutorial 7
//
// Additional mesh data: internal edges and faces
//
// -----------------------------------------------------------------------------
#include <iostream>
#include <set>
#include <map>
#include <gmsh.h>
int main(int argc, char **argv)
{
gmsh::initialize(argc, argv);
gmsh::model::add("x7");
// Meshes are fully described in Gmsh by nodes and elements, both associated
// to model entities. The API can be used to generate and handle other mesh
// entities, i.e. mesh edges and faces, which are not stored by default.
// Let's create a simple model and mesh it:
gmsh::model::occ::addBox(0, 0, 0, 1, 1, 1);
gmsh::model::occ::synchronize();
gmsh::option::setNumber("Mesh.MeshSizeMin", 2.);
gmsh::model::mesh::generate(3);
// Like elements, mesh edges and faces are described by (an ordered list of)
// their nodes. Let us retrieve the edges and the (triangular) faces of all
// the first order tetrahedra in the mesh:
int elementType = gmsh::model::mesh::getElementType("tetrahedron", 1);
std::vector<std::size_t> edgeNodes, faceNodes;
gmsh::model::mesh::getElementEdgeNodes(elementType, edgeNodes);
gmsh::model::mesh::getElementFaceNodes(elementType, 3, faceNodes);
// Edges and faces are returned for each element as a list of nodes
// corresponding to the canonical orientation of the edges and faces for a
// given element type.
// Gmsh can also identify unique edges and faces (a single edge or face
// whatever the ordering of their nodes) and assign them a unique tag. This
// identification can be done internally by Gmsh (e.g. when generating keys
// for basis functions), or requested explicitly as follows:
gmsh::model::mesh::createEdges();
gmsh::model::mesh::createFaces();
// Edge and face tags can then be retrieved by providing their nodes:
std::vector<std::size_t> edgeTags, faceTags;
std::vector<int> edgeOrientations, faceOrientations;
gmsh::model::mesh::getEdges(edgeNodes, edgeTags, edgeOrientations);
gmsh::model::mesh::getFaces(3, faceNodes, faceTags, faceOrientations);
// Since element edge and face nodes are returned in the same order as the
// elements, one can easily keep track of which element(s) each edge or face
// is connected to:
std::vector<std::size_t> elementTags, elementNodeTags;
gmsh::model::mesh::getElementsByType(elementType, elementTags, elementNodeTags);
std::map<std::size_t, std::vector<std::size_t>> edges2Elements, faces2Elements;
for(std::size_t i = 0; i < edgeTags.size(); i++) // 6 edges per tetrahedron
edges2Elements[edgeTags[i]].push_back(elementTags[i / 6]);
for(std::size_t i = 0; i < faceTags.size(); i++) // 4 faces per tetrahedron
faces2Elements[faceTags[i]].push_back(elementTags[i / 4]);
// New unique lower dimensional elements can also be easily created given the
// edge or face nodes. This is especially useful for numerical methods that
// require integrating or interpolating on internal edges or faces (like
// e.g. Discontinuous Galerkin techniques), since creating elements for the
// internal entities will make this additional mesh data readily available
// (see `x6.cpp'). For example, we can create a new discrete surface...
int s = gmsh::model::addDiscreteEntity(2);
// ... and fill it with unique triangles corresponding to the faces of the
// tetrahedra:
std::set<std::size_t> uniqueFaceTags;
std::vector<std::size_t> tagsForTriangles, faceNodesForTriangles;
std::size_t maxElementTag;
gmsh::model::mesh::getMaxElementTag(maxElementTag);
for(std::size_t i = 0; i < faceTags.size(); i++) {
if(uniqueFaceTags.find(faceTags[i]) == uniqueFaceTags.end()) {
uniqueFaceTags.insert(faceTags[i]);
tagsForTriangles.push_back(faceTags[i] + maxElementTag);
faceNodesForTriangles.push_back(faceNodes[3 * i]);
faceNodesForTriangles.push_back(faceNodes[3 * i + 1]);
faceNodesForTriangles.push_back(faceNodes[3 * i + 2]);
}
}
int elementType2D = gmsh::model::mesh::getElementType("triangle", 1);
gmsh::model::mesh::addElementsByType(s, elementType2D, tagsForTriangles,
faceNodesForTriangles);
// Since the tags for the triangles have been created based on the face tags,
// the information about neighboring elements can also be readily created,
// useful e.g. in Finite Volume or Discontinuous Galerkin techniques:
for(auto t : tagsForTriangles) {
std::cout << "triangle " << t << " is connected to tetrahedra ";
for(auto tt : faces2Elements[t - maxElementTag]) std::cout << tt << " ";
std::cout << "\n";
}
// If all you need is the list of all edges or faces in terms of their nodes, you
// can also directly call:
gmsh::model::mesh::getAllEdges(edgeTags, edgeNodes);
gmsh::model::mesh::getAllFaces(3, faceTags, faceNodes);
// Launch the GUI to see the results:
std::set<std::string> args(argv, argv + argc);
if(!args.count("-nopopup")) gmsh::fltk::run();
gmsh::finalize();
return 0;
}
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