triangulation.h

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// ------------------------------------------------------------------------
//
// SPDX-License-Identifier: LGPL-2.1-or-later
// Copyright (C) 2022 - 2025 by the deal.II authors
//
// This file is part of the deal.II library.
//
// Part of the source code is dual licensed under Apache-2.0 WITH
// LLVM-exception OR LGPL-2.1-or-later. Detailed license information
// governing the source code and code contributions can be found in
// LICENSE.md and CONTRIBUTING.md at the top level directory of deal.II.
//
// ------------------------------------------------------------------------
 
#ifndef dealii_cgal_triangulation_h
#define dealii_cgal_triangulation_h
 
#include <deal.II/base/config.h>
 
#include <deal.II/base/exceptions.h>
#include <deal.II/base/function.h>
 
#include <deal.II/cgal/utilities.h>
 
#include <deal.II/grid/tria.h>
#include <deal.II/grid/tria_description.h>
 
#ifdef DEAL_II_WITH_CGAL
#  include <deal.II/cgal/surface_mesh.h>
 
#  include <boost/hana.hpp>
 
#  include <CGAL/version.h>
#  if CGAL_VERSION_MAJOR >= 6
#    include <CGAL/Installation/internal/disable_deprecation_warnings_and_errors.h>
#  endif
#  include <CGAL/Complex_2_in_triangulation_3.h>
#  include <CGAL/IO/facets_in_complex_2_to_triangle_mesh.h>
#  include <CGAL/Implicit_surface_3.h>
#  include <CGAL/Labeled_mesh_domain_3.h>
#  include <CGAL/Mesh_complex_3_in_triangulation_3.h>
#  include <CGAL/Mesh_criteria_3.h>
#  include <CGAL/Mesh_triangulation_3.h>
#  include <CGAL/Polyhedron_3.h>
#  include <CGAL/Surface_mesh.h>
#  include <CGAL/Surface_mesh_default_triangulation_3.h>
#  include <CGAL/Triangulation_2.h>
#  include <CGAL/Triangulation_3.h>
#  include <CGAL/make_mesh_3.h>
#  include <CGAL/make_surface_mesh.h>
 
DEAL_II_NAMESPACE_OPEN
 
namespace CGALWrappers
{
  template <int spacedim, typename CGALTriangulation>
  void
  add_points_to_cgal_triangulation(const std::vector<Point<spacedim>> &points,
                                   CGALTriangulation &triangulation);

  template <typename CGALTriangulation, int dim, int spacedim>
  void
  cgal_triangulation_to_dealii_triangulation(
    const CGALTriangulation      &cgal_triangulation,
    Triangulation<dim, spacedim> &dealii_triangulation);

  template <typename CGALTriangulationType,
            typename CornerIndexType,
            typename CurveIndexType>
  void
  cgal_triangulation_to_dealii_triangulation(
    const CGAL::Mesh_complex_3_in_triangulation_3<CGALTriangulationType,
                                                  CornerIndexType,
                                                  CurveIndexType>
                     &cgal_triangulation,
    Triangulation<3> &dealii_triangulation);

  template <typename CGAL_MeshType>
  void
  cgal_surface_mesh_to_dealii_triangulation(const CGAL_MeshType &cgal_mesh,
                                            Triangulation<2, 3> &triangulation);
 
 
 
#  ifndef DOXYGEN
  // Template implementation
  template <int spacedim, typename CGALTriangulation>
  void
  add_points_to_cgal_triangulation(const std::vector<Point<spacedim>> &points,
                                   CGALTriangulation &triangulation)
  {
    Assert(triangulation.is_valid(),
           ExcMessage(
             "The triangulation you pass to this function should be a valid "
             "CGAL triangulation."));
 
    std::vector<typename CGALTriangulation::Point> cgal_points(points.size());
    std::transform(points.begin(),
                   points.end(),
                   cgal_points.begin(),
                   [](const auto &p) {
                     return CGALWrappers::dealii_point_to_cgal_point<
                       typename CGALTriangulation::Point>(p);
                   });
 
    triangulation.insert(cgal_points.begin(), cgal_points.end());
    Assert(triangulation.is_valid(),
           ExcMessage(
             "The Triangulation is no longer valid after inserting the points. "
             "Bailing out."));
  }
 
 
 
  template <typename CGALTriangulationType,
            typename CornerIndexType,
            typename CurveIndexType>
  void
  cgal_triangulation_to_dealii_triangulation(
    const CGAL::Mesh_complex_3_in_triangulation_3<CGALTriangulationType,
                                                  CornerIndexType,
                                                  CurveIndexType>
                     &cgal_triangulation,
    Triangulation<3> &dealii_triangulation)
  {
    using C3T3 = CGAL::Mesh_complex_3_in_triangulation_3<CGALTriangulationType,
                                                         CornerIndexType,
                                                         CurveIndexType>;
 
    // Extract all vertices first
    std::vector<Point<3>> dealii_vertices;
    std::map<typename C3T3::Vertex_handle, unsigned int>
      cgal_to_dealii_vertex_map;
 
    std::size_t inum = 0;
    for (auto vit = cgal_triangulation.triangulation().finite_vertices_begin();
         vit != cgal_triangulation.triangulation().finite_vertices_end();
         ++vit)
      {
        if (vit->in_dimension() <= -1)
          continue;
        cgal_to_dealii_vertex_map[vit] = inum++;
        dealii_vertices.emplace_back(
          CGALWrappers::cgal_point_to_dealii_point<3>(vit->point()));
      }
 
    // Now build cell connectivity
    std::vector<CellData<3>> cells;
    for (auto cgal_cell = cgal_triangulation.cells_in_complex_begin();
         cgal_cell != cgal_triangulation.cells_in_complex_end();
         ++cgal_cell)
      {
        CellData<3> cell(ReferenceCells::Tetrahedron.n_vertices());
        for (unsigned int i = 0; i < 4; ++i)
          cell.vertices[i] = cgal_to_dealii_vertex_map[cgal_cell->vertex(i)];
        cell.manifold_id = cgal_triangulation.subdomain_index(cgal_cell);
        cells.push_back(cell);
      }
 
    // Do the same with surface patches, if possible
    SubCellData subcell_data;
    if constexpr (std::is_integral_v<typename C3T3::Surface_patch_index>)
      {
        for (auto face = cgal_triangulation.facets_in_complex_begin();
             face != cgal_triangulation.facets_in_complex_end();
             ++face)
          {
            const auto &[cgal_cell, cgal_vertex_face_index] = *face;
            CellData<2> dealii_face(ReferenceCells::Triangle.n_vertices());
            // A face is identified by a cell and by the index within the cell
            // of the opposite vertex. Loop over vertices, and retain only those
            // that belong to this face.
            int j = 0;
            for (int i = 0; i < 4; ++i)
              if (i != cgal_vertex_face_index)
                dealii_face.vertices[j++] =
                  cgal_to_dealii_vertex_map[cgal_cell->vertex(i)];
            dealii_face.manifold_id =
              cgal_triangulation.surface_patch_index(cgal_cell,
                                                     cgal_vertex_face_index);
            subcell_data.boundary_quads.emplace_back(dealii_face);
          }
      }
    // and curves
    if constexpr (std::is_integral_v<typename C3T3::Curve_index>)
      {
        for (auto edge = cgal_triangulation.edges_in_complex_begin();
             edge != cgal_triangulation.edges_in_complex_end();
             ++edge)
          {
            const auto &[cgal_cell, v1, v2] = *edge;
            CellData<1> dealii_edge(ReferenceCells::Line.n_vertices());
            dealii_edge.vertices[0] =
              cgal_to_dealii_vertex_map[cgal_cell->vertex(v1)];
            dealii_edge.vertices[1] =
              cgal_to_dealii_vertex_map[cgal_cell->vertex(v2)];
            dealii_edge.manifold_id =
              cgal_triangulation.curve_index(cgal_cell->vertex(v1),
                                             cgal_cell->vertex(v2));
            subcell_data.boundary_lines.emplace_back(dealii_edge);
          }
      }
 
    dealii_triangulation.create_triangulation(dealii_vertices,
                                              cells,
                                              subcell_data);
  }
 
 
 
  template <typename CGALTriangulation, int dim, int spacedim>
  void
  cgal_triangulation_to_dealii_triangulation(
    const CGALTriangulation      &cgal_triangulation,
    Triangulation<dim, spacedim> &dealii_triangulation)
  {
    AssertThrow(cgal_triangulation.dimension() == dim,
                ExcMessage("The dimension of the input CGAL triangulation (" +
                           std::to_string(cgal_triangulation.dimension()) +
                           ") does not match the dimension of the output "
                           "deal.II triangulation (" +
                           std::to_string(dim) + ")."));
 
    Assert(dealii_triangulation.n_cells() == 0,
           ExcMessage("The output triangulation object needs to be empty."));
 
    // deal.II storage data structures
    std::vector<Point<spacedim>> vertices(
      cgal_triangulation.number_of_vertices());
    std::vector<CellData<dim>> cells;
    SubCellData                subcell_data;
 
    // CGAL storage data structures
    std::map<typename CGALTriangulation::Vertex_handle, unsigned int>
      vertex_map;
    {
      unsigned int i = 0;
      for (const auto &v : cgal_triangulation.finite_vertex_handles())
        {
          vertices[i] =
            CGALWrappers::cgal_point_to_dealii_point<spacedim>(v->point());
          vertex_map[v] = i++;
        }
    }
 
    const auto has_faces = boost::hana::is_valid(
      [](auto &&obj) -> decltype(obj.finite_face_handles()) {});
 
    const auto has_cells = boost::hana::is_valid(
      [](auto &&obj) -> decltype(obj.finite_cell_handles()) {});
 
    // Different loops for Triangulation_2 and Triangulation_3 types.
    if constexpr (decltype(has_faces(cgal_triangulation)){})
      {
        // This is a non-degenerate Triangulation_2
        if (cgal_triangulation.dimension() == 2)
          for (const auto &f : cgal_triangulation.finite_face_handles())
            {
              CellData<dim> cell(ReferenceCells::Triangle.n_vertices());
              for (unsigned int i = 0;
                   i < ReferenceCells::Triangle.n_vertices();
                   ++i)
                cell.vertices[i] = vertex_map[f->vertex(i)];
              cells.push_back(cell);
            }
        else if (cgal_triangulation.dimension() == 1)
          // This is a degenerate Triangulation_2, made of edges
          for (const auto &e : cgal_triangulation.finite_edges())
            {
              // An edge is identified by a face and a vertex index in the
              // face
              const auto   &f = e.first;
              const auto   &i = e.second;
              CellData<dim> cell(ReferenceCells::Line.n_vertices());
              unsigned int  id = 0;
              // Since an edge is identified by a face (a triangle) and the
              // index of the opposite vertex to the edge, we can use this
              // logic to infer the indices of the vertices of the edge: loop
              // over all vertices, and keep only those that are not the
              // opposite vertex of the edge.
              for (unsigned int j = 0;
                   j < ReferenceCells::Triangle.n_vertices();
                   ++j)
                if (j != i)
                  cell.vertices[id++] = vertex_map[f->vertex(j)];
              cells.push_back(cell);
            }
        else
          {
            DEAL_II_ASSERT_UNREACHABLE();
          }
      }
    else if constexpr (decltype(has_cells(cgal_triangulation)){})
      {
        // This is a non-degenerate Triangulation_3
        if (cgal_triangulation.dimension() == 3)
          for (const auto &c : cgal_triangulation.finite_cell_handles())
            {
              CellData<dim> cell(ReferenceCells::Tetrahedron.n_vertices());
              for (unsigned int i = 0;
                   i < ReferenceCells::Tetrahedron.n_vertices();
                   ++i)
                cell.vertices[i] = vertex_map[c->vertex(i)];
              cells.push_back(cell);
            }
        else if (cgal_triangulation.dimension() == 2)
          // This is a degenerate Triangulation_3, made of triangles
          for (const auto &facet : cgal_triangulation.finite_facets())
            {
              // A facet is identified by a cell and the opposite vertex index
              // in the face
              const auto   &c = facet.first;
              const auto   &i = facet.second;
              CellData<dim> cell(ReferenceCells::Triangle.n_vertices());
              unsigned int  id = 0;
              // Since a face is identified by a cell (a tetrahedron) and the
              // index of the opposite vertex to the face, we can use this
              // logic to infer the indices of the vertices of the face: loop
              // over all vertices, and keep only those that are not the
              // opposite vertex of the face.
              for (unsigned int j = 0;
                   j < ReferenceCells::Tetrahedron.n_vertices();
                   ++j)
                if (j != i)
                  cell.vertices[id++] = vertex_map[c->vertex(j)];
              cells.push_back(cell);
            }
        else if (cgal_triangulation.dimension() == 1)
          // This is a degenerate Triangulation_3, made of edges
          for (const auto &edge : cgal_triangulation.finite_edges())
            {
              // An edge is identified by a cell and its two vertices
              const auto &[c, i, j] = edge;
              CellData<dim> cell(ReferenceCells::Line.n_vertices());
              cell.vertices[0] = vertex_map[c->vertex(i)];
              cell.vertices[1] = vertex_map[c->vertex(j)];
              cells.push_back(cell);
            }
        else
          {
            DEAL_II_ASSERT_UNREACHABLE();
          }
      }
    dealii_triangulation.create_triangulation(vertices, cells, subcell_data);
  }
 
 
 
  template <typename CGAL_MeshType>
  void
  cgal_surface_mesh_to_dealii_triangulation(const CGAL_MeshType &cgal_mesh,
                                            Triangulation<2, 3> &triangulation)
  {
    Assert(triangulation.n_cells() == 0,
           ExcMessage(
             "Triangulation must be empty upon calling this function."));
 
    const auto is_surface_mesh =
      boost::hana::is_valid([](auto &&obj) -> decltype(obj.faces()) {});
 
    const auto is_polyhedral =
      boost::hana::is_valid([](auto &&obj) -> decltype(obj.facets_begin()) {});
 
    // Collect Vertices and cells
    std::vector<::Point<3>> vertices;
    std::vector<CellData<2>>      cells;
    SubCellData                   subcell_data;
 
    // Different loops for Polyhedron or Surface_mesh types
    if constexpr (decltype(is_surface_mesh(cgal_mesh)){})
      {
        AssertThrow(cgal_mesh.num_vertices() > 0,
                    ExcMessage("CGAL surface mesh is empty."));
        vertices.reserve(cgal_mesh.num_vertices());
        std::map<typename CGAL_MeshType::Vertex_index, unsigned int> vertex_map;
        {
          unsigned int i = 0;
          for (const auto &v : cgal_mesh.vertices())
            {
              vertices.emplace_back(CGALWrappers::cgal_point_to_dealii_point<3>(
                cgal_mesh.point(v)));
              vertex_map[v] = i++;
            }
        }
 
        // Collect CellData
        for (const auto &face : cgal_mesh.faces())
          {
            const auto face_vertices =
              CGAL::vertices_around_face(cgal_mesh.halfedge(face), cgal_mesh);
 
            AssertThrow(face_vertices.size() == 3 || face_vertices.size() == 4,
                        ExcMessage("Only triangle or quadrilateral surface "
                                   "meshes are supported in deal.II"));
 
            CellData<2> c(face_vertices.size());
            auto        it_vertex = c.vertices.begin();
            for (const auto &v : face_vertices)
              *(it_vertex++) = vertex_map[v];
 
            // Make sure the numberfing is consistent with the one in deal.II
            if (face_vertices.size() == 4)
              std::swap(c.vertices[3], c.vertices[2]);
 
            cells.emplace_back(c);
          }
      }
    else if constexpr (decltype(is_polyhedral(cgal_mesh)){})
      {
        AssertThrow(cgal_mesh.size_of_vertices() > 0,
                    ExcMessage("CGAL surface mesh is empty."));
        vertices.reserve(cgal_mesh.size_of_vertices());
        std::map<decltype(cgal_mesh.vertices_begin()), unsigned int> vertex_map;
        {
          unsigned int i = 0;
          for (auto it = cgal_mesh.vertices_begin();
               it != cgal_mesh.vertices_end();
               ++it)
            {
              vertices.emplace_back(
                CGALWrappers::cgal_point_to_dealii_point<3>(it->point()));
              vertex_map[it] = i++;
            }
        }
 
        // Loop over faces of Polyhedron, fill CellData
        for (auto face = cgal_mesh.facets_begin();
             face != cgal_mesh.facets_end();
             ++face)
          {
            auto               j                 = face->facet_begin();
            const unsigned int vertices_per_face = CGAL::circulator_size(j);
            AssertThrow(vertices_per_face == 3 || vertices_per_face == 4,
                        ExcMessage("Only triangle or quadrilateral surface "
                                   "meshes are supported in deal.II. You "
                                   "tried to read a mesh where a face has " +
                                   std::to_string(vertices_per_face) +
                                   " vertices per face."));
 
            CellData<2> c(vertices_per_face);
            auto        it = c.vertices.begin();
            for (unsigned int i = 0; i < vertices_per_face; ++i)
              {
                *(it++) = vertex_map[j->vertex()];
                ++j;
              }
 
            if (vertices_per_face == 4)
              std::swap(c.vertices[3], c.vertices[2]);
 
            cells.emplace_back(c);
          }
      }
    else
      {
        AssertThrow(false,
                    ExcInternalError(
                      "Unsupported CGAL surface triangulation type."));
      }
    triangulation.create_triangulation(vertices, cells, subcell_data);
  }
} // namespace CGALWrappers
#  endif // doxygen
 
DEAL_II_NAMESPACE_CLOSE
 
#else
 
// Make sure the scripts that create the C++20 module input files have
// something to latch on if the preprocessor #ifdef above would
// otherwise lead to an empty content of the file.
DEAL_II_NAMESPACE_OPEN
DEAL_II_NAMESPACE_CLOSE
 
#endif
#endif