Cgal triangulation

doc/news/changes/major/20220426FederHeltai

@@ -1,3 +1,12 @@
-New: Added support for the CGAL library (www.cgal.org).
+New: Added support for the CGAL library (www.cgal.org). The following features 
+are supported:
+<ul>
+  <li>Conversion from deal.II to CGAL point types and viceversa</li>
+  <li>Conversion from deal.II cell types to CGAL::Surface_mesh and viceversa</li>
+  <li>Conversion from deal.II Triangulation to CGAL::Surface_mesh and viceversa</li>
+  <li>Insertion of deal.II points in CGAL triangulation types</li>
+  <li>Conversion from CGAL::Triangulation_2 to Triangulation<dim, 2></li>
+  <li>Conversion from CGAL::Triangulation_3 to Triangulation<dim, 3></li>
+</ul>
 <br>
 (Marco Feder, Luca Heltai, 2022/04/26)

include/deal.II/cgal/triangulation.h

@@ -0,0 +1,279 @@
+// ---------------------------------------------------------------------
+//
+// Copyright (C) 2022 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// The deal.II library is free software; you can use it, redistribute
+// it, and/or modify it under the terms of the GNU Lesser General
+// Public License as published by the Free Software Foundation; either
+// version 2.1 of the License, or (at your option) any later version.
+// The full text of the license can be found in the file LICENSE.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>
+
+#ifdef DEAL_II_WITH_CGAL
+
+#  include <deal.II/grid/tria.h>
+
+#  include <boost/hana.hpp>
+
+#  include <CGAL/Surface_mesh.h>
+#  include <CGAL/Triangulation_3.h>
+#  include <deal.II/cgal/utilities.h>
+
+DEAL_II_NAMESPACE_OPEN
+
+namespace CGALWrappers
+{
+  /**
+   * Build any CGAL triangulation from a list of deal.II points.
+   *
+   * The CGAL library offers several types of triangulations, including surface
+   * triangulations, cell complexes, Delaunay triangulations in two and three
+   * dimensions, and many others. For simple cases, all of these triangulations
+   * allow you to incrementally build them from a list of points. This function
+   * provides a convenient way to add a vector of deal.II points to any CGAL
+   * triangulation that admits the insertion of new points via iterators.
+   *
+   * The resulting triangulation will be the convex-hull of the input points. If
+   * you need to build more complicated triangulations, you can use the
+   * functions that build a triangulation from a polyhedral surface domain.
+   *
+   * More information on the available CGAL triangulation classes is available
+   * at https://doc.cgal.org/latest/Triangulation_2/index.html for two
+   * dimensional triangulations, and at
+   * https://doc.cgal.org/latest/Triangulation_3/index.html for three
+   * dimensional triangulations.
+   *
+   * Notice that CGAL distinguishes between a triangulation and a polygonal or
+   * polyhedral mesh. Generally speaking, a triangulation is made of simplices,
+   * whereas a polygonal or polyhedral mesh is made of general polygons or
+   * general polyhedrons. While CGAL implements the two concepts in a similar
+   * fashion using half-edge data structures, some optimizations are performed
+   * for triangulations, where only triangles or tetrahedra are used.
+   *
+   * @param[in] points The input points to build the triangulation from.
+   * @param[out] triangulation The output triangulation.
+   */
+  template <int spacedim, typename CGALTriangulation>
+  void
+  add_points_to_cgal_triangulation(const std::vector<Point<spacedim>> &points,
+                                   CGALTriangulation &triangulation);
+
+  /**
+   * Convert any compatible CGAL triangulation type to a deal.II triangulation.
+   *
+   * The conversion is done by looping over the finite vertices and finite cells
+   * of the CGAL triangulation, and building with them a deal.II Triangulation
+   * object.
+   *
+   * CGAL considers triangulations to be a partition of the entire
+   * spacedim-dimensional space, where every face (including boundary faces) is
+   * shared between neighboring cells. This is obtained by adding to the list of
+   * vertices a special vertex (the vertex at infinity, representing a point on
+   * the spacedim-dimensional sphere with infinite radius), and adding a
+   * neighboring infinite cell to each boundary face.
+   *
+   * Valid conversions require that the capacity of the deal.II triangulation
+   * matches that of the input CGAL triangulation, i.e., if the input CGAL
+   * triangulation is a CGAL::Triangulation_2, then spacedim must be greater
+   * or equal than two. If the input CGAL triangulation is a
+   * CGAL::Triangulation_3, then spacedim must be greater or equal than three.
+   *
+   * Both CGAL::Triangulation_2 and CGAL::Triangulation_3 can store degenerate
+   * two dimensional triangulations (i.e., a one-dimensional triangulation), or
+   * a degenerate three dimensional triangulation (i.e., a two-dimensional
+   * surface triangulation). The dimension of the triangulation, as returned by
+   * `cgal_triangulation.dimension()`, must match the dimension `dim` of the
+   * input Triangulation `dealii_triangulation`. If this is not the case, an
+   * exception is thrown.
+   *
+   * @param[in] cgal_triangulation The input CGAL triangulation.
+   * @param[out] dealii_triangulation The output deal.II triangulation.
+   */
+  template <typename CGALTriangulation, int dim, int spacedim>
+  void
+  cgal_triangulation_to_dealii_triangulation(
+    const CGALTriangulation &     cgal_triangulation,
+    Triangulation<dim, spacedim> &dealii_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 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 idenfied 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
+          {
+            Assert(false, ExcInternalError());
+          }
+      }
+    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 idenfied 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 idenfied 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
+          {
+            Assert(false, ExcInternalError());
+          }
+      }
+    dealii_triangulation.create_triangulation(vertices, cells, subcell_data);
+  }
+#  endif
+} // namespace CGALWrappers
+
+DEAL_II_NAMESPACE_CLOSE
+
+#endif
+#endif

tests/cgal/cgal_triangulation_01.cc

@@ -0,0 +1,56 @@
+// ---------------------------------------------------------------------
+//
+// Copyright (C) 2022 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// The deal.II library is free software; you can use it, redistribute
+// it, and/or modify it under the terms of the GNU Lesser General
+// Public License as published by the Free Software Foundation; either
+// version 2.1 of the License, or (at your option) any later version.
+// The full text of the license can be found in the file LICENSE.md at
+// the top level directory of deal.II.
+//
+// ---------------------------------------------------------------------
+
+// Convert a vector of deal.II points to a cgal Triangulation
+
+#include <deal.II/base/config.h>
+
+#include <deal.II/grid/grid_generator.h>
+#include <deal.II/grid/tria.h>
+
+#include <CGAL/IO/io.h>
+#include <CGAL/Triangulation_3.h>
+#include <deal.II/cgal/triangulation.h>
+
+#include "../tests.h"
+
+using namespace CGALWrappers;
+
+using K                 = CGAL::Simple_cartesian<double>;
+using CGALTriangulation = CGAL::Triangulation_3<K>;
+
+template <int dim, int spacedim>
+void
+test()
+{
+  Triangulation<dim, spacedim> tria;
+  GridGenerator::hyper_cube(tria);
+  CGALTriangulation tr;
+  add_points_to_cgal_triangulation(tria.get_vertices(), tr);
+  deallog << "dim " << dim << ", spacedim " << spacedim << std::endl
+          << tr << std::endl;
+}
+
+int
+main()
+{
+  initlog();
+  test<1, 1>();
+  test<1, 2>();
+  test<1, 3>();
+  test<2, 2>();
+  test<2, 3>();
+  test<3, 3>();
+}