From 2795a6374c60d0381253bc6d07f7c83d376bb03a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Loriot?= <sebastien.loriot@cgal.org>
Date: Thu, 12 May 2022 16:19:28 +0200
Subject: [PATCH] add an example to simplify a mesh in parallel

---
 .../CMakeLists.txt                            |  13 ++
 .../collapse_small_edges_in_parallel.cpp      | 157 ++++++++++++++++++
 2 files changed, 170 insertions(+)
 create mode 100644 Surface_mesh_simplification/examples/Surface_mesh_simplification/collapse_small_edges_in_parallel.cpp

diff --git a/Surface_mesh_simplification/examples/Surface_mesh_simplification/CMakeLists.txt b/Surface_mesh_simplification/examples/Surface_mesh_simplification/CMakeLists.txt
index 2f0a61cc06e..e26d9e72d0a 100644
--- a/Surface_mesh_simplification/examples/Surface_mesh_simplification/CMakeLists.txt
+++ b/Surface_mesh_simplification/examples/Surface_mesh_simplification/CMakeLists.txt
@@ -51,3 +51,16 @@ if(OpenMesh_FOUND)
   create_single_source_cgal_program("edge_collapse_OpenMesh.cpp")
   target_link_libraries(edge_collapse_OpenMesh PRIVATE ${OPENMESH_LIBRARIES})
 endif()
+
+find_package(METIS)
+include(CGAL_METIS_support)
+
+find_package(TBB)
+include(CGAL_TBB_support)
+
+if(TARGET CGAL::TBB_support AND TARGET CGAL::METIS_support)
+  create_single_source_cgal_program("collapse_small_edges_in_parallel.cpp")
+  target_link_libraries(collapse_small_edges_in_parallel PUBLIC CGAL::TBB_support CGAL::METIS_support)
+else()
+  message(STATUS "collapse_small_edges_in_parallel, which use the METIS and TBB libraries will not be compiled.")
+endif()
diff --git a/Surface_mesh_simplification/examples/Surface_mesh_simplification/collapse_small_edges_in_parallel.cpp b/Surface_mesh_simplification/examples/Surface_mesh_simplification/collapse_small_edges_in_parallel.cpp
new file mode 100644
index 00000000000..17eebbe0314
--- /dev/null
+++ b/Surface_mesh_simplification/examples/Surface_mesh_simplification/collapse_small_edges_in_parallel.cpp
@@ -0,0 +1,157 @@
+#include <CGAL/Simple_cartesian.h>
+#include <CGAL/Surface_mesh.h>
+
+#include <CGAL/Surface_mesh_simplification/edge_collapse.h>
+#include <CGAL/Surface_mesh_simplification/Policies/Edge_collapse/Edge_length_stop_predicate.h>
+#include <CGAL/Surface_mesh_simplification/Policies/Edge_collapse/Edge_length_cost.h>
+#include <CGAL/Surface_mesh_simplification/Policies/Edge_collapse/Constrained_placement.h>
+
+#include <CGAL/boost/graph/Face_filtered_graph.h>
+
+#include <CGAL/boost/graph/partition.h>
+#include <CGAL/Real_timer.h>
+#include <CGAL/Polygon_mesh_processing/polygon_soup_to_polygon_mesh.h>
+#include <CGAL/Polygon_mesh_processing/stitch_borders.h>
+
+#include <tbb/task_group.h>
+
+#include <iostream>
+#include <fstream>
+
+
+typedef CGAL::Simple_cartesian<double>                                      Kernel;
+
+typedef Kernel::Point_3                                                     Point_3;
+typedef CGAL::Surface_mesh<Point_3>                                         Triangle_mesh;
+
+
+namespace SMS = CGAL::Surface_mesh_simplification;
+namespace PMP = CGAL::Polygon_mesh_processing;
+
+struct Border_is_constrained_edge_map{
+  const Triangle_mesh& sm;
+  typedef Triangle_mesh::Edge_index key_type;
+  typedef bool value_type;
+  typedef value_type reference;
+  typedef boost::readable_property_map_tag category;
+  Border_is_constrained_edge_map(const Triangle_mesh& sm)
+    : sm(sm)
+  {}
+  friend bool get(Border_is_constrained_edge_map m, const key_type& edge) {
+    return  CGAL::is_border(edge, m.sm);
+  }
+};
+
+struct Simplify
+{
+  Triangle_mesh* tm_ptr;
+  double edge_length;
+
+  Simplify()
+    :tm_ptr(NULL), edge_length(0)
+  {}
+
+  Simplify(Triangle_mesh& tm, double edge_length)
+    : tm_ptr(&tm), edge_length(edge_length)
+  {}
+
+  void operator()() const
+  {
+
+    SMS::Edge_length_cost<Triangle_mesh> cost;
+    SMS::Edge_length_stop_predicate<double> stop(edge_length);
+    Border_is_constrained_edge_map eicm(*tm_ptr);
+    SMS::Constrained_placement<SMS::LindstromTurk_placement<Triangle_mesh>,
+                               Border_is_constrained_edge_map > placement(eicm);
+
+    SMS::edge_collapse(*tm_ptr, stop,
+                       CGAL::parameters::edge_is_constrained_map(eicm)
+                        .get_placement(placement)
+                        .get_cost(cost));
+  }
+};
+
+int main(int argc, char** argv)
+{
+  typedef boost::graph_traits<Triangle_mesh>::face_descriptor face_descriptor;
+
+  std::ifstream in((argc>1) ? std::string(argv[1]) : CGAL::data_file_path("meshes/elephant.off"));
+  double edge_length = (argc>2) ? std::atof(argv[2]) : 0.02;
+  int number_of_parts = (argc>3) ? std::atoi(argv[3]) : 8;
+
+  Triangle_mesh tm;
+  in >> tm;
+
+  std::cerr << "Input: #V = "<< num_vertices(tm) << " #E = "<< num_edges(tm)
+            << " #F = " << num_faces(tm) << std::endl;
+
+  CGAL::Real_timer t;
+  t.start();
+
+  // Partition ID map
+  Triangle_mesh::Property_map<face_descriptor, std::size_t> fpmap
+    = tm.add_property_map<face_descriptor, std::size_t>("f:partition").first;
+
+  // Set some custom options for METIS
+  idx_t options[METIS_NOPTIONS];
+  METIS_SetDefaultOptions(options);
+  options[METIS_OPTION_CONTIG] = 1; // need to have contiguous subdomains
+  options[METIS_OPTION_UFACTOR] = 1;
+
+  CGAL::METIS::partition_dual_graph(tm, number_of_parts, CGAL::parameters::METIS_options(&options)
+                                    // .vertex_index_map(get(boost::vertex_index,tm))
+                                                                          .face_partition_id_map(fpmap));
+
+  SMS::LindstromTurk_placement<Triangle_mesh> placement;
+  SMS::Edge_length_cost<Triangle_mesh> cost;
+  SMS::Edge_length_stop_predicate<double> stop(edge_length);
+
+  CGAL::Face_filtered_graph<Triangle_mesh> fg(tm);
+  std::vector<Triangle_mesh> meshes(number_of_parts);
+  for (int i=0; i< number_of_parts; ++i)
+  {
+    fg.set_selected_faces(i, fpmap);
+    CGAL::copy_face_graph(fg, meshes[i]);
+  }
+
+  tbb::task_group tasks;
+  for(int id=0; id<number_of_parts; ++id)
+  {
+    tasks.run(Simplify(meshes[id], edge_length));
+  }
+  tasks.wait();
+
+  // reassemble the mesh
+  Triangle_mesh final_mesh;
+  std::size_t nv=0, nf=0, ne=0;
+  for (int i=0; i< number_of_parts; ++i)
+  {
+    meshes[i].collect_garbage();
+    nv += num_vertices(meshes[i]);
+    nf += num_faces(meshes[i]);
+    ne += num_edges(meshes[i]);
+  }
+  final_mesh.reserve(nv, ne, nf);
+  for (int i=0; i< number_of_parts; ++i)
+  {
+    CGAL::copy_face_graph(meshes[i], final_mesh);
+  }
+  PMP::stitch_borders(final_mesh);
+
+  // final pass (could be applied only on interface boundaries)
+  SMS::edge_collapse(final_mesh, stop,
+                     CGAL::parameters::get_placement(placement)
+                     .get_cost(cost));
+
+  final_mesh.collect_garbage();
+  t.stop();
+
+  std::cerr << "  Total time: " << t.time() << " sec.\n"
+          << "  Result: #V = " << num_vertices(final_mesh)
+          << " #E = " << num_edges(final_mesh)
+          << " #F = " << num_faces(final_mesh) << std::endl;
+
+  std::ofstream("simplified_mesh.off") << std::setprecision(17) << final_mesh;
+
+  return EXIT_SUCCESS;
+}