From: Marc Fehling <mafehling.git@gmail.com>
Date: Mon, 21 Aug 2023 23:36:19 +0000 (-0600)
Subject: Disable eliminate_unrefined_islands in create_geometric_coarsening_sequence().
X-Git-Tag: relicensing~561^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=refs%2Fpull%2F15907%2Fhead;p=dealii.git

Disable eliminate_unrefined_islands in create_geometric_coarsening_sequence().
---

diff --git a/doc/news/changes/incompatibilities/20230822Fehling b/doc/news/changes/incompatibilities/20230822Fehling
new file mode 100644
index 0000000000..8a1ae184eb
--- /dev/null
+++ b/doc/news/changes/incompatibilities/20230822Fehling
@@ -0,0 +1,8 @@
+Changed: For Triangulations created with
+MGTransferGlobalCoarseningTools::create_geometric_coarsening_sequence(),
+the MeshSmoothing flag Triangulation::eliminate_unrefined_islands will
+be removed. It caused unintentional refinement during coarsening which
+led to problems in GlobalCoarseningFineDoFHandlerView. See also
+<a href="https://github.com/dealii/dealii/issues/15541">#15541</a>.
+<br>
+(Marc Fehling, 2023/08/22)
diff --git a/include/deal.II/multigrid/mg_transfer_global_coarsening.templates.h b/include/deal.II/multigrid/mg_transfer_global_coarsening.templates.h
index dfb99460fc..0f973922cb 100644
--- a/include/deal.II/multigrid/mg_transfer_global_coarsening.templates.h
+++ b/include/deal.II/multigrid/mg_transfer_global_coarsening.templates.h
@@ -2380,8 +2380,7 @@ namespace MGTransferGlobalCoarseningTools
             &fine_triangulation_in))
         return std::make_shared<
           parallel::distributed::Triangulation<dim, spacedim>>(
-          fine_triangulation->get_communicator(),
-          fine_triangulation->get_mesh_smoothing());
+          fine_triangulation->get_communicator());
       else
 #endif
 #ifdef DEAL_II_WITH_MPI
@@ -2390,23 +2389,29 @@ namespace MGTransferGlobalCoarseningTools
               &fine_triangulation_in))
         return std::make_shared<parallel::shared::Triangulation<dim, spacedim>>(
           fine_triangulation->get_communicator(),
-          fine_triangulation->get_mesh_smoothing(),
+          Triangulation<dim, spacedim>::none,
           fine_triangulation->with_artificial_cells());
       else
 #endif
-        return std::make_shared<Triangulation<dim, spacedim>>(
-          fine_triangulation_in.get_mesh_smoothing());
+        return std::make_shared<Triangulation<dim, spacedim>>();
     };
 
     const unsigned int max_level = fine_triangulation_in.n_global_levels() - 1;
 
+    // clear 'eliminate_unrefined_islands' from MeshSmoothing flags
+    // to prevent unintentional refinement during coarsen_global()
+    const auto mesh_smoothing =
+      static_cast<typename Triangulation<dim, spacedim>::MeshSmoothing>(
+        fine_triangulation_in.get_mesh_smoothing() &
+        ~(Triangulation<dim, spacedim>::eliminate_unrefined_islands));
+
     // create coarse meshes
     for (unsigned int l = max_level; l > 0; --l)
       {
         // copy triangulation
         auto new_tria = create_new_empty_triangulation();
-
         new_tria->copy_triangulation(*coarse_grid_triangulations[l]);
+        new_tria->set_mesh_smoothing(mesh_smoothing);
 
         // coarsen mesh
         new_tria->coarsen_global();
@@ -2447,9 +2452,6 @@ namespace MGTransferGlobalCoarseningTools
 
     const auto comm = fine_triangulation->get_communicator();
 
-    parallel::distributed::Triangulation<dim, spacedim> temp_triangulation(
-      comm, fine_triangulation->get_mesh_smoothing());
-
     if (keep_fine_triangulation == true &&
         repartition_fine_triangulation == false)
       {
@@ -2492,12 +2494,23 @@ namespace MGTransferGlobalCoarseningTools
     if (fine_triangulation_in.n_global_levels() == 1)
       return coarse_grid_triangulations;
 
+    parallel::distributed::Triangulation<dim, spacedim> temp_triangulation(
+      comm);
+
     if (keep_fine_triangulation == true)
       temp_triangulation.copy_triangulation(*fine_triangulation);
 
     auto *temp_triangulation_ptr =
       keep_fine_triangulation ? &temp_triangulation : fine_triangulation;
 
+    // clear 'eliminate_unrefined_islands' from MeshSmoothing flags
+    // to prevent unintentional refinement during coarsen_global()
+    const auto mesh_smoothing =
+      static_cast<typename Triangulation<dim, spacedim>::MeshSmoothing>(
+        temp_triangulation_ptr->get_mesh_smoothing() &
+        ~(Triangulation<dim, spacedim>::eliminate_unrefined_islands));
+    temp_triangulation_ptr->set_mesh_smoothing(mesh_smoothing);
+
     const unsigned int max_level = fine_triangulation->n_global_levels() - 1;
 
     // create coarse meshes
@@ -2525,6 +2538,12 @@ namespace MGTransferGlobalCoarseningTools
         // save mesh
         coarse_grid_triangulations[l - 1] = level_triangulation;
       }
+
+    // recover MeshSmoothing flags in case we used the fine_triangulation
+    // to build the sequence
+    if (keep_fine_triangulation == false)
+      fine_triangulation->set_mesh_smoothing(
+        coarse_grid_triangulations.back()->get_mesh_smoothing());
 #endif
 
     AssertDimension(coarse_grid_triangulations[0]->n_global_levels(), 1);