From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Thu, 28 Apr 2022 21:29:08 +0000 (-0600)
Subject: Also deal with internal faces.
X-Git-Tag: v9.4.0-rc1~277^2~3
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3cfb9f0463808f1335767b5c8581bbf001805d1f;p=dealii.git

Also deal with internal faces.
---

diff --git a/include/deal.II/numerics/data_postprocessor.h b/include/deal.II/numerics/data_postprocessor.h
index 40710dcfa9..d518ac6f92 100644
--- a/include/deal.II/numerics/data_postprocessor.h
+++ b/include/deal.II/numerics/data_postprocessor.h
@@ -1293,6 +1293,15 @@ namespace DataPostprocessors
    * @note This class is intended for use with DataOutFaces, not DataOut.
    *   This is because it provides information about the *faces* of a
    *   triangulation, not about cell-based information.
+   *
+   * By default, the DataOutFaces class function only generates
+   * output for faces that lie on the boundary of the domain, and on these
+   * faces, boundary indicators are available. But one can also
+   * instruct DataOutFaces to run on internal faces as
+   * well (by providing an argument to the constructor of the class).
+   * At these internal faces, no boundary indicator is available because,
+   * of course, the face is not actually at the boundary. For these
+   * faces, the current class then outputs -1 as an indicator.
    */
   template <int dim>
   class BoundaryIds : public DataPostprocessorScalar<dim>
diff --git a/source/numerics/data_postprocessor.cc b/source/numerics/data_postprocessor.cc
index 946be4b1ed..dc10f61b17 100644
--- a/source/numerics/data_postprocessor.cc
+++ b/source/numerics/data_postprocessor.cc
@@ -219,7 +219,15 @@ namespace DataPostprocessors
     for (auto &output : computed_quantities)
       {
         AssertDimension(output.size(), 1);
-        output(0) = cell->face(face)->boundary_id();
+
+        // By default, DataOutFaces is only run on faces at the boundary of the
+        // domain. But one can instruct it to also run on internal faces, and in
+        // that case we cannot ask for the boundary id. Rather, we output -1, as
+        // described in the documentation.
+        if (cell->at_boundary(face))
+          output(0) = cell->face(face)->boundary_id();
+        else
+          output(0) = -1;
       }
   }
 } // namespace DataPostprocessors