Merge branch '11-add-subsampling-vtk-writer' into 'master'

Resolve "add subsampling VTK writer"

Closes #11

See merge request !68

CHANGELOG.md

@@ -4,6 +4,8 @@
 ## Unreleased
 
 ### Added
+* DORiE now employs a `SubsamplingVTKWriter` for higher output precision.
+    Subsampling levels are controlled via the new key `output.subsamplingLevel`.
 * Automatic deployment of `devel` and stable DORiE Docker Images to Docker Hub
 * Automatic update of the DUNE environment Docker Image for `master` builds
 * CI pipeline now also compiles a Debug build with the LLVM Clang compiler
@@ -23,6 +25,11 @@
 * Decentralization of the header system so that in most of the header files are 
 	independent of each other
 
+### Deprecated
+* `output.subsamplingLevel` is optional and defaults to 0 if not given
+    explicitly in the config file. The key will become mandatory in future
+    versions.
+
 ### Fixed
 * The source code is now compatible to the Clang LLVM compiler.
     DORiE can now be compiled with the onboard compiler on macOS.

doc/default_files/parameters.xml

@@ -61,6 +61,18 @@ adding an empty line, make text **bold** or ``monospaced``.
       <suggestion> false </suggestion>
     </parameter>
 
+    <parameter name="subsamplingLevel">
+      <definition> Plot VTK files with virtually refined grids. VTK only
+        supports bilinear triangulations and displays higher-order solutions
+        inappropriately. Use level 0 for order 1, and level N+1 for order N.
+        For level &gt; 0, the printed grid does not resemble the actual grid.
+        This parameter defaults to 0 if not given in the config file. Notice
+        that subsampling significantly increases the output file size!
+      </definition>
+      <values> int </values>
+      <suggestion> 0 </suggestion>
+    </parameter>
+
     <parameter name="asciiVtk">
       <definition> Defines whether VTK files should be written as ASCII (``true``)
         or binary (``false``). ASCII is easier to parse in case you want to write

dune/dorie/interface/output.hh

@@ -9,6 +9,7 @@
 #include <dune/common/parametertree.hh>
 
 #include <dune/grid/io/file/vtk/vtksequencewriter.hh>
+#include <dune/grid/io/file/vtk/subsamplingvtkwriter.hh>
 
 #include <dune/pdelab/gridfunctionspace/gridfunctionspaceutilities.hh>
 
@@ -46,6 +47,7 @@ private:
 	using GV = typename Traits::GV;
 	
 	using VTKWriter = Dune::VTKSequenceWriter<GV>;
+	using SubVTK = Dune::SubsamplingVTKWriter<GV>;
 	using UDGF = Dune::PDELab::DiscreteGridFunction<GFS,U>;
 	using FluxDGF = Dune::Dorie::GradientFluxAdapter<GFS,U,Param>;
 	using ConDGF = Dune::Dorie::ConductivityAdapter<Traits,Param>;
@@ -72,11 +74,12 @@ public:
 	 */
 	OutputWriter (Dune::ParameterTree& _inifile, GV& gv, GFS& gfs, Param& param, U& unew)
 	: OutputWriterBase<Traits>(),
-		vtkwriter(gv,
+		vtkwriter(std::make_shared<SubVTK>(gv,
+			Dune::refinementLevels(_inifile.get<int>(
+				"output.subsamplingLevel", 0))),
 			_inifile.get<std::string>("output.fileName"),
 			_inifile.get<std::string>("output.outputPath"),
-			"./",
-			Dune::VTK::nonconforming),
+			"./"),
 		udgf(gfs,unew),
 		fluxdgf(gfs,unew,param,udgf),
 		condgf(gv,param),