Merge branch 'feature/data_reader' into 'master'

Added data reader to VtkReader and a grid-function representation of the read data

See merge request iwr/dune-vtk!24

README.md

@@ -2,14 +2,14 @@
 File reader and writer for the VTK Format
 
 ## Summary
-Provides structured and unstructured file writers for the VTK XML File Formats 
+Provides structured and unstructured file writers for the VTK XML File Formats
 that can be opened in the popular ParaView visualization application. Additionally
 a file reader is provided to import VTK files into Dune grid and data objects.
 
 ## Usage
-The VTK writer works similar to the dune-grid `VTKWriter`. It needs to be bound 
+The VTK writer works similar to the dune-grid `VTKWriter`. It needs to be bound
 to a GridView and then data can be added to the points or cells in the grid.
-Points are not necessarily grid vertices, but any coordinates placed inside the 
+Points are not necessarily grid vertices, but any coordinates placed inside the
 grid cells, so the data must be provided as GridViewFunction to allow the local
 evaluation in arbitrary local coordinates.
 
@@ -21,11 +21,11 @@ class Vtk[Type]Writer
 public:
   // Constructor
   Vtk[Type]Writer(GridView, Vtk::FormatTypes = Vtk::BINARY, Vtk::DataTypes = Vtk::FLOAT32);
-  
+
   // Bind data to the writer
   Vtk[Type]Writer& addPointData(Function [, std::string name, int numComponents, Vtk::FormatTypes]);
   Vtk[Type]Writer& addCellData(Function [, std::string name, int numComponents, Vtk::FormatTypes]);
-  
+
   // Write file with filename
   void write(std::string filename);
 };
@@ -64,74 +64,114 @@ proposed one. A comparions:
 | Timeseries         | -             | **x**        |
 
 ## Writers and Readers
-Dune-Vtk provides nearly all file formats specified in VTK + 2 time series formats: 
+Dune-Vtk provides nearly all file formats specified in VTK + 2 time series formats:
 PVD and VTK-Timeseries.
 
 ### VtkUnstructuredGridWriter
-Implements a VTK file format for unstructured grids with arbitrary element types 
-in 1d, 2d, and 3d. Coordinates are specified explicitly and a connectivity table + 
-element types are specified for all grid elements (of codim 0). Can be used with 
+Implements a VTK file format for unstructured grids with arbitrary element types
+in 1d, 2d, and 3d. Coordinates are specified explicitly and a connectivity table +
+element types are specified for all grid elements (of codim 0). Can be used with
 all Dune grid types.
 
 ### VtkStructuredGridWriter
-Implements a writer for grid composed of cube elements (lines, pixels, voxels) with 
-local numbering similar to Dunes `cube(d)` numbering. The coordinates of the vertices 
-can be arbitrary but the connectivity is implicitly given and equals that of 
-`Dune::YaspGrid` or `Dune::SPGrid`. Might be chosen as writer for a transformed 
-structured grid, using, e.g., a `GeometryGrid` meta-grid. See `src/geometrygrid.cc` 
+Implements a writer for grid composed of cube elements (lines, pixels, voxels) with
+local numbering similar to Dunes `cube(d)` numbering. The coordinates of the vertices
+can be arbitrary but the connectivity is implicitly given and equals that of
+`Dune::YaspGrid` or `Dune::SPGrid`. Might be chosen as writer for a transformed
+structured grid, using, e.g., a `GeometryGrid` meta-grid. See `src/geometrygrid.cc`
 for an example.
 
 ### VtkRectilinearGridWriter
-Rectilinear grids are tensor-product grids with given coordinates along the x, y, 
-and z axes. Therefore, the grid must allow to extract these 1d coordinates and a 
-specialization for a `StructuredDataCollector` must be provided, that implements 
-the `ordinates()` function. By default, it assumes constant grid spacing starting 
-from a lower left corner. For `YaspGrid` a specialization is implemented if the 
-coordinates type is `TensorProductCoordinates`. See `src/structuredgridwriter.cc` 
+Rectilinear grids are tensor-product grids with given coordinates along the x, y,
+and z axes. Therefore, the grid must allow to extract these 1d coordinates and a
+specialization for a `StructuredDataCollector` must be provided, that implements
+the `ordinates()` function. By default, it assumes constant grid spacing starting
+from a lower left corner. For `YaspGrid` a specialization is implemented if the
+coordinates type is `TensorProductCoordinates`. See `src/structuredgridwriter.cc`
 for an example.
 
 ### VtkImageDataWriter
-The *most structured* grid is composed of axis-parallel cube elements with constant 
-size along each axis. The is implemented in the VtkImageDataWriter. A specialization 
-of the `StructuredDataCollector` must implement `origin()` for the lower left corner, 
-`wholeExtent()` for the range of cell numbers along each axis in the global grid, 
-`extent()` for the range in the local grid, and `spacing()` for the constant grid 
+The *most structured* grid is composed of axis-parallel cube elements with constant
+size along each axis. The is implemented in the VtkImageDataWriter. A specialization
+of the `StructuredDataCollector` must implement `origin()` for the lower left corner,
+`wholeExtent()` for the range of cell numbers along each axis in the global grid,
+`extent()` for the range in the local grid, and `spacing()` for the constant grid
 spacing in each direction.
 
 ### PvdWriter
-A sequence writer, i.e. a collection of timestep files, in the ParaView Data (PVD) 
-format. Supports all VtkWriters for the timestep output. In each timestep a collection 
+A sequence writer, i.e. a collection of timestep files, in the ParaView Data (PVD)
+format. Supports all VtkWriters for the timestep output. In each timestep a collection
 (.pvd) file is created.
 
 ### VtkTimseriesWriter
-A timeseries is a collection of timesteps stored in one file, instead of separate 
-files for each timestep value. Since in the `Vtk::APPENDED` mode, the data is written 
-as binary blocks in the appended section of the file and references by an offset 
-in the XML DataArray attributes, it allows to reuse written data. An example of 
-usage is when the grid points and cells do not change over time, but just the 
+A timeseries is a collection of timesteps stored in one file, instead of separate
+files for each timestep value. Since in the `Vtk::APPENDED` mode, the data is written
+as binary blocks in the appended section of the file and references by an offset
+in the XML DataArray attributes, it allows to reuse written data. An example of
+usage is when the grid points and cells do not change over time, but just the
 point-/cell-data. Then, the grid is written only once and the data is just appended.
 
-Timeseries file are create a bit differently from other Vtk file. There, in the 
-first write the grid points and cells are stored in a separate file, and in each 
-timestep just the data is written also to temporary files. When you need the timeseries 
-file, these stored temporaries are collected and combined to one VTK file. Thus, 
-only the minimum amount of data is written in each timestep. The intermediate files 
+Timeseries file are create a bit differently from other Vtk file. There, in the
+first write the grid points and cells are stored in a separate file, and in each
+timestep just the data is written also to temporary files. When you need the timeseries
+file, these stored temporaries are collected and combined to one VTK file. Thus,
+only the minimum amount of data is written in each timestep. The intermediate files
 are stored, by default, in a `/tmp` folder, with (hopefully) fast write access.
 
-### VtkReader
-Read in unstructured grid files (.vtu files) and create a new grid, using a GridFactory.
-The reader allows to create the grid in multiple ways, by providing a `GridCreator`
-template parameter. The `ContinuousGridCreator` reads the connectivity of the grid
-as it is and assumes that the elements are already connected correctly. On the other
-hand, a `DiscontinuousGridCreator` reconnects separated elements, by identifying 
-matching coordinates of the cell vertices.
+## VtkReader
+Reading unstructured grid files (.vtu files) and creating a new grid, using a GridFactory,
+can be performed using the `VtkReader` class. The reader allows to create the grid in
+multiple ways, by providing a `GridCreator` template parameter. The `ContinuousGridCreator`
+reads the connectivity of the grid as it is and assumes that the elements are already
+connected correctly. On the other hand, a `DiscontinuousGridCreator` reconnects separated
+elements, by identifying matching coordinates of the cell vertices. See more possible
+grid-creators in the directory `dune/vtk/gridcreators`.
 
-The VtkReader supports grid creation in parallel. If a partition file .pvtu is 
-provided, all partitions can be read by 1. one processor and distributed later on
-(`SerialGridCreator`) or read directly in parallel (`ParallelGridCreator`). The later
-is currently only available in dune-alugrid 2.6.
+General interface of a VtkReader
+```c++
+template <class Grid, class GridCreator = ContinuousGridCreator<Grid>, class FieldType = double>
+class VtkReader
+{
+public:
+  // Constructors
+  VtkReader();                    // Construct a GridCreator with internal stored GridFactory
+  VtkReader(GridFactory<Grid>&);  // Construct a GridCreator referencing the passed GridFactory
+  VtkReader(GridCreator&);        // Reference the passed GridCreator
 
-**TODO:**
+  // Read the data from a file with filename
+  void read(std::string filename);
 
-- Provide an interface to read the points-/cell-data from the file
-- Extent the implementation to other file formats
+  // Construct the Grid from the data read before.
+  std::unique_ptr<Grid> createGrid() const;
+
+  // Static method to construct the Grid directly
+  static std::unique_ptr<Grid> createGridFromFile(std::string file);
+
+  // Extract data from the reader
+  _PointDataGridFunction_ getPointData(std::string name) const;
+  _CellDataGridFunction_  getCellData(std::string name) const;
+};
+```
+where `Grid` is a dune grid type providing a `GridFactory` specialization, `GridCreator is
+the policy type implementing how the raw data from the file is transformed in data that can
+be passed to the GridFactory and `FieldType` is the data for for internal storage of value
+data from the file, i.e. point-data or cell-data.
+
+The grid can either be created using the static method `createGridFromFile()` or by first
+constructing the `VtkReader` with its `GridCreator`, calling `read()` and finally `createGrid()`.
+The latter allows to access data stored in the reader, like point-data or cell-data.
+
+Value from point-data or cell-data cannot be accessed directly, but through the interface
+of a grid-function. These grid-functions are provided by the `getPointData()` or `getCellData()`
+member functions of the reader. The interface of a dune-functions grid-function concept is
+implemented by these two types. The reason why the reader does not provide the data directly
+is, that it is quiet complicated to associate the specific value to a DOF in the grid, since
+the GridFactory is allows to change the global indexing and even to change to local indexing
+in the elements such that even the local element coordinates might need a transformation
+compared to that of the element stored in the file. All these renumbering and coordinate
+transformations are performed by the grid-functions internally.
+
+The VtkReader supports grid creation in parallel. If a partition file .pvtu is
+provided, all partitions can be read by either one processor and distributed later on
+(`SerialGridCreator`) or read directly in parallel (`ParallelGridCreator`). The later
+is currently only available in dune-alugrid 2.6.

dune/vtk/CMakeLists.txt

@@ -27,6 +27,7 @@ install(FILES
 
 add_subdirectory(datacollectors)
 add_subdirectory(gridcreators)
+add_subdirectory(gridfunctions)
 add_subdirectory(test)
 add_subdirectory(utility)
 add_subdirectory(writers)

dune/vtk/defaultvtkfunction.hh

@@ -67,13 +67,22 @@ namespace Dune
         return comp < N ? vec[comp] : 0.0;
       }
 
-      // Return the scalar values
-      template <class T>
-      double evaluateImpl (int comp, T const& value) const
-      {
-        assert(comp == 0);
-        return value;
-      }
+    // Evaluate a component of a vector valued data
+    template <class T,
+      std::enable_if_t<IsIndexable<T,int>::value, int> = 0>
+    double evaluateImpl (int comp, T const& value) const
+    {
+      return value[comp];
+    }
+
+    // Return the scalar values
+    template <class T,
+      std::enable_if_t<not IsIndexable<T,int>::value, int> = 0>
+    double evaluateImpl (int comp, T const& value) const
+    {
+      assert(comp == 0);
+      return value;
+    }
 
     private:
       LocalFunction localFct_;

dune/vtk/gridcreatorinterface.hh

@@ -29,9 +29,21 @@ namespace Dune
       using Derived = DerivedType;
 
     public:
-      /// Constructor. Stores a reference to the passed GridFactory
+      /// Constructor. Stores a reference to the passed GridFactory.
       GridCreatorInterface (GridFactory<Grid>& factory)
-        : factory_(&factory)
+        : factory_(stackobject_to_shared_ptr(factory))
+      {}
+
+      /// Constructor. Store the shared_ptr to the GridFactory.
+      GridCreatorInterface (std::shared_ptr<GridFactory<Grid>> factory)
+        : factory_(std::move(factory))
+      {}
+
+      /// Constructor. Construct a new GridFactory from the passed arguments.
+      template <class... Args,
+        std::enable_if_t<std::is_constructible<GridFactory<Grid>, Args...>::value,int> = 0>
+      GridCreatorInterface (Args&&... args)
+        : factory_(std::make_shared<GridFactory<Grid>>(std::forward<Args>(args)...))
       {}
 
       /// Insert all points as vertices into the factory
@@ -112,7 +124,7 @@ namespace Dune
       }
 
     protected:
-      GridFactory<Grid>* factory_;
+      std::shared_ptr<GridFactory<Grid>> factory_;
     };
 
   } // end namespace Vtk

dune/vtk/gridcreators/continuousgridcreator.hh

@@ -11,6 +11,7 @@
 
 #include <dune/vtk/types.hh>
 #include <dune/vtk/gridcreatorinterface.hh>
+#include <dune/vtk/gridfunctions/continuousgridfunction.hh>
 
 namespace Dune
 {
@@ -26,10 +27,7 @@ namespace Dune
       using Super = GridCreatorInterface<Grid, ContinuousGridCreator>;
       using GlobalCoordinate = typename Super::GlobalCoordinate;
 
-      ContinuousGridCreator (GridFactory<Grid>& factory)
-        : Super(factory)
-      {}
-
+      using Super::Super;
       using Super::factory;
 
       void insertVerticesImpl (std::vector<GlobalCoordinate> const& points,
@@ -69,5 +67,16 @@ namespace Dune
       }
     };
 
+    // deduction guides
+    template <class Grid>
+    ContinuousGridCreator(GridFactory<Grid>&)
+      -> ContinuousGridCreator<Grid>;
+
+    template <class GridType, class FieldType, class Context>
+    struct AssociatedGridFunction<ContinuousGridCreator<GridType>, FieldType, Context>
+    {
+      using type = ContinuousGridFunction<GridType, FieldType, Context>;
+    };
+
   } // end namespace Vtk
 } // end namespace Dune

dune/vtk/gridcreators/derivedgridcreator.hh

@@ -22,9 +22,11 @@ namespace Dune
       using Grid = typename GridCreator::Grid;
       using GlobalCoordinate = typename Super::GlobalCoordinate;
 
-      DerivedGridCreator (GridFactory<Grid>& factory)
-        : Super(factory)
-        , gridCreator_(factory)
+      template <class... Args,
+        disableCopyMove<DerivedGridCreator, Args...> = 0>
+      DerivedGridCreator (Args&&... args)
+        : Super(std::forward<Args>(args)...)
+        , gridCreator_(Super::factory())
       {}
 
       void insertVerticesImpl (std::vector<GlobalCoordinate> const& points,

dune/vtk/gridcreators/discontinuousgridcreator.hh

@@ -40,11 +40,9 @@ namespace Dune
         }
       };
 
-      DiscontinuousGridCreator (GridFactory<Grid>& factory)
-        : Super(factory)
-      {}
-
+      using Super::Super;
       using Super::factory;
+
       void insertVerticesImpl (std::vector<GlobalCoordinate> const& points,
                               std::vector<std::uint64_t> const& /*point_ids*/)
       {
@@ -97,5 +95,10 @@ namespace Dune
       std::map<GlobalCoordinate, std::size_t, CoordLess> uniquePoints_;
     };
 
+    // deduction guides
+    template <class Grid>
+    DiscontinuousGridCreator(GridFactory<Grid>&)
+      -> DiscontinuousGridCreator<Grid>;
+
   } // end namespace Vtk
 } // end namespace Dune

dune/vtk/gridcreators/lagrangegridcreator.hh

@@ -15,6 +15,7 @@
 
 #include <dune/vtk/types.hh>
 #include <dune/vtk/gridcreatorinterface.hh>
+#include <dune/vtk/gridfunctions/lagrangegridfunction.hh>
 #include <dune/vtk/utility/lagrangepoints.hh>
 
 namespace Dune
@@ -58,11 +59,11 @@ namespace Dune
       class LocalFunction;
 
     public:
-      using Super::factory;
+      using LocalGeometry = MultiLinearGeometry<typename Element::Geometry::ctype,Element::dimension,Element::dimension>;
 
-      LagrangeGridCreator (GridFactory<GridType>& factory)
-        : Super(factory)
-      {}
+    public:
+      using Super::Super;
+      using Super::factory;
 
       /// Implementation of the interface function `insertVertices()`
       void insertVerticesImpl (std::vector<GlobalCoordinate> const& points,
@@ -163,22 +164,41 @@ namespace Dune
       * The returned LocalParametrization is a mapping `GlobalCoordinate(LocalCoordinate)`
       * where `LocalCoordinate is w.r.t. the local coordinate system in the passed element
       * and `GlobalCoordinate` a world coordinate in the parametrized grid.
-      *
-      * Note, when an element is passed, it might have a different local coordinate system
-      * than the coordinate system used to defined the element parametrization. Thus
-      * coordinate transform is internally chained to the evaluation of the local
-      * parametrization. This local geometry transform is obtained by figuring out the
-      * permutation of corners in the element corresponding to the inserted corner
-      * vertices.
       **/
       LocalParametrization localParametrization (Element const& element) const
       {
-        assert(!nodes_.empty() && !parametrization_.empty());
+        VTK_ASSERT(!nodes_.empty() && !parametrization_.empty());
 
         unsigned int insertionIndex = factory().insertionIndex(element);
         auto const& localParam = parametrization_.at(insertionIndex);
-        assert(element.type() == localParam.type);
+        VTK_ASSERT(element.type() == localParam.type);
+
+        return {nodes_, localParam, order(localParam), localGeometry(element, localParam)};
+      }
+
+      /// \brief Construct a transformation of local element coordinates
+      /**
+      * An element might have a different local coordinate system than the coordinate
+      * system used to defined the element parametrization. Thus coordinate transform
+      * of the local parametrization is needed for element-local evaluations. This
+      * local geometry transform is obtained by figuring out the permutation of corners
+      * in the element corresponding to the inserted corner vertices.
+      **/
+      LocalGeometry localGeometry (Element const& element) const
+      {
+        VTK_ASSERT(!nodes_.empty() && !parametrization_.empty());
+
+        unsigned int insertionIndex = factory().insertionIndex(element);
+        auto const& localParam = parametrization_.at(insertionIndex);
+        VTK_ASSERT(element.type() == localParam.type);
+
+        return localGeometry(element, localParam);
+      }
 
+    private:
+      // implementation details of localGeometry()
+      LocalGeometry localGeometry (Element const& element, ElementParametrization const& localParam) const
+      {
         // collect indices of vertices
         std::vector<unsigned int> indices(element.subEntities(GridType::dimension));
         for (unsigned int i = 0; i < element.subEntities(GridType::dimension); ++i)
@@ -188,31 +208,41 @@ namespace Dune
         std::vector<unsigned int> permutation(indices.size());
         for (std::size_t i = 0; i < indices.size(); ++i) {
           auto it = std::find(localParam.corners.begin(), localParam.corners.end(), indices[i]);
-          assert(it != localParam.corners.end());
+          VTK_ASSERT(it != localParam.corners.end());
           permutation[i] = std::distance(localParam.corners.begin(), it);
         }
 
-        return LocalParametrization{nodes_, localParam, order(localParam), permutation};
+        auto refElem = referenceElement<typename Element::Geometry::ctype,Element::dimension>(localParam.type);
+        std::vector<LocalCoordinate> corners(permutation.size());
+        for (std::size_t i = 0; i < permutation.size(); ++i)
+          corners[i] = refElem.position(permutation[i], Element::dimension);
+
+        return {localParam.type, corners};
       }
 
+    public:
+
       /// Determine lagrange order from number of points
-      template <class LocalParam>
-      int order (LocalParam const& localParam) const
+      int order (GeometryType type, std::size_t nNodes) const
       {
-        GeometryType type = localParam.type;
-        int nNodes = localParam.nodes.size();
-        for (int o = 1; o <= nNodes; ++o)
+        for (int o = 1; o <= int(nNodes); ++o)
           if (numLagrangePoints(type.id(), type.dim(), o) == std::size_t(nNodes))
             return o;
 
         return 1;
       }
 
+      int order (ElementParametrization const& localParam) const
+      {
+        return order(localParam.type, localParam.nodes.size());
+      }
+
       /// Determine lagrange order from number of points from the first element parametrization
       int order () const
       {
         assert(!parametrization_.empty());
-        return order(parametrization_.front());
+        auto const& localParam = parametrization_.front();
+        return order(localParam);
       }
 
     public:
@@ -274,6 +304,16 @@ namespace Dune
       Parametrization parametrization_;
     };
 
+    // deduction guides
+    template <class Grid>
+    LagrangeGridCreator(GridFactory<Grid>&)
+      -> LagrangeGridCreator<Grid>;
+
+    template <class GridType, class FieldType, class Context>
+    struct AssociatedGridFunction<LagrangeGridCreator<GridType>, FieldType, Context>
+    {
+      using type = LagrangeGridFunction<GridType, FieldType, Context>;
+    };
 
     template <class Grid>
     class LagrangeGridCreator<Grid>::LocalParametrization
@@ -300,16 +340,11 @@ namespace Dune
       }
 
       /// Construct a local element parametrization for elements with permuted corners
-      template <class Nodes, class LocalParam, class Permutation>
-      LocalParametrization (Nodes const& nodes, LocalParam const& param, int order, Permutation const& permutation)
+      template <class Nodes, class LocalParam, class LG>
+      LocalParametrization (Nodes const& nodes, LocalParam const& param, int order, LG&& localGeometry)
         : LocalParametrization(nodes, param, order)
       {
-        auto refElem = referenceElement<ctype,Grid::dimension>(param.type);
-        std::vector<LocalCoordinate> corners(permutation.size());
-        for (std::size_t i = 0; i < permutation.size(); ++i)
-          corners[i] = refElem.position(permutation[i], Grid::dimension);
-
-        localGeometry_.emplace(param.type, corners);
+        localGeometry_.emplace(std::forward<LG>(localGeometry));
       }
 
       /// Evaluate the local parametrization in local coordinates

dune/vtk/gridcreators/parallelgridcreator.hh

@@ -27,9 +27,9 @@ namespace Dune
       // The GridFactory must support insertion of global vertex IDs
       static_assert(Std::is_detected<HasInsertVertex, GridFactory<Grid>, GlobalCoordinate, VertexId>{}, "");
 
-      ParallelGridCreator (GridFactory<Grid>& factory)
-        : Super(factory)
-      {}
+    public:
+
+      using Super::Super;
 
       void insertVerticesImpl (std::vector<GlobalCoordinate> const& points,
                                std::vector<std::uint64_t> const& point_ids)
@@ -48,5 +48,10 @@ namespace Dune
       }
     };
 
+    // deduction guides
+    template <class Grid>
+    ParallelGridCreator(GridFactory<Grid>&)
+      -> ParallelGridCreator<Grid>;
+
   } // end namespace Vtk
 } // end namespace Dune

dune/vtk/gridcreators/serialgridcreator.hh

@@ -21,9 +21,9 @@ namespace Dune
       using Super = GridCreatorInterface<Grid, Self>;
       using GlobalCoordinate = typename Super::GlobalCoordinate;
 
-      SerialGridCreator (GridFactory<Grid>& factory)
-        : Super(factory)
-      {}
+    public:
+
+      using Super::Super;
 
       void insertVerticesImpl (std::vector<GlobalCoordinate> const& points,
                                std::vector<std::uint64_t> const& /*point_ids*/)
@@ -72,5 +72,10 @@ namespace Dune
       std::vector<std::int64_t> shift_;
     };
 
+    // deduction guides
+    template <class Grid>
+    SerialGridCreator(GridFactory<Grid>&)
+      -> SerialGridCreator<Grid>;
+
   } // end namespace Vtk
 } // end namespace Dune

dune/vtk/gridfunctions/CMakeLists.txt

+#install headers
+install(FILES
+  common.hh
+  continuousgridfunction.hh
+  lagrangegridfunction.hh
+  DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/vtk/gridfunctions)

dune/vtk/gridfunctions/common.hh

+#pragma once
+
+namespace Dune
+{
+  namespace Vtk
+  {
+    /// Context indicating that a GridFunction generates a local-function from point data
+    struct PointContext {};
+
+    /// Context indicating that a GridFunction generates a local-function from cell data
+    struct CellContext {};
+
+    /// \brief Type-Traits to associate a GridFunction to a GridCreator.
+    /**
+     * Each GridCreator type should specialize this template and set `type` to the
+     * corresponding GridFunction type, e.g. Vtk::ContinuousGridFunction or
+     * Vtk::LagrangeGridFunction.
+     *
+     * \tparam GridCreator   A Type implementing the GridCreatorInterface
+     * \tparam FieldType     Coefficient type of the data extracted from the file.
+     * \tparam Context       A context-type for specialization of the local-function, e.g.,
+     *                       Vtk::PointContext or Vtk::CellContext.
+     **/
+    template <class GridCreator, class FieldType, class Context>
+    struct AssociatedGridFunction
+    {
+      using type = void;
+    };
+
+  } // end namespace Vtk
+} // end namespace Dune

dune/vtk/gridfunctions/continuousgridfunction.hh

+#pragma once
+
+#include <vector>
+
+#include <dune/common/dynvector.hh>
+#include <dune/localfunctions/lagrange/lagrangelfecache.hh>
+#include <dune/vtk/gridfunctions/common.hh>
+
+namespace Dune
+{
+  namespace Vtk
+  {
+    /// \brief A GridFunction representing data stored on the grid vertices in a continuous manner.
+    template <class GridType, class FieldType, class Context>
+    class ContinuousGridFunction
+    {
+      using Grid = GridType;
+      using Field = FieldType;
+