Saving cell data for Fides

[lahwaacz]

Since 0771024 there are metadata for the Fides Paraview reader, but the result is very different from the VTX reader opening the same dataset/directory:

• The VTK attribute specifies the data arrays as CellData:

  ADIOS2-Examples/source/cpp/gray-scott/simulation/writer.cpp

  Lines 17 to 23 in 86ed110

  	<CellData Scalars="U">
  	<DataArray Name="U" />
  	<DataArray Name="V" />
  	<DataArray Name="TIME">
  	step
  	</DataArray>
  	</CellData>

• Opening the dataset/directory with the VTX reader results in the following extents (for all x,y,z): 0 to 64 (dimension: 65)
• However, the Fides attributes specify the data as "points":

  ADIOS2-Examples/source/cpp/gray-scott/simulation/writer.cpp

  Lines 67 to 70 in 86ed110

  	std::vector<std::string> varList = {"U", "V"};
  	std::vector<std::string> assocList = {"points", "points"};
  	io.DefineAttribute<std::string>("Fides_Variable_List", varList.data(), varList.size());
  	io.DefineAttribute<std::string>("Fides_Variable_Associations", assocList.data(), assocList.size());

• Opening the same dataset/directory with the Fides reader results in different extents than the VTX reader: 0 to 63 (dimension: 64)
• The VTK and Fides attributes specify different spacing (1 vs 0.1):

  • ADIOS2-Examples/source/cpp/gray-scott/simulation/writer.cpp

    Line 14 in 86ed110

    R"(" Origin="0 0 0" Spacing="1 1 1">

  • ADIOS2-Examples/source/cpp/gray-scott/simulation/writer.cpp

    Lines 63 to 64 in 86ed110

    	double spacing[3] = {0.1, 0.1, 0.1};
    	io.DefineAttribute<double>("Fides_Spacing", &spacing[0], 3);

• Moreover, when the example is run in a distributed manner, e.g. with mpirun -np 8, the Fides dataset contains "gaps" between the subdomains:
  

How can I specify CellData properly for the Fides reader? I tried changing Fides_Variable_Associations from points to cells for all variables, but then the reader fails with the following error and nothing is loaded in Paraview:

ERROR: In vtkFidesReader.cxx, line 697
vtkFidesReader (0x5d464a9f2b20): Couldn't find index of time value 0

It seems that PointData are somewhat necessary for proper dataset loading (maybe Fides_Dimension_Variable?) but there are in fact datasets without any PointData in the real world and it should be possible to load them with Fides.