[vtkusers] [EXTERNAL] Higher order elements in VTU files

[Mikael Öhman]

Hi David and thank you for the quick reply.

On Thu, Jun 21, 2012 at 6:20 PM, David Thompson <dcthomp at sandia.gov> wrote:
> Hi Mikael,
>
>> Hi. I'm a developer for a finite element code ...
>>
>> I found both in vtkCellType.h  and on
>> http://www.paraview.org/Wiki/ParaView/Users_Guide/VTK_Data_Model
>> a lists many of the types i would wish to find, but they are not
>> documented in the file format guide.
>
>
> The bi- and tri-quadratic elements were implemented by Soeren Gebbert and
> can be members of a vtkUnstructuredGrid. The vtkExodusIIReader will
> certainly read them and I believe the legacy and XML readers also handle
> them properly.

Good to know!

>
>
>> In fact, all cell types from VTK_PARAMETRIC_* and VTK_HIGHER_* doesn't
>> seem to be implemented.
>
>
> NURBS are not implemented.

Would NURBS be part of the unstructured grids?

> The VTK_HIGHER_* types are for use with the
> vtkGenericDataSet mesh format, not vtkUnstructuredGrids. The
> vtkGenericDataSet mesh was meant to serve as a way for novel PDE
> discretization techniques to be rendered with VTK -- without VTK having to
> understand the interpolation strategy or choose a sampling strategy. Thus,
> the class serves as an "adaptor" between visualization code and the FE
> solver providing the interpolation functions. Subclasses of
> vtkGenericAdaptorCell were intended to provide their own linearized
> approximations for rendering. However, the filters for this mesh type are
> not as well tested as those that work with unstructured grids.

So it can't go into VTU-files, but is there any means to store this
such data in any file format which could be loaded into Paraview?

>
>
>> I can only imagine how hard it would be to try and fit general
>> parametric functions into the same procedure, as the nodes can have
>> arbitrary positions in the mapped coordinate system.
>>
>> 1. Are they just placeholders?
>
>
> As above, they are intended for use by cells in a vtkGenericDataSet.
> However, there are not currently open-sourced, higher-order finite elements.
>
>
>> 2. Is there any plan to implement these parametric and higher order
>> elements?
>
>
> Kitware has written one set of "adaptors" for an existing finite element
> package and Sandia developed another, but neither are open-source at the
> moment. The Sandia version was hampered by the fact that there were not
> robust polynomial system solvers at the time (which are needed to identify
> the critical points of interpolants, in turn needed to guarantee true maxima
> and minima are shown).
>
>
>> 3. Can I export the bi-quadratic and tri-quadratic element types to
>> *.vtu files (even though they are not listed in file format
>> documentation)? i.e. the file format documentation should probably be
>> updated.
>
>
> Yes, the documentation should probably be updated. I believe the
> documentation of the various cell types includes the ordering expected for
> the volume-, face-, and edge-centered nodes in a vtkCellArray.
>
>        Regards,
>        David
>
>