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

[Mikael Öhman]

Hi again David,

On Thu, Jun 21, 2012 at 11:27 PM, David Thompson <dcthomp at sandia.gov> wrote:
> Hi Mikael,
>
>
>>> NURBS are not implemented.
>>
>> Would NURBS be part of the unstructured grids?
>
>
> Should someone implement NURBS cells, the barrier that keeps a
> vtkUnstructuredGrid from holding them is that
> vtkGenericCell::InstantiateCell enumerates all of the subclasses of vtkCell
> that are supported -- and Kitware is loathe to increase the number of cell
> types here. Because vtkGenericCell(**) is the mechanism used by a lot of
> filters to perform per-cell operations such as isocontouring, clipping, and
> cutting, these operations fail when a mesh contains cells of a type it
> cannot handle.

I thought NURBS would go under for example
VTK_PARAMETRIC_QUAD_SURFACE. If not, then what else are they for?

>
> (**) Note that vtkGenericCell is not related to the vtkGenericDataSet
> described below. Its equivalent in vtkGenericDataSet objects is
> vtkGenericAdaptorCell. The names clash a little because vtkGenericCell was
> around long before the other vtkGenericXXX classes.
>
>
>>> 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?
>
>
> It is possible to create your own reader and use the vtkGenericDataSet
> filters from ParaView, but this is a lot of work. It is also possible to
> hack vtkGenericCell to include your own cell type, but this is hard to
> maintain. Sorry there is not an easy solution,
>
>        David
>