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

[David Thompson]

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.

(**) 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