[Paraview] Quadrangles and Q_1,1 basis functions

[Torquil Macdonald Sørensen]

Hi!

I have defined a 2d quadrangle mesh consisting of 2x2 elements, with
Q_1,1 basis functions in GetFEM++, and exported an example function to
ASCII VTK format. The only node at which the example function is nonzero
is the middle one.

When I open the VTK file in Paraview, it looks as if it has used some
kind of triangle interpolation. Instead of its support being the entire
mesh, it has become zero near two opposite domain corners. In
particular, the result should have been 90-degree rotation symmetric
about the centre node.

How do I get Paraview to display this Q_1,1 function correctly, or at
least without it seemingly being interpolated onto a triangular mesh?

I'm using Paraview 4.3.1 and the ASCI VTK file generated by GetFEM++ is:

# vtk DataFile Version 2.0
Exported by getfem++
ASCII
DATASET UNSTRUCTURED_GRID
POINTS 9 float
 0 0 0
 0.5 0 0
 0 0.5 0
 0.5 0.5 0
 1 0 0
 1 0.5 0
 0 1 0
 0.5 1 0
 1 1 0

CELLS 4 20
 4 0 1 3 2
 4 1 4 5 3
 4 2 3 7 6
 4 3 5 8 7

CELL_TYPES 4
 9
 9
 9
 9

POINT_DATA 9


SCALARS scalarField float 1
LOOKUP_TABLE default
 0 0 0 1 0 0 0 0 0

Thanks and best regards
Torquil Sørensen
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