[aspect-devel] VTK output vs. actual solution

[Wolfgang Bangerth]

Hi Marie,

> There are several 'limitations' with Paraview that you should be aware of.
>
>  1. My understanding is that the data that you (ASPECT) is putting  in
>     the *.vtk files is only at the (square?) cell nodes.  Paraview uses
>     this data in some way to produce visualizations and manipulate your
>     data in other ways.
>
>      1. In particular, if you are using a 'high-order' finite element
>         spacial discretization of your problem, the order of accuracy in
>         Paraview will necessary be reduced, probably to the equivalent
>         of a Q_1 element.

Correct. Internally, ASPECT uses a piecewise quadratic representation of 
data. However, because of file format limitations, we only output a 
piecewise linear representation of data.


>          1. There are others who are more familiar with the specific
>             details of these issues, so some of these details may not be
>             entirely correct.  (I'm sure you'll hear comments about them
>             here.) However, personally I would not rely on Paraview to
>             compute graphs, etc. that require a high-order of accuracy.

Correct. Paraview or Visit are good for visualizing fields, but not for 
extracting summary information. That's what the ASPECT postprocessors 
are there for that have access to the internal representation of data.


>  2. The data that is put in the Paraview *.vtk file is truncated. In
>     other words, even though in ASPECT you are obtaining values that
>     are, in principal, are accurate to 1e-16, which is the limit of
>     64bit IEEE floating point accuracy (you don't really expect 16
>     digits of accuracy, but something small, say 12 digits) a certain
>     additional number of digits of accuracy are lost when those values
>     are written to the *.vtk file.

That is true for the .vtk file format. But the .vtu file format we use 
by default actually provides 16 digits of accuracy.

Best
  W.

-- 
------------------------------------------------------------------------
Wolfgang Bangerth          email:                 bangerth at colostate.edu
                            www: http://www.math.colostate.edu/~bangerth/