[Paraview] Gradient filter: boundary effects

[Moreland, Kenneth]

Matthias,

The gradient is estimated with finite differences. Thus, it is not wholly unexpected that there could be differences at the boundaries. However, the algorithm does not assume zero for adjacent cells at the boundaries.

The way the unstructured gradient filter works is that it computes the local gradient in each cell at each of the cell’s points. Then for every point it averages the gradient from all incident cells at that point. (If you have the Faster Approximation option on, then the filter only computes one gradient per cell in the center and averages those. Faster, but more artifacts, particularly at the edges.)

If this averaging is causing you an issue, you might try the Compute Derivatives filter. This does a wholly local operation within each cell, so you should not see any artifacts (unless the field itself has artifacts at the boundaries).

-Ken


From: ParaView <paraview-bounces at paraview.org> on behalf of "Zenker, Dr. Matthias" <Matthias.Zenker at erbe-med.com>
Date: Tuesday, September 27, 2016 at 5:45 AM
To: "paraview at paraview.org" <paraview at paraview.org>
Subject: [EXTERNAL] [Paraview] Gradient filter: boundary effects

Hi,

when I use the gradient filter (unstructured dataset), I observe edge effects which are IMO unphysical. For the nodes on the outer boundary of my domain, the gradient magnitude is smaller than I would expect. The behavior is like the filter tries to use the adjacent nodes to calculate the gradient, and since there are none  outside the domain, it assumes zero and finds a lower result.
If so, I would consider this a bug – is there a fix or workaround?

Thanks,

Matthias
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