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<p class="MsoNormal">Hi Kenneth,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">thank you for the quick answer!<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">So it seems not that trivial to calculate an electrical field between a rod and a plane given the potential…<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The compute derivatives filter does not give me the problem at the boundary indeed. Instead it gives a value per cell, not per node, which does not look nice.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Is there a friendly filter or switch which would give me the smoothed surface representation back?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Matthias<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">Von:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Moreland, Kenneth [mailto:kmorel@sandia.gov]
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<b>Gesendet:</b> Dienstag, 27. September 2016 14:14<br>
<b>An:</b> Zenker, Dr. Matthias; paraview@paraview.org<br>
<b>Betreff:</b> Re: [Paraview] Gradient filter: boundary effects<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">Matthias,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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
<i>not</i> assume zero for adjacent cells at the boundaries.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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.)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">-Ken<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="color:black">From: </span></b><span lang="EN-US" style="color:black">ParaView <<a href="mailto:paraview-bounces@paraview.org">paraview-bounces@paraview.org</a>> on behalf of "Zenker, Dr. Matthias" <<a href="mailto:Matthias.Zenker@erbe-med.com">Matthias.Zenker@erbe-med.com</a>><br>
<b>Date: </b>Tuesday, September 27, 2016 at 5:45 AM<br>
<b>To: </b>"<a href="mailto:paraview@paraview.org">paraview@paraview.org</a>" <<a href="mailto:paraview@paraview.org">paraview@paraview.org</a>><br>
<b>Subject: </b>[EXTERNAL] [Paraview] Gradient filter: boundary effects</span><span lang="EN-US" style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-family:"Times New Roman","serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">If so, I would consider this a bug – is there a fix or workaround?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Matthias<o:p></o:p></span></p>
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