[Paraview] Difference between "Compute Derivatives" and "Gradient (Unstructured)"...

[Stefan Melber]

Hi Jacques,

currently i use tau2plt, then generate "EnsightGold" output and read 
that in. Would be very nice if you can share your NetCDF-Reader for 
paraview...

You working at ARA in Bedford? I was there last December for a 
Solar-Training - there i had my first contact with paraview - it is a 
really nice tool - thats why i am now working with it more and more

Best regards,

     Stefan
> Hi Stefan,
>  
> Are you using ParaView to load in Tau datasets ?
> I have been using it, for that purpose, and have written a Tau data 
> loader for ParaView.
> I find it very useful.
>  
> Regards,
> Jacques
>  
> Aircraft Research Association
>
> 2009/1/8 Stefan Melber <Stefan.Melber at dlr.de 
> <mailto:Stefan.Melber at dlr.de>>
>
>     Hi Ken,
>
>
>         I took a look at this.  To answer the first question, compute
>         derivatives and gradient (unstructured) are fairly similar.
>          The compute derivatives filter takes point scalars and
>         computes the gradient in the centroid of each cell (thus
>         producing cell data).  This is a fairly straightforward
>         operation as the vtkCell classes can compute the gradient
>         anywhere in the cell from point data.
>
>         The gradient filter can take point data and find the data at
>         the points or take cell data and find (an estimate of) the
>         gradient at the cell centroids.  The algorithm for finding the
>         cell-centered is similar to that in compute derivatives and
>         should take about the same amount of time.  The algorithm for
>         finding point-centered data is to find the gradient at each
>         point of each cell and average the results at each point.
>
>         On your prompting, I can the gradient filter through a
>         performance monitor and realized that it was spending about
>         half its time checking for degenerate cells.  I just checked
>         in a change that makes the check much faster.  However,
>         because the gradient filter is doing more derivative
>         calculations, it will always be slower than compute derivatives.
>
>     So its possible to change the gradient-filter to get a speedup? -
>     nice. Please let me know when this change is checked in into the
>     cvs-version of paraview - i will check it here again.
>
>     The last statement i dont understand: using the "compute
>     derivatives" i get overall a tensor with 9 derivatives, using the
>     "gradient" filter i get only three. So in my case, because i need
>     the complete tensor (to compute the strain-rate of the flow) i
>     have to call three times the "gradient"-filter. So overall, in
>     both cases 9 derivatives are calculated - or am i wrong?
>
>         That said, I think it should be fairly easy to add a mode that
>         approximates point gradients by computing cell gradients using
>         the point data and then doint a point-to-cell conversion much
>         like you were doing.  Would anyone want that?
>
>     I have done it by combining both steps in a custom filter ... so
>     thats enough for me. If other users need it - why not.
>
>     Yust a small remark regarding paraview overall: its really a nice
>     tool! I currently do much comparison work between paraview,
>     fieldview and ensight. We have all packages here at DLR (the
>     german aerospace center) and i want to find out if all features we
>     need from fv and ensight can be done although in pv ... it seems,
>     thats the case. And pv is much more flexible then the other
>     packages. Further on, the speed is critical, because we have huge
>     unstructured datasets (typical: > 20e6 points up to 50e6,
>     sometimes time dependend). So the complete parallel setup of pv
>     can be a great help here - with (serial) fv for example we run all
>     the time on the limits of our workstations (16 GB main memory, 4
>     cores) ...
>
>     Best regards,
>
>         Stefan
>
>
>
>         -Ken
>
>
>         On 1/7/09 2:44 AM, "Stefan Melber" <Stefan.Melber at DLR.de> wrote:
>
>            Hi,
>
>            i have a question regarding the filters "Compute
>         Derivatives" and
>            "Gradient (Unstructured)". I have to calculate a equation on an
>            unstructured data set with some derivatives of the velocity.
>
>            Using the "Gradient (Unstructured)" it works, but it is
>         really slow.
>            Using the "Compute Derivatives" and the convert the result
>         back from
>            cell centers to points i can get nearly the same results -
>         but much
>            faster (the most time takes the conversion from cell center to
>            points!).
>
>            So i made a comparison of both results with an isosurface
>         of the
>            magnitude of the difference between both gradients an i can
>         only find
>            minor changes. So my question to the developers: Where is the
>            difference
>            between both filters? Why is the "Gradient (Unstructured)"
>         so much
>            slower?
>
>            Best regards and thank you in advance,
>
>                 Stefan
>
>
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