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" <[email protected]> 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
_______________________________________________
ParaView mailing list
[email protected]
http://www.paraview.org/mailman/listinfo/paraview
