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.
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?
-Ken
On 1/7/09 2:44 AM, "Stefan Melber" <stefan.mel...@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
---------------------------------------------------------------
Stefan Melber-Wilkending
_/
_/_/ Fields of activities:
_/ _/ -) High-Lift Research
_/_/_/_/_/_/_/_/_/_/ -) Complex Configurations
_/ _/ _/ _/ -) Numerical Wind-Tunnel
_/ _/ _/ _/ -) Supercomputing
_/_/_/_/_/_/_/_/_/_/ -) Glider-Aerodynamics
_/ _/ _ _
_/_/ | \ | |_| German Aerospace Center, DLR
_/ |_/ |_ | \ in the Helmholtz-Association
Phone ..: +49 531/295-2836 Institute of Aerodynamics
Fax. ...: +49 531/295-2320 and Flow Technology
Email ..: stefan.mel...@dlr.de Lilienthalplatz 7
Web ....: http://www.dlr.de/AS D-38106 Braunschweig/Germany
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**** Kenneth Moreland
*** Sandia National Laboratories
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