[vtkusers] algorithm performance

[Jeff Lee]

The cutter just evaluates the implicit function at every point in the input
dataset, then contours that scalar field at the requested value (usually
0).  It is equivalent to generating your own point scalar field of the
implicit function on the dataset and running it through a contourfilter,
and ScalarTreeOn should produce the speedup of subsequent contour values
assuming you are just changing the contour value.  For multiple cuts of the
same function you would set multiple values in the filter, that would get
you the multiple parallel slice case.  The scalar tree won't help if you
are changing the implicit function (i.e. the scalar field), but extracting
different contour values for a single field should show improvement.  The
contour values would be the offset from the zero contour, so values of 0.0,
.1, .2, etc.. would generate multiple planes including the original
implicit definition.


On Fri, Aug 2, 2013 at 4:03 PM, Bill Lorensen <bill.lorensen at gmail.com>wrote:

> I'll try to find an example that does not use a cut function, but uses the
> scalar point data. I know I did this years ago when we first wrote
> vtkCutter and vtkClipPolyData.
>
> You should be able to compute point data scalars for your dataset and use
> those scalars for cutting rtaher than a cut function.
> At least that is how it used to work.
>
>
>
> On Fri, Aug 2, 2013 at 3:44 PM, Gerrick Bivins <
> Gerrick.Bivins at halliburton.com> wrote:
>
>>  Bill,Jeff,****
>>
>> Currently, the slices will be orthogonal slices, parallel to the
>> Cartesian axes (x,y,z).****
>>
>> Next request will be multiple arbitrarily oriented slices but probably
>> parallel to each other (same normal).****
>>
>> ** **
>>
>> Still absorbing this. It’s not quite clear how I’d generate a planar
>> slice using vtkCutter from the scalar data. ****
>>
>> Can either of you point me to some example code? ****
>>
>> Gerrick****
>>
>> ****
>>
>> ** **
>>
>> *From:* Jeff Lee [mailto:jlee1549 at gmail.com]
>> *Sent:* Friday, August 02, 2013 12:01 PM
>> *To:* Bill Lorensen
>> *Cc:* Gerrick Bivins; vtkusers at vtk.org
>>
>> *Subject:* Re: [vtkusers] algorithm performance****
>>
>> ** **
>>
>> IIRC the unstructured grid algorithm doesn't use a cell locator to find
>> candidate cells, it runs over all cells and checks the scalar range of the
>> cell and then contours it if the range is satisfied.  Its faster than
>> blindly contouring every cell but not as good as it could be.  A better
>> approach would be to use a scalar tree (where the function value is the
>> scalar) and use that to locate candidate cells.  It will be expensive to
>> build the tree the first time, but subsequent contours of the same function
>> will typically be much faster, as the candidate cells have been sorted in
>> bins by range in the scalar tree.  You would have to modify the algorithm a
>> bit to do this, but i've had success with that approach in the past...***
>> *
>>
>> ** **
>>
>> On Fri, Aug 2, 2013 at 11:46 AM, Bill Lorensen <bill.lorensen at gmail.com>
>> wrote:****
>>
>> Are the slice planes parallel to each other, or are they arbitrarily
>> oriented?****
>>
>> If they are parallel, then you could compute point data scalars once and
>> leave out the implicit function. Then Cutter will use the scalar data for
>> cutting. This would avoid recomputing the scalar field for each slice.***
>> *
>>
>> ** **
>>
>> ** **
>>
>> On Fri, Aug 2, 2013 at 11:31 AM, Gerrick Bivins <
>> Gerrick.Bivins at halliburton.com> wrote:****
>>
>> Hi Bill,****
>>
>> We are building release. Our grids are large at least 10’s – 100’s of
>> thousands of cells.****
>>
>> I’m seeing the behavior in a Paraview  install as well,****
>>
>> with auto-apply on. Trying to move the slice plane through ****
>>
>> the dataset becomes less and less interactive as the dataset cell count
>> increases.****
>>
>> So I was hoping there were some ways to speed this up.****
>>
>> Any other suggestions?****
>>
>> Gerrick****
>>
>>  ****
>>
>> *From:* Bill Lorensen [mailto:bill.lorensen at gmail.com]
>> *Sent:* Friday, August 02, 2013 10:18 AM
>> *To:* Gerrick Bivins
>> *Cc:* vtkusers at vtk.org
>> *Subject:* Re: [vtkusers] algorithm performance****
>>
>>  ****
>>
>> Also be sure you build VTK and your app Release and not Debug.****
>>
>>  ****
>>
>>  ****
>>
>> On Fri, Aug 2, 2013 at 11:08 AM, Gerrick Bivins <
>> Gerrick.Bivins at halliburton.com> wrote:****
>>
>> Hi All,****
>>
>> I’m looking for suggestions on how to speed up an algorithm on large
>> unstructured grids, like vtkCutter.****
>>
>> I thought I could build an octree (or similar structure) from the input
>>  and execute the vtkCutter on the octree****
>>
>> but it’s not obvious to me how to do it. Is something like this possible?
>> ****
>>
>> How can I improve the speed of the algorithm? Performance seems to go
>> down as cell count grows.****
>>
>> Gerrick****
>>
>>  ****
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