[Paraview] Gauss points

[Burlen Loring]

> 1. What the numbers 0 4 8 12 (line 24) represent?
This is the offset array used by VTK for random access into data on the 
quadrature points data. To generate it you go cell by cell and 
accumulate the number of quadrature points used by each cell. The next 
cell's data offset is the sum of the previous cell's number of 
quadrature points.

note that the documentation says that each array has a quadrature scheme 
dictionary, but this changed. the dictionaries are stored in the offeset 
array. we did this so that you only need to define one quadrature scheme 
dictionary for many arrays.

> 2. What the QuadratureWeights are all zero (line 37)?
Those aren't currently used in VTK but we have a place holder for them 
in case they are needed in user developed custom filters or later in VTK.

> 3. In your example there are 4 cells with 4 Gauss points in each cell. 
> In total 16 Gauss points. If I want to assign to each Gauss point one 
> value of stress or strain how I have to write those 16 values in xml 
> format.
in my example you would read this file into PV and apply "interpolate to 
quadrature points" and then "generate quadrature points".

You're asking about writing the data on the Gauss points into the file. 
You have to store them in VTK FieldData and add an information key to 
each array that names the offset array. You have to order them correctly 
according to the offsets array and quadrature scheme. for example the 
pdf I sent before shows how each cell's quadrature points are ordered 
for one scheme. you won't write the points themselves, just the data values.

the T FieldData array in the following file is an example of containing 
scalar values at quadrature points (watch out there's also a PointData 
array named T that stores the nodal values), 
http://www.hpcvis.com/vis/images/vtk-quadrature-points-example/quadrature-points-example-2.vtu

after reading such a file into PV, you could see the arrays in the 
spreadsheet view but to see them in the 2d/3d view you'll need to apply 
the "generate quadrature points" filter to produce the point set and 
transfer the data onto it.

On 12/01/2013 10:26 PM, Marios Mavros wrote:
> Hi Burlen,
>
> Your example was really helpful to understand many thing about the xml 
> format and I want to thank you about that, but I still have some 
> questions.
>
> 1. What the numbers 0 4 8 12 (line 24) represent?
> 2. What the QuadratureWeights are all zero (line 37)?
> 3. In your example there are 4 cells with 4 Gauss points in each cell. 
> In total 16 Gauss points. If I want to assign to each Gauss point one 
> value of stress or strain how I have to write those 16 values in xml 
> format. Can you please sent me an example with this information?
>
> Thank you very much
>
> Marios
>
>
> On Sat, Nov 30, 2013 at 4:28 PM, Burlen Loring <bloring at lbl.gov 
> <mailto:bloring at lbl.gov>> wrote:
>
>     Hi Mario,
>
>     sending again to the list. the weights have 2 dimensions i (node
>     id) and j (Gauss point id), so to generate ShapeFunctionWeights
>     for a new QuadratureSchemeDefinition  what you do is evaluate each
>     shape function at each Gauss point. For the linear quadrilateral
>     you'll end up with 16 weights. For example see the example
>
>     http://www.hpcvis.com/vis/images/vtk-quadrature-points-example/vtk-quadrature-points-example.pdf
>     http://www.hpcvis.com/vis/images/vtk-quadrature-points-example/quadrature-points-example.vtu
>
>     Burlen
>
>
>     On 11/29/2013 3:34 PM, Marios Mavros wrote:
>>     Hi Burlen,
>>
>>     I want to thank you for the information that you send me. I read
>>     the paper and I try to made a simple example in xml format for a
>>     4 node quadrilateral element but it didn't work for me. Attached
>>     you will find the element formulation that I used with the shape
>>     functions and a .vtu file with my example. If I want to give 4
>>     stress values in the 4 Gauss points ( lets say 0.1 0.2 0.4 -0.1)
>>     can you please write in the .vtu file the missing information in
>>     order to understand haw I have to write a correct file in xml format.
>>
>>     Thank you very much
>>
>>     Marios Mavros
>>
>>
>>     On Wed, Nov 27, 2013 at 2:56 PM, Burlen Loring <bloring at lbl.gov
>>     <mailto:bloring at lbl.gov>> wrote:
>>
>>         You'll need to use an XML file format. There's some
>>         documentation in the following link. It's slightly dated but
>>         should get you started.
>>         http://www.vtk.org/Wiki/images/7/78/VTK-Quadrature-Point-Design-Doc.pdf
>>         The ctest,
>>         VTK/Filters/General/Testing/Cxx/TestQuadraturePoints.cxx,
>>         could be used as an example.
>>
>>
>>         On 11/27/2013 02:32 PM, Marios Mavros wrote:
>>>         Hi
>>>
>>>         I have wrote a simple program in Fortran that transform the
>>>         output of a finite element software in vtk Legacy Format. I
>>>         don't know how to handle the Gauss points values. For
>>>         example in my case my cells have 4 Gauss points and I have 4
>>>         values (for example strain in x-direction) for each cell.
>>>         How I have to write the vtk file taking into account that
>>>         each cell has many Gauss points?
>>>
>>>         Thank you
>>>
>>>         Marios
>>>
>>>
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>>
>
>

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