[vtkusers] unstructured grid + tetrahedrons

[Jérôme]

You are right, polydata is intended to describe 0D (vertices), 1D (lines)
and 2D (polygons) cell. Tetrahedrons are the "simplest" 3D cells, such as
triangles are the "simplest" 2D cells, but vtkPolyData cannot have 3D cells.
However, a tetrahedron is composed of 4 vertices, 6 edges (lines) and 4
triangles (polygons) that you can *represents* thanks to a vtkPolyData. when
you render a tetrahedral mesh through a polydata, you don't "see" the
tetrahedrons, but their bounds.


Jerome

2009/12/11 Baliki B. <balikivtk at gmail.com>

> I tried vtkShrinkFilter and it worked.  My question is why using
> vtkShrinkFilter and pressing w key shows tetrahedrons and without it(e.g.
> using the original unstructured grid) doesn't? Is it something with the fact
> you say it results in "polydata" ?
>
> Why you give emphasis in "results in polydata" in both first two methods
> (vtkShrinkFilter and vtkExtractGeometry) ? Is something "wrong" the polydata
> we get?
>
> Also, i read in the book that polydata is types(vertices, lines, polygons
> and triangle strips) which is limited and that's why we use unstructured
> grid to represent a tetrahedron.
> Am i missing something?
>
>
> 2009/12/10 Dominik Szczerba <dominik at itis.ethz.ch>
>
>> Unfortunately, I will not have a possibility to write those examples this
>> time. A few quick comments follow below.
>>
>>
>> David Doria wrote:
>>
>>> On Thu, Dec 10, 2009 at 8:52 AM, Baliki B. <balikivtk at gmail.com> wrote:
>>>
>>>> Could you explain (with some code) a little bit more?
>>>>
>>>>
>>>> 2009/12/10 Dominik Szczerba <dominik at itis.ethz.ch>
>>>>
>>>>> You can see your tets in multiple ways:
>>>>>
>>>>> - extract cells by region
>>>>>
>>>>
>> There is a filter (vtkExtractGeometry) to extract parts of a volume mesh
>> NOT cutting the cells. Resulting is a polydata of course, but you see the
>> inside cells otherwise hidden. Combined with an interactive widget (e.g.
>> vtkBoxWidget) and edge visibility it gives a very good view.
>>
>>  - cell shrinking
>>>>>
>>>>
>> You can shrink all the cells by a factor (vtkShrinkFilter). This will
>> result in a polydata again, but you will see the separate cells (by their 4
>> shrinked triangles).
>>
>>
>>
>>>>> To see the associated data:
>>>>> - volume rendering
>>>>>
>>>>
>> see vtkVolume
>>
>>  - cut slices
>>>>>
>>>>
>> see vtkCutter
>>
>>  - clip
>>>>>
>>>>
>> see vtkClipDataSet
>>
>>  - contouring
>>>>>
>>>>
>> see vtkContourFilter
>>
>> Dominik
>>
>>
>>
>>>>> Dominik
>>>>>
>>>>>
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>>>>
>>> It's sounding like this may call for a couple of examples...
>>>
>>> Thanks,
>>>
>>> Captain Example
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>
>
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