[vtkusers] unstructured grid + tetrahedrons

[Dominik Szczerba]

Baliki B. wrote:
> 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" ?

AFAIR 'w' is a shortcut for wireframe representation.

> 
> Why you give emphasis in "results in polydata" in both first two methods 
> (vtkShrinkFilter and vtkExtractGeometry) ? Is something "wrong" the 
> polydata we get?

Nothing wrong, just what you get is formally a polydata. You do not see 
your tets, at least not directly: you see them as their faces.

> 
> 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?

See above. How would you actually like to see your tet? If you just want 
to see its faces, shrink or extract geometry filter is enough, even if 
producing polygons (tet's faces/triangles)

Dominik

> 
> 2009/12/10 Dominik Szczerba <dominik at itis.ethz.ch 
> <mailto: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
>         <mailto: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
>             <mailto: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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>