[vtkusers] Fwd: problems with GetCellNeighbors

[Andy Bauer]

You got it.  Obviously you'll get cells repeated though since some of your
neighbor cells will share more than 1 point with your source/specified cell.

On Fri, Jan 22, 2010 at 3:50 AM, Bill Chivas <noo134 at googlemail.com> wrote:

> Many thanks Andy.
>
> If i consider a neighbor any cell that shares at least one point with the
> specified cell, then i'd have to call GetCellNeighbors() 4 times where the
> cellId is the vtkIdType of the cell you want the neighbors of, the ptIds
> "list" for each call would be {cell point 0}, {cell point 1}, {cell point 2}
> and {cell point 3}? Assuming i have no 2D cells in my mesh that use any of
> the "list" of points and i have a valid topological grid, the cellIds list
> returned would have the cell Ids of the 3D cells that also use the
> given point?
>
> VTK User's Guide is sold out (3 months now).
> I've got VTK TextBook.
>
>
> 2010/1/21 Andy Bauer <andy.bauer at kitware.com>
>
>>  To clear up some confusion, the only topological grid structures that
>> vtkUnstructuredGrid has are points and cells.  For vtkUnstructuredGrid
>> though there are a bunch of cell types (e.g. lines, quads, tris, vertices,
>> tets, hexes, etc.).  So neighbors/adjacency is really only determined
>> through cells that share the same points (if two points have the same
>> coordinate location this still does not count in the neighbor calculation).
>>
>>
>> Also, you may want to clarify your concept of neighbors.  It looks like
>> when you say face neighbors of a tet you mean another 3D cell that shares 3
>> points with that tet (could be another tet, a prism, or a pyramid).  Of
>> course it's possible that someone specified a tri cell in the grid that also
>> uses those same 3 points and then according to VTK this cell would also be
>> considered a neighbor of that tet.  Other people may consider a neighbor any
>> cell that shares at least one point with the specified cell.
>>
>> So to hopefully finally answer your question (assuming I can infer the
>> specifics of your question), an algorithm to get the 3D neighbor cells of a
>> tet you'd have to call GetCellNeighbors() 4 times where the cellId is the
>> vtkIdType of the cell you want the neighbors of, the ptIds list for each
>> call would be {cell point 0, cell point 1, cell point 2}, {cell point 0,
>> cell point 1, cell point 3}, {cell point 0, cell point 2, cell point 3}, and
>> {cell point 1, cell point 2, cell point 3}.  Assuming you have no 2D cells
>> in your mesh that use any of the list of points and you have a valid
>> topological grid, the cellIds list returned would either have size 0 (no
>> neighbor for the given list of points) or 1 (the cell Id of the 3D cell that
>> also uses the given list of points).
>>
>> You may want to get the VTK User's Guide to get a better idea of the grid
>> data structures inside of VTK (p. 334 & 335 have the cell definitions in UG
>> #5).
>>
>> Andy
>>
>>
>>> Date: Thu, 21 Jan 2010 10:06:04 +0200
>>> From: Bill Chivas <noo134 at googlemail.com>
>>> Subject: [vtkusers] Fwd: problems with GetCellNeighbors
>>> To: vtkusers at vtk.org
>>> Message-ID:
>>>        <1668c8ff1001210006g76bbe198ve5ed73c75addfc3e at mail.gmail.com>
>>> Content-Type: text/plain; charset="iso-8859-1"
>>>
>>>
>>>  *GetCellNeighbors*   (  vtkIdType  *cellId*,    vtkIdList *  *ptIds*,
>>> vtkIdList *  *cellIds*   )
>>>
>>>
>>> 1) cellid is the cell i want its neighbors. As "cell" is a e.g.
>>> tetrahedron
>>> cell or one of its faces?
>>> 2) ptids are cell's points or one of its faces' points?
>>> 3) cellids are cells (e.g. neighboring tetrahedra) or faces (neighboring
>>> faces)?
>>>
>>> I'm a little confused!
>>>
>>>
>>> 2010/1/20 Dominik Szczerba <domi at vision.ee.ethz.ch>
>>>
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>>> >
>>> > Yes, GetCellNeighbours is the way to go. AFAIR it expect cell FACE ids
>>> > and will return all cells containing all given points.
>>> >
>>> > Dominik
>>> >
>>> > Bill Chivas wrote:
>>> > > It is an old post (/Tue Jun 27 04:01:59 EDT 2006/).
>>> > > Is it correct?
>>> > > How can i get all neighbors (for cell NOT cell face)?
>>> > >
>>> > >>OK just for others in future - I figured it out.
>>> > >>It is nowhere clearlystated but GetCellNeighbours needs
>>>
>>> > >>cell face points and not cell points.
>>> > >>It returns cell face neighbours and not all neighbours.
>>> > >>Dominik
>>> >
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