[vtk-developers] bug in vtkPointSet::FindCell

[Will Schroeder]

Mathieu-

I am going to look this over on the weekend. There are several known issues
due to a variety of complex reasons; and if I remember right there are
pathological cases with what you propose (walking across neighbors) because
if there is topological separation/cracks between neighboring cells it's
possible to fail too.

Anyway I'll have a chance to look this weekend and we'll talk soon. I think
the bottom line is that to 100% guarantee to find the containing cell a
cell locator is needed, but for performance/memory reasons a point locator
has been used in the past (unfortunately a performance tradeoff that comes
back to bite us repeatedly). There may be more intelligent ways to consider
the N closest points from FindPoint which may produce better results and
don't cost too much in performance/memory.

W

On Thu, Jul 9, 2015 at 6:10 AM, Mathieu Westphal <
mathieu.westphal at kitware.com> wrote:

> There is a bug in vtkPointCell::FindCell
>
> I copy here my mantis bug and associated notes:
> http://www.vtk.org/Bug/view.php?id=15573
>
> The current implementation of FindCell/ FindCellWalk is just wrong and not
> working in some case :
>
> See atached image.
> In the current implementation there is three pass :
>
> 1 : FindPoint, Check CellPoint
> 2 : Check first neighbor
> 3 : Check CellPoint within tolerance
>
> In the associated image, one can see the the first pass will find red star
> and only check Cell 1
> the second pass will only check cell 2
> the third pass will not check anything more
>
> Cell 3 will never be found.
>
> I propose a solution based on vtkStreamTracer surface implementation,
> which walk among neighbors more smartly using pcoords and cell boundary.
>
> See :
>
> https://gitlab.kitware.com/vtk/vtk/blob/master/Filters/FlowPaths/vtkAbstractInterpolatedVelocityField.cxx#L289
> [^
> <https://gitlab.kitware.com/vtk/vtk/blob/master/Filters/FlowPaths/vtkAbstractInterpolatedVelocityField.cxx#L289>
> ]
>
> What do you think ?
>
> Mathieu Westphal
>
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-- 
William J. Schroeder, PhD
Kitware, Inc.
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