[vtkusers] vtkImageData::FindCell

[David Gobbi]

On Tue, Nov 30, 2010 at 10:41 AM, David Gobbi <david.gobbi at gmail.com> wrote:

> On Tue, Nov 30, 2010 at 9:25 AM, David Doria <daviddoria at gmail.com> wrote:
>
>>
>> So it considers a point on the boundary to be "outside" no matter the
>> tolerance? Doesn't that seem silly? Often you may know the coordinates
>> of the grid points and want to get the cell id they are associated
>> with. This method should work for all such points, but it seems to
>> work only on points that aren't on the boundary faces of the
>> structured grid?
>>
>
> Hmm... I looked at the FindCell implementation in vtkImageData, and it
> ignores
> the tolerance completely.  This is not good.  As you said, boundary points
> should be considered to be inside the cell unless the tolerance is strictly
> set
> to zero (and even then, it should be an even 50/50 chance that the boundary
> points are considered to be inside).
>
> In fact this is precisely the sort of boundary issue that tolerances are
> for:
> to ensure that points on the boundary are not mistakenly marked as
> "outside"
> due to roundoff error.
>

David, can you put this into the bug tracker?  The entry doesn't have to say
much, it just has to say that vtkImageData::FindCell ignores the tolerance
and that, because of this, there is no way to ensure that FindCell will
return
a cell when given a point that lies on the boundary of the data set.

There is a good chance that I can do a fix for this in the coming weeks.

  David
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