[ITK] [ITK-users] Symmetric eigen analysis discrepancy

[Matt McCormick]

Hi Olivier,

Thanks for digging down and submitting the patches! The split is fine
-- it is nice to have one topic per commit.  I have rebased them on
the release branch before merging to help get them into the next
bugfix release.

Yes, we should re-use code as much as possible!  I am not sure about
source of the thread-safe warning, be it can be attacked another time
:-).

Thank you for your contribution,
Matt

On Mon, Aug 17, 2015 at 4:10 AM, Olivier Commowick
<Olivier.Commowick at inria.fr> wrote:
> Hi Matt,
>
> Yes, those are very small numbers indeed :) After digging in the code, I
> found two discrepancies with respect to the vnl symmetric eigensystem, so I
> took the vnl code as the reference and did the corrections. I am submitting
> the two patches on gerrit (I didn’t know they would be separated for review,
> I can merge them if necessary):
>
> - http://review.source.kitware.com/#/c/20092/
> - http://review.source.kitware.com/#/c/20093/
>
> That being said, when digging in the code, it is not clear to me why
> SymmetricEigenAnalysis is not just a wrapper on vnl_symmetric_eigensystem.
> This would avoid duplicated code, and I do not see any place in the
> eigensystem code that wouldn’t be thread safe. But again, I am not an expert
> of that code and so I may be wrong. In any case, may be something to think
> of for future work :)
>
> Thanks for the help,
> Olivier
>
> On 13 Aug 2015, at 22:59, Matt McCormick <matt.mccormick at kitware.com> wrote:
>
> Hi Olivier,
>
> Those are some pretty small numbers ;-).  Thanks for sharing your
> experiences.
>
> It may be necessary to build both ITK and vnl/netlib instances with
> debugging symbols and walk through them in a debugger to see where the
> deviation occurs.
>
> As always, patches are welcome :-)
>
> Thanks,
> Matt
>
> On Thu, Aug 13, 2015 at 9:35 AM, Olivier Commowick
> <Olivier.Commowick at inria.fr> wrote:
>
> Hello all,
>
> I have been experiencing a problem when using the itk SymmetricEigenAnalysis
> class in ITK. When using this class, it sometimes produces NaN
> eigenvalues/vectors. On the contrary, the vnl_symmetric_eigensystem doesn’t
> seem to have a problem (and neither does matlab). Looking at the
> documentation, it seems both use QL decomposition but not implemented in the
> same place. In addition, the vnl one is said to be not thread safe, although
> it’s the one working for me.
>
> In fact, I have isolated one 3x3 matrix for which I always have the problem.
>
> A =
> [7.503107128706855667859804009212206062302e-53
> 1.870107888834986371214720644344813262375e-52
> -8.662047753682068190087788895125120421821e-27;
> 1.870107888834986371214720644344813262375e-52
> 4.661140319458029838073740773908360767728e-52
> -2.15896741973055935210319454393408888029e-26;
> -8.662047753682068190087788895125120421821e-27
> -2.15896741973055935210319454393408888029e-26 1]
>
> It’s a weird one I know, but again vnl (and matlab) works great on it. Would
> there be anyone who knows the QL decomposition code enough to help me
> understand and possibly correct the ITK class so that it does the same as
> the VNL one ?
>
> Thanks a lot in advance.
>
> ---
>
> Olivier Commowick, Ph.D.
> Research Scientist
> INRIA Rennes - Bretagne Atlantique, VISAGES Team
> Campus de Beaulieu
> 35042 Rennes
> FRANCE
>
> Phone: +33 2 99 84 25 92
> Email: Olivier.Commowick at inria.fr
> Web: http://olivier.commowick.org/
>
>
>
>
>
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>
> ---
>
> Olivier Commowick, Ph.D.
> Research Scientist
> INRIA Rennes - Bretagne Atlantique, VISAGES Team
> Campus de Beaulieu
> 35042 Rennes
> FRANCE
>
> Phone: +33 2 99 84 25 92
> Email: Olivier.Commowick at inria.fr
> Web: http://olivier.commowick.org/
>
>
>
>
>
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