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

[Matt McCormick]

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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