[ITK-users] Problem with ITK eigensystem

[Olivier Commowick]

Hello,

I think I have found a bug in the behaviour of ITK’s symmetric eigensystem. In the attached link (http://olivier.commowick.org/tmp/Test.zip <http://olivier.commowick.org/tmp/Test.zip>) you will find an example code and matrix that fails with it and not with VNL’s symmetric eigensystem (run on macOS 10.12, Xcode 8.2.1).

It seems to me that there are subtle differences in between ITK and VNL versions, VNL being more robust (and the same as what Matlab gives, I didn’t test python). Here is what I have found so far :

- the hypothenuse function (hypot for ITK and pythag for VNL) do not get always the exact same result up to a very far away decimal
- some loop indices were wrong I think in the ITK eigensystem. Some do vary in the VNL version, not in ITK’s equivalent

In the patch file attached, I have made a set of corrections that make things in ITK eigensystem closer to VNL but still the ITK version fails. I have to say I am getting completely lost here so I am calling upon the community for help. If anyone knows this enough, could you try the test attached and see if you have any ideas of what is going on. That would help us a lot.

The test attached can be compiled with cmake and run with the text file included : ./test_eigensystem data.txt 

Thanks in advance
Olivier



---

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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