[MOAB-dev] Matrix3 EigenDecomp Bug

[Grindeanu, Iulian R.]

Ouch
only obb tree is using it. 
So only obb tree is affected by it

indeed, it is returning cartVect's as you said
It should be returning the transpose

strange nobody noticed 

I remember somebody did a study of our obb tree and it was not scaling properly

Could this be the reason?


________________________________________
From: moab-dev-bounces at mcs.anl.gov [moab-dev-bounces at mcs.anl.gov] on behalf of shriwise [shriwise at wisc.edu]
Sent: Tuesday, August 18, 2015 3:56 PM
To: moab-dev at mcs.anl.gov
Subject: [MOAB-dev] Matrix3 EigenDecomp Bug

Hey all,

After a day or so of working on this, I found an error in the Eigenvalue
decomposition function provided by Matrix3.

I believe that its computation is correct, but the way that it returns
the Eigenvectors is at the very least ambiguous, and I would argue that
it is simply incorrect.

Typically the matrix produced by this decomposition contains the vectors
in a column-based manner, but the vectors are being returned in an array
of CartVects where each CartVect contains a row of the resulting matrix.
The problem being that the solution matrix in this case is indeed column
based. So while the correct information is available, it is returned in
such a way that leads one to believe each CartVect contains one of the
EigenVectors when this is not the case.

I found this by comparing the results of this function to that of a
common linear algebra library, Armadillo.

I've attached the test program I was using if anyone is curious.

Normally, I would simply fix it and create a PR but I'm not quite
certain how to proceed. I think probably the best way is to fix this at
the source in Matrix3.hpp but I don't want any reliant functionality to
be affected without notice. This is a long standing bug affecting the
OBBTree. I'll start working on a fix tomorrow morning, perhaps by
finding anywhere this is used and double checking the use of the
returned Eigenvectors. Does that sound amenable to all?

Cheers,

--
Patrick C. Shriwise
Research Fellow
University of Wisconsin - Madison
Engineering Research Building - Rm. 428
1500 Engineering Drive
Madison, WI 53706
(608) 446-8173