[Insight-users] Orthogonality tolerance test

[Luis Ibanez]

Hi Andy,

Thanks for pointing this out.

We have struggled in the past with the selection
of values to be used as tolerances...

We tend to put tolerances that are too stringent,
and sometimes unrealistic.  I recall that recently
Hans Johnson fixed several classes that were
using tolerances that were too exigent.

You may be pointing out to a case that also needs
reevaluation.

Any suggestions on what could be a realistic value,
that is still making sure that we don't take in matrices
that are too deformed ?

Also, another option to consider is that we have code
that computes the Orthogonal part of a Matrix that
is not fully orthogonal. This is done via Polar decomposition,

See for example
http://www.itk.org/pipermail/insight-users/2006-August/019025.html

You could use this code for cleaning up that Matrix,
before passing it to the Rigid Transform.


Please let us know what you think is a better approach,


        Thanks


             Luis


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On Thu, Jul 2, 2009 at 11:43 AM, Andriy Fedorov <fedorov at bwh.harvard.edu>wrote:

> Hi,
>
> What is the motivation for setting the tolerance in matrix orthogonality
> test to 1e-10 in itkRigid3DTransform.cxx?
>
> I have a transform produced by FSL FLIRT tool, which is orthogonal with
> tolerance 1e-5. I am trying to re-shape this transform to an ITK
> transform (FSL uses different origin conventions), so that I can import
> it into Slicer. With the current tolerance setting this is not possible,
> the code breaks when I am trying to set the matrix.
>
> Is the current check with 1e-10 tolerance level a correct and necessary
> precaution? Isn't it too strict for practical purposes?
>
> Thanks
>
> Andriy Fedorov
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