On 28 August 2016 at 06:59, Knut J Bjuland <knutjorgen@outlook.com> wrote:

I found this paper about named “Reproducibility of neuroimaging analyses
across operating systems” (Glatard et al., 2015).   There were differences
in cortical morphometry like cortical thicknes between  images proscced in
the same version of freesurfer with different version of Linux or glibc for
Freesurfer, and according to the paper, it was caused by how different
version of glibc handles small single precssion floating numbers. I have
therefore some questions.

Are there plans for freesurfer to use double floating numbers in steps that
may be affected by numerical stability? Or can this problem be solved by
using staticall linking glibc into Freesurfer?

Freesurfer does use double precision numbers extensively on the CPU
side (on the GPU side of things, I generally went back to singles).
The issue is not one of numeric stability, and going to all-double (or
all-quad) would not remove the differences - they are fundamental to
how floating point arithmetic works. Put simply, for floating point
numbers:
a+(b+c) != (a+b)+c
Any attempt to ignore this fact is going to come to grief at some point.

Freesurfer often has to do things such as "find voxels exceeding
threshold" or "find most likely label for voxel." Operations like this
are always going to be vulnerable to differences in the least
significant bit.

Speaking from a numerical background (I have no experience in
neuroscience), I would suggest embracing this behaviour rather than
trying avoid it (arguably a fool's errand). Use the differences
between different library versions (not just libc, but all the other
numerical libraries Freesurfer uses) to provide an estimate of the
confidence levels/error bounds on your Freesurfer runs. If your
conclusion depends critically on using version x.y.z of a library,
rather than x.y.z+1, then unless you can point to a specific bug in
the library, you probably don't really have a conclusion.

Regards,

Richard

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numerical libraries Freesurfer uses) to provide an estimate of the confidence levels/error bounds on your Freesurfer runs"