Hi Tony
typically you don't measure distortion by total displacement as if you
rotate the sphere the displacement would be large but you would not incur
any distortion.
There are a bunch of measures that try to summarize distortion, but
basically there is areal and angular distortion that can be combined in
different summary measures. Areal you can measure by first normalizing the
two surfaces to have the same total area (not needed in the sphere
comparison, but if you want to compare to e.g. white you would need to),
then look at the ratio of the area of each triangle. Angular you can
compare directly. There are other measures like harmonic energy you can
also look at.
cheers
Bruce
On Fri, 10 Nov 2017, tony han wrote:
Hi Bruce,
Thanks for your quick response! So I have several follow-up questions:
1. As I load the two files (.sphere and .sphere.reg), there are two variables
for each file, i.e.
coordinates and facets. If I want to measure the distortion during
registration, I will only need to
look at coordinates? If so, the most appropriate way is simply calculating the
Euclidean distance
for each vertex between the two files (i.e., compute the distance between the
first row
of coordinates in .sphere file and the first row of coordinates in .sphere.reg,
and so forth)?
2. What file I can use to get the distortion by the sphere inflation, as you
mentioned?
3. The measure of vertex-wise distortion is basically how far each vertex has
been warped from its
original coordinate (native space) to the new coordinate (fsaverage space)?
Thanks!
Best,
Tony
____________________________________________________________________________________________________
From: freesurfer-boun...@nmr.mgh.harvard.edu
<freesurfer-boun...@nmr.mgh.harvard.edu> on behalf of
Bruce Fischl <fis...@nmr.mgh.harvard.edu>
Sent: Thursday, November 9, 2017 12:28:13 PM
To: Freesurfer support list
Subject: Re: [Freesurfer] comparing sphere and sphere.reg
Hi Tony
you can do that but it won't measure the distortion induced by the sphere
mapping itself.
cheers
Bruce
On Thu, 9 Nov 2017, tony han wrote:
>
> Hi,
>
>
> I'm trying evaluating the distortion for each subject from their native space
to fsaverage atlas
> space. Is it a appropriate measure to compare ?h.sphere and ?h.sphere.reg? I
assume that by
> comparison, I may get the how each vertex warped during the registration?
Thanks!
>
>
> Best,
>
> Tony
>
>
>
>
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