[Insight-users] TPS approximation

[Luis Ibanez]

Hi Martin,

Please do the following:

1) reduce this to a minimal case,
2) make an entry in the bug tracker, and
3) upload the source code that illustrates
    the problem the bug entry

This behavior may well be a bug,... and the
way to verify it is by doing first (1,2,3).


    Thanks


          Luis

-------------------------------------------------------------
On Mon, Sep 13, 2010 at 5:26 AM, <M.Baiker at lumc.nl> wrote:

> Dear all,
>
>
>
> I have two corresponding 3D point sets and tried to map one domain towards
> the other using
>
> “itkThinPlateSplineKernelTransform”. Since I want to allow small landmark
> localization errors, I set
>
> the stiffness parameter > 0. However, the results do not make sense at all
> (I also have a Matlab
>
> implementation where I can compare it with).
>
> Having a look at the code I noticed, that the G matrix is computed as r
> i.e. the Euclidean distance
>
> between landmarks. Shouldn’t that be –r instead? While for the
> interpolation case it shouldn’t
>
> matter (since u(x) = A*x + B + I* sum_i=1_N(wi * ri) and if ri is –ri, the
> warping coefficients wi
>
> become –wi as well), it does matter for the approximation case.
>
> To give it a try, I changed r to –r and the results are already much
> better, however still not
>
> correct. Finally I tried m_Stiffness = 0 for –r and I saw that the result
> is different from the result
>
> using r, which shouldn’t be the case (see above).
>
> Could anyone comment on this? Did anyone use the TPS mapping with setting
> m_Stiffness > 0
>
> in the past? I tried to step through “itkKernelTransform” but couldn’t find
> an error so far. However
>
> I also have to admit that I had quite some trouble getting through the
> code…
>
>
>
> One further remark: What is in the current implementation is a smoothing
> TPS rather than a TPS
>
> approximation since m_Stiffness has to be specified as being the same for
> all landmarks. For the
>
> approximation case, as demonstrated in the Rohr et al. paper, it should be
> possible to set
>
> m_Stiffness for all landmarks individually.
>
>
>
> Thanks in advance.
>
>
>
> Greetings,
>
> Martin
>
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