[Insight-users] gaussian filtering in itkSymmetricForcesDemonsRegistrationFilter

[Luis Ibanez]

Hi Corinne,

The purpose of the smoothing in the implementation of
the Demons algorithm is to regularize the deformation
field.

In principle, what would be expected from this filter
is to return the deformation field that minimizes the
sum of squared differences between the fixed image and
the deformed moving image.  Since the algorithm is
stopped when the changed in this difference is under
a threshold, it is reasonable to return the latest
deformation field that produced this minimum value.

If the filter is changed to smooth the last deformation
field, a user will find the inconsistent situation in
which by using this (smoothed) deformation fields for
resampling the moving image, and recomputing the sum
of squared differences she/he will not be able to find
the same last metric value returned by the filter.

In addition, it is expected that if the algorithm really
converged, no further modification of the deformation
field should be necessary.


Do you find any particular reason for which the
deformation field should have this extra smoothing
after convergence ?


Please let us know,


    Thanks


      Luis



-------------------------
Corinne Mattmann wrote:

> Hi,
> 
> I've just noticed, that in itkSymmetricForcesDemonsRegistrationFilter
> the deformable field is always smoothed before performing an iteration
> (line 70 in itkSymmetricForcesDemonsRegistrationFilter.txx). Like this,
> the output field (the one after the last iteration) is never filtered.
> Is there a reason not doing this?
> 
> Thanks,
> Corinne
> 
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