[ITK] [ITK-users] Problem Using ScaleEstimator in V4 Registration

[Dženan Zukić]

Hi Sepide,

if I remember correctly, registration framework is advertised as using
random sampling. And the function you mention is not used for sampling the
registration metric, but rather for parameter scale estimation. So even if
random sampling usage for scale estimation is unintentional, I don't
believe it is a major problem. Perhaps someone who participated in writing
the registration framework can chime in?

Regards,
Dženan

On Wed, Aug 24, 2016 at 1:52 PM, Sepide Movaghati <
sepideh.movaghati at gmail.com> wrote:

> Hi,
>
> I found a problem while I was using ScaleEstimator in v4 registration:
>
> In itkRegistrationParametersScaleEstimator class there are two functions
> SetScaleSamplingStrategy and SetStepScaleSamplingStrategy. In both the
> sampling strategy is chosen based on the Transformation type. For example
> if the transformation is of type displacement field or BSpline, it uses all
> of the samples. If transformation is any regular affine or rigid
> transformation (child of class MatrixOffsetTransformBase) it performs
> corner sampling. However, if the transformation is none of the known ones,
> it uses Random Sampling.
>
> Here is the problem: in itkImageRegistrationMethodv4.hxx the Metric
> transformation is set using ->SetMovingTransform() and it is always a
> CompositeTransform! Therefore, ScaleEstimator cannot detect the
> transformation type and it always fall into "else" which is using Random
> Sampling! It does not matter if you do a multilevel registration or not.
> The transformation is always of type CompositeTransform even if you do just
> a one level rigid transform!
>
> I would appreciate if you let me know if I was correct about this issue
> and whether we can consider this a bug and hope to get it fixed for the
> upcoming versions.
>
> Thanks a lot for your consideration in advance,
> Sepide
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