[Insight-users] itk::MultiResolutionPDEDeformableRegistration

[jef vdmb]

Hi everyone,

I noticed some unexpected behavior using the itk::MultiResolutionPDEDeformableRegistration filter. It took me some time to figure ou what was happeingn so I thought it might be usefull to share my experience.

First of all when  starting a registration at the coarsest level, no iteration event is invoked. The event is only invoked after completion of this level. This is different from the MultiResolutionImageRegistrationMethod, where one is invoked at the beginning, thus allowing to set parameter in an observer class even for this level. It suffices to place the call:     

I used the  itk::MultiResolutionPDEDeformableRegistration in combination with the itk::RecursiveMultiResolutionPyramidImageFilter. Since this is not the default filter some inconsistenties can occur: the registration filter and the pyramid filter may be working with a different number of resolution levels if the SetNumberOfLevels() is called before setting the pyramid filters. This is what happens (or at least what I have deduced from going over the code)

*The (default) pyramid filters are initialized to a default number of levels in the  constructor of itk::MultiResolutionPDEDeformableRegistration
*When explicitly setting the number of levels using    itk::MultiResolutionPDEDeformableRegistration->SetNumberOfLevels(), the number of levels to the (default) pyramid filter are updated.
*When setting the pyramid filters afterwards, at no time the number of levels is updated to these filters and so they can be inconsistent with the registration filter (which was the case for me)

Of course this behavior can simply be avoided by calling    itk::MultiResolutionPDEDeformableRegistration->SetNumberOfLevels() as last, but since no warnings are given I thought I'd report it anyway. Alternatively a call this->SetNumberOfLevels() can be made in   itk::MultiResolutionPDEDeformableRegistration->GenerateData(). This function already verifies if any of the members needs to be updated, so won't do anything if not necessary.  

Regards,
Jef

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