[Insight-developers] Question regarding itk::SymmetricDemonsForceRegistrationFunction::ComputeUpdate()

[Torsten Rohlfing]

Hi --

This is not particularly important, but I was wondering if someone can 
help me understand something I find slightly sketchy about the 
implementation of 
itk::SymmetricDemonsForceRegistrationFunction::ComputeUpdate().

Towards the end of the function, there is a block labeled "update the 
metric with the latest deformable field". What happens there is that in 
the global data structure, the mean squared difference between fixed and 
moving image is updated with the metric value AFTER applying the current 
update. Or I should say: supposedly that's what happens.

I am wondering whether this is really necessary (it does take some extra 
CPU time after all)? The non-symmetric force class does not do this, and 
I can't figure out where this is ever used? Also, it does not seem 
particularly meaningful, since the update computed in ComputeUpdate is 
not necessarily applied fully in the respective registration filter's 
ApplyUpdate() member function. There may be smoothing, for example, of 
either the update or the updated deformation field. So the value that is 
computed in ComputeUpdate() for the MSD after applying the update is not 
actually guaranteed to be correct anyway.

Any thoughts would be greatly appreciated.

Best,
  Torsten

-- 
Torsten Rohlfing, PhD          SRI International, Neuroscience Program
 Research Scientist             333 Ravenswood Ave, Menlo Park, CA 94025
  Phone: ++1 (650) 859-3379      Fax: ++1 (650) 859-2743
   torsten at synapse.sri.com        http://www.stanford.edu/~rohlfing/

     "Though this be madness, yet there is a method in't"