[Insight-developers] Question regardingitk::SymmetricDemonsForceRegistrationFunction::ComputeUpdate()

[Torsten Rohlfing]

Jim:

Thanks for the explanation. I can almost see your point, except that I 
disagree with this statement:

>The symmetric case calculates the SumOfSquaredDifference as the 
>difference between the fixed and moving image using the deformation
>field calculated by the CURRENT iteration, i.e. the SumOfSquaredDifferences
>AFTER the new update field is calculate.
>
>So, the SumOfSquaredDifference in the non-symmetric case is the alignment
>error at the beginning of an iteration, whereas for the symmetric case, it is 
>the alignment error at the end of an iteration.
>  
>
As I mentioned earlier, the registration function does not actually know 
(or have control over) how much of the computed update is actually 
applied (see time step parameter "dt" to ApplyUpdate), and whether the 
update or the resulting deformation field is smoothed. So what I've been 
seeing here when I played around with things was that the symmetric 
force actually produced oscillating values, even though the actual 
metric was actually decreasing monotonically. That's because the 
registration function in general applies too aggressive an update 
internally, so it overshoots.

Again, thanks for your comments!
  Torsten