[Insight-developers] Unit tests - Smoothing difference between standard and diffeomorphic demons

[Tom Vercauteren]

Hi Luis and Hans,

Thanks for your feedback. I'll try and comment below.

> I can see, however, how the final smoothing may be a problem,
> since it is an extra smoothing after the last version of the
> field has been computed.
>
> This raises the interesting point that probably a better
> behavior of the overall algorithm will be to perform progressively
> less and less smoothing as the iterations progress, under the
> assumption that the field is converging to a stable solution,
> and at that point, the smoothing, more than regularizing the
> solution is actually perturbing it.

I don't think that the final smoothing is perturbing the results.
Indeed the "quality" of the registration is a trade-off between the
image similarity and the field regularity. The test is only measuring
one of these metrics.

Concerning a progressive reduction of the smoothing, I think it is out
of the scope of this discussion and needs some research :)
Anyhow, it should at least be done in conjunction with a progressive
reduction of the update step length.

> The good news is that given that the new code is in the Review
> directory, we have the freedom to rework its API and behavior
> to do the right thing. Changes are acceptable at this point if
> we can make the case that they relate to bug fixes, and not
> to adding new functionality.
>
> The three options that you suggest sound very reasonable.
> They don't seem to be exclusive, so we could actually opt
> for all of them.
>
> Adding a flag to allow both behaviors in the DemonsRegistration
> filters (smoothing first or smoothing after), documenting the flag and
> its effect, and adding tests for the flag ON and flag OFF, so we
> exercise both types of usage. Note that then we can relax the tests
> cases corresponding to the 'smooth after' case.
>
> If you have some time available for making the appropriate
> changes inside your Demons classes, we will take care of the
> documentation and the additional testing. We probably shouldn't
> touch the current Demons class in Algorithms.