[ITK] Fwd: Image Pyramid Speed

[Stephen Parsons]

I have been attempting to use the MultiResolutionImageRegistrationMethod.
As I understand it, the input images are blurred and then resampled to
create smaller internal images on which the registration operates for the
lower levels. I have set the number of levels and the starting shrink
factors and by outputting the shrink factor at each level I have confirmed
that it seems to be set up correctly (shrink factors are halved with each
new level).

The program will run and produce correctly registered output, but each
iteration takes just as long as it did before I implemented the image
pyramid, when it was operating on full size images. Running smaller images
through the pipeline leads to a dramatic increase in speed which made me
think that the image pyramid in MultiResolutionImageRegistrationMethod
would allow for the same speed increase, at least in the beginning levels.
With the image pyramid there is no speed decrease as it proceeds through
the pyramid levels, the first takes just as long as the last.

What could I have missed that would make the image pyramid just as slow as
not having it implemented at all? Have I misunderstood something about the
way it is implemented or is intended to be used?

Thanks,
Stephen Parsons
University of Kentucky
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