[ITK] Image Registration Initialization

[Brian Eastwood]

Hi Folks,



I really like the changes implemented for ITK v4 registration. The composite transform makes multistage registration much easier to implement. Nice work!



Is there a way to explicitly set the fixed and moving images to be used at each level of a multiresolution registration? How about the gradient images used by the image metric? I see you can set gradient filters on the metric, but what if you already have the gradients precomputed?



I get the feeling that v4 registration runs slower than similar v3 registration, but I think it's mainly down to the initialization where the fixed and moving images are smoothed and the metric computes the image gradients.



It seems like there could be some real advantages to using precomputed smoothed and gradient images. For example, the smoothing could be implemented with a cascade of Gaussians where the standard deviation used at a level k is given by s^2 = s_k^2 - s_{k-1}^2. Or the smoothing could be implemented on the GPU. Or we could be reusing a resolution level in a different registration stage, perhaps optimizing a different set of transform parameters.



Have I missed something in the API where it is possible to explicitly set the images and gradients to use at each level of the registration? I could probably create an itk::Command that swaps the fixed and moving images before each registration iteration, but is there a way to disable the smoothing step?



Regards,

Brian
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