Hi!

I am trying to add the VirtualDomain as an option to the SimpleITK 
ImageRegistrationMethod. I have does so here [1].

This method for setting the virtual domain follows the 
SimpleImageRegistrationTest / Example by setting the VirtualDomain on the 
Metric[2].

The test created in SimpleITK registers two gaussian blobs with a translation 
transform. The fixed image has it’s origin set to (5,100), and the 
FixedInitalTransform is set to a translation transform with the same 5,100 
offset. This should map the 0-origined virtual domain to the same index on the 
fixed image, to essentially be an identity translation between then virtual 
domain and the fixed image when the origin and transform is take into 
consideration. This test fails with the following error:

MeanSquaresImageToImageMetricv4 (0x66f8930): No valid points were found during 
metric evaluation. For image metrics, verify that the images overlap 
appropriately. For instance, you can align the image centers by translation. 
For point-set metrics, verify that the fixed points, once transformed into the 
virtual domain space, actually lie within the virtual domain.

This is an indication the transforms are not being applied as I expect. My 
expectation of how the transforms should be applied here [3].

Digging into the code, I see that the ImageRegistrationMethod always sets the 
VirtualDomain here [4], to the FixedImage. This clobbers the value set directly 
to the Metric as in the example/test[2]. Hacking the code to use the 
GetCurrentLevelVirtualDomainImage() does get my one case to work.

Suggestions on how to correct this issue, or a patch are welcomed!

Thanks,
Brad


[1] http://review.source.kitware.com/#/c/21540/2
[2] 
https://github.com/InsightSoftwareConsortium/ITK/blob/master/Modules/Registration/RegistrationMethodsv4/test/itkSimpleImageRegistrationTest.cxx#L162
[3] 
http://insightsoftwareconsortium.github.io/SimpleITK-Notebooks/61_Registration_Introduction_Continued.html#Version-1.1
[4] 
https://github.com/InsightSoftwareConsortium/ITK/blob/master/Modules/Registration/RegistrationMethodsv4/include/itkImageRegistrationMethodv4.hxx#L409-L415
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