[ITK] Metric NaN and multi resolution registration

[Luigi Riba]

Dear ITK community,

today I was trying to implement a multi-resolution registration process for
3d images.

I started from the example in the tutorial. Unfortunately, I've received
the following warning:

> WARNING: In
> f:\libs\insighttoolkit-4.8.2\modules\numerics\optimizersv4\include\itkObjectToObjectMetric.hxx,
> line 529
> MeanSquaresImageToImageMetricv4 (000000006506C020): 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.


These are the parameters I have used for the multi resolution process:
> const unsigned int numberOfLevels = 3;
> RegistrationType::ShrinkFactorsArrayType shrinkFactorsPerLevel;
> shrinkFactorsPerLevel.SetSize(numberOfLevels);
> shrinkFactorsPerLevel[0] = 4;
> shrinkFactorsPerLevel[1] = 2;
> shrinkFactorsPerLevel[2] = 1;
> RegistrationType::SmoothingSigmasArrayType smoothingSigmasPerLevel;
> smoothingSigmasPerLevel.SetSize(numberOfLevels);
> smoothingSigmasPerLevel[0] = 2;
> smoothingSigmasPerLevel[1] = 1;
> smoothingSigmasPerLevel[2] = 0;
> registration->SetNumberOfLevels(numberOfLevels);
> registration->SetSmoothingSigmasPerLevel(smoothingSigmasPerLevel);
> registration->SetShrinkFactorsPerLevel(shrinkFactorsPerLevel);


Could you please me explain what is the meaning of this warning?

By the way, I am working with affine transformations and I have used the
transform initializer.

Best,

Luigi
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