MattesMutualInformationv4 throws exception when fixed image has a single value.

[zivy]

PR #3679 provides a test illustrating the issue.

Issue originally reported here.

[agarcia-ruiz]

Hi, I have encountered this same issue with MI metric when running antsSliceRegularizedRegistration (2D slices), but as far as I can tell, no slices have a single value? Some may have a lot of zeroes. Setting CC as metric works, also replacing zeroes with random noise works with MI too.

I got this warning first:
Warning: bad time point - too little intensity variation0 0

Then the next exception:
terminate called after throwing an instance of 'itk::ExceptionObject' what(): /home/runner/work/_temp/build/staging/include/ITK-5.3/itkImageBase.hxx:77: ITK ERROR: Image(0x55a2a1f15e30): Zero-valued spacing is not supported and may result in undefined behavior. Refusing to change spacing from [1, 1] to [0, 0]

On other cases, I got a different error message (Joint PDF summed to zero), but also fixed by the workaround above.
Many of these:
/home/runner/work/_temp/build/ITKv5/Modules/ThirdParty/VNL/src/vxl/core/vnl/algo/vnl_svd.hxx: suspicious return value (2) from SVDC /home/runner/work/_temp/build/ITKv5/Modules/ThirdParty/VNL/src/vxl/core/vnl/algo/vnl_svd.hxx: M is 2x2 M = [ ... -nan -nan -nan -nan ]

And then:
Exception caught: itk::ExceptionObject (0x555cb90599a0) Location: "unknown" File: /home/runner/work/_temp/build/staging/include/ITK-5.3/itkMattesMutualInformationImageToImageMetricv4.hxx Line: 315 Description: ITK ERROR: MattesMutualInformationImageToImageMetricv4(0x555cbc91ee60): Joint PDF summed to zero

Just reporting I think it happens with very little intensity variation, and in case someone else is getting these exceptions, so they can try the workaround to know if that is the issue.

[orena1]

any update on this one?
Thanks