[ITK] Virtual image and TMetricTraits in Joint histogram mutual information

[Constantinus Spanakis]

Hello. In the ITK PDFs tutorials, the basic components of the ITKv4
framework, in addition to the components that existed in the previous
framework, there is a new component, the Virtual image, which provides
more flexibility in our efforts to registrate two images. I tried to
use the Joint histrogram mutual information metric and its template is
the following one:

itk::JointHistogramMutualInformationImageToImageMetricv4< TFixedImage,
TMovingImage, TVirtualImage, TInternalComputationValueType,
TMetricTraits > Class Template Reference

In the case where the virtual image is the same as the fixed image, we
can define our metric:

itk::JointHistogramMutualInformationImageToImageMetricv4< TFixedImage,
TMovingImage, TFixedImage, TInternalComputationValueType,
TMetricTraits > Class Template Reference

But what happens when they aren't the same? How can we define our
different-from-fixed-image virtual image? And last but not least, what
are the TMetricTraits and how are they defined?

Thank you