Sampler in Elastix and ITK

[pypi20200320]

trafficstars

When studying the source code of Elastix, I have always had a question regarding the design of the sampler mechanism, which is based on the fixed image. What is the reason behind this design choice and what benefits does it bring? Why doesn't it allow flexible setting of a VirtualDomainImage like in ITK, and then perform sampling based on the VirtualDomainImage? I'm wondering if someone can help clarify my confusion.