[Insight-users] Parametric vs. Non-parametric image registration?

[motes motes]

The terms Parametric and Non-parametric Image registration are used in
many articles but what is the exact difference?

I know that optimizing a deformation field using eg. B-Splines is
called parametric (the coefficients/parameters of the deformation
vectors are optimized).

But what are equivalent examples of non-parametric methods?