  We provide a new angle and obtain new results on a class of metrics on
length-normalized curves in $d$ dimensions, represented by their unit tangents
expressed as a function of arc-length, which are functions from the unit
interval to the $(d-1)$-dimensional unit sphere. These metrics are derived from
the combined action of diffeomorphisms (change of parameters) and
arc-length-dependent rotation acting on the tangent. Minimizing a Riemannian
metric balancing a right-invariant metric on diffeomorphisms and an $L^2$ norm
on the motion of tangents leads to a special case of "metamorphosis", which
provides a general framework adapted to similar situations when Lie groups acts
on Riemannian manifolds. Within this framework and using a Sobolev norm with
order 1 on the diffeomorphism group, we generalize previous results from the
literature that provide explicit geodesic distances on parametrized curves.
