  Waveplates having spatially varying fast-axis orientation and retardance
provide an elegant and easy way to locally manipulate different attributes of
light beams namely, polarization, amplitude and phase, leading to the
generation of exotic structured light beams. The fabrication of such doubly
inhomogeneous waveplates (d-plates) is more complex, compared to that of singly
inhomogeneous waveplates (s-plates) having uniform retardance, which can be
easily fabricated by different means such as photoalignment of liquid crystals,
metasurfaces etc. Here, exploiting the SU(2) formalism, we establish
analytically that any d-plate can be equivalently implemented using a pair of
quarter-wave s-plates and a half-wave s-plate. An important advantage of this
method is that it gives the flexibility to realize a whole family of distinct
d-plates using the same triplet of s-plates. To underline the scope of this
method, we propose novel d-plates for spatially tailoring the phase and complex
amplitudes of light beams. Towards complex amplitude shaping, we present a
generic method for carving out higher-order eigenmodes of light using a d-plate
in conjugation with a polarizer. A generalized q-plate-like gadget, for
imparting a polarization-dependent phase profile to a scalar light beam, is
proposed as a demonstration of phase-polarization interplay. For these two
illustrations, the corresponding three-s-plate gadget is constructed, and its
functioning is validated with extensive numerical simulations. The main result
and its illustrations are generic and agnostic to the way the s-plates are
fabricated and we believe they carry the potential to push the current state of
the art in interdisciplinary applications involving structured light beams.
