FFTW's “advanced” interface supplements the basic interface with four
new planner routines, providing a new level of flexibility: you can plan
a transform of multiple arrays simultaneously, operate on non-contiguous
(strided) data, and transform a subset of a larger multi-dimensional
array. Other than these additional features, the planner operates in
the same fashion as in the basic interface, and the resulting
fftw_plan is used in the same way (see Using Plans).