vendors/fftw3/doc/html/Guru-Real_002dto_002dreal-Transforms.html
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fftw_plan fftw_plan_guru_r2r(int rank, const fftw_iodim *dims,
int howmany_rank,
const fftw_iodim *howmany_dims,
double *in, double *out,
const fftw_r2r_kind *kind,
unsigned flags);
Plan a real-to-real (r2r) multi-dimensional FFTW_FORWARD transform with transform dimensions given by (rank, dims) over a multi-dimensional vector (loop) of dimensions (howmany_rank, howmany_dims). dims and howmany_dims should point to fftw_iodim arrays of length rank and howmany_rank, respectively.
The transform kind of each dimension is given by the kind parameter, which should point to an array of length rank. Valid fftw_r2r_kind constants are given in Real-to-Real Transform Kinds.
in and out point to the real input and output arrays; they may be the same, indicating an in-place transform.
flags is a bitwise OR (‘|’) of zero or more planner flags, as defined in Planner Flags.