4.5.4 Guru Real-data DFTs

     fftw_plan fftw_plan_guru_dft_r2c(
          int rank, const fftw_iodim *dims,
          int howmany_rank, const fftw_iodim *howmany_dims,
          double *in, fftw_complex *out,
          unsigned flags);
     
     fftw_plan fftw_plan_guru_split_dft_r2c(
          int rank, const fftw_iodim *dims,
          int howmany_rank, const fftw_iodim *howmany_dims,
          double *in, double *ro, double *io,
          unsigned flags);
     
     fftw_plan fftw_plan_guru_dft_c2r(
          int rank, const fftw_iodim *dims,
          int howmany_rank, const fftw_iodim *howmany_dims,
          fftw_complex *in, double *out,
          unsigned flags);
     
     fftw_plan fftw_plan_guru_split_dft_c2r(
          int rank, const fftw_iodim *dims,
          int howmany_rank, const fftw_iodim *howmany_dims,
          double *ri, double *ii, double *out,
          unsigned flags);

Plan a real-input (r2c) or real-output (c2r), multi-dimensional DFT 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. As for the basic and advanced interfaces, an r2c transform is FFTW_FORWARD and a c2r transform is FFTW_BACKWARD.

The last dimension of dims is interpreted specially: that dimension of the real array has size dims[rank-1].n, but that dimension of the complex array has size dims[rank-1].n/2+1 (division rounded down). The strides, on the other hand, are taken to be exactly as specified. It is up to the user to specify the strides appropriately for the peculiar dimensions of the data, and we do not guarantee that the planner will succeed (return non-NULL) for any dimensions other than those described in Real-data DFT Array Format and generalized in Advanced Real-data DFTs. (That is, for an in-place transform, each individual dimension should be able to operate in place.)

in and out point to the input and output arrays for r2c and c2r transforms, respectively. For split arrays, ri and ii point to the real and imaginary input arrays for a c2r transform, and ro and io point to the real and imaginary output arrays for an r2c transform. in and ro or ri and out may be the same, indicating an in-place transform. (In-place transforms where in and io or ii and out are the same are not currently supported.)

flags is a bitwise OR (‘|’) of zero or more planner flags, as defined in Planner Flags.

In-place transforms of rank greater than 1 are currently only supported for interleaved arrays. For split arrays, the planner will return NULL.