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<h4 class="subsection">4.5.3 Guru Complex DFTs</h4>
<pre class="example"> fftw_plan fftw_plan_guru_dft(
int rank, const fftw_iodim *dims,
int howmany_rank, const fftw_iodim *howmany_dims,
fftw_complex *in, fftw_complex *out,
int sign, unsigned flags);
fftw_plan fftw_plan_guru_split_dft(
int rank, const fftw_iodim *dims,
int howmany_rank, const fftw_iodim *howmany_dims,
double *ri, double *ii, double *ro, double *io,
unsigned flags);
</pre>
<p><a name="index-fftw_005fplan_005fguru_005fdft-248"></a><a name="index-fftw_005fplan_005fguru_005fsplit_005fdft-249"></a>
These two functions plan a complex-data, multi-dimensional DFT
for the interleaved and split format, respectively.
Transform dimensions are given by (<code>rank</code>, <code>dims</code>) over a
multi-dimensional vector (loop) of dimensions (<code>howmany_rank</code>,
<code>howmany_dims</code>). <code>dims</code> and <code>howmany_dims</code> should point
to <code>fftw_iodim</code> arrays of length <code>rank</code> and
<code>howmany_rank</code>, respectively.
<p><a name="index-flags-250"></a><code>flags</code> is a bitwise OR (‘<samp><span class="samp">|</span></samp>’) of zero or more planner flags,
as defined in <a href="Planner-Flags.html#Planner-Flags">Planner Flags</a>.
<p>In the <code>fftw_plan_guru_dft</code> function, the pointers <code>in</code> and
<code>out</code> point to the interleaved input and output arrays,
respectively. The sign can be either -1 (=
<code>FFTW_FORWARD</code>) or +1 (= <code>FFTW_BACKWARD</code>). If the
pointers are equal, the transform is in-place.
<p>In the <code>fftw_plan_guru_split_dft</code> function,
<code>ri</code> and <code>ii</code> point to the real and imaginary input arrays,
and <code>ro</code> and <code>io</code> point to the real and imaginary output
arrays. The input and output pointers may be the same, indicating an
in-place transform. For example, for <code>fftw_complex</code> pointers
<code>in</code> and <code>out</code>, the corresponding parameters are:
<pre class="example"> ri = (double *) in;
ii = (double *) in + 1;
ro = (double *) out;
io = (double *) out + 1;
</pre>
<p>Because <code>fftw_plan_guru_split_dft</code> accepts split arrays, strides
are expressed in units of <code>double</code>. For a contiguous
<code>fftw_complex</code> array, the overall stride of the transform should
be 2, the distance between consecutive real parts or between
consecutive imaginary parts; see <a href="Guru-vector-and-transform-sizes.html#Guru-vector-and-transform-sizes">Guru vector and transform sizes</a>. Note that the dimension strides are applied equally to the
real and imaginary parts; real and imaginary arrays with different
strides are not supported.
<p>There is no <code>sign</code> parameter in <code>fftw_plan_guru_split_dft</code>.
This function always plans for an <code>FFTW_FORWARD</code> transform. To
plan for an <code>FFTW_BACKWARD</code> transform, you can exploit the
identity that the backwards DFT is equal to the forwards DFT with the
real and imaginary parts swapped. For example, in the case of the
<code>fftw_complex</code> arrays above, the <code>FFTW_BACKWARD</code> transform
is computed by the parameters:
<pre class="example"> ri = (double *) in + 1;
ii = (double *) in;
ro = (double *) out + 1;
io = (double *) out;
</pre>
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