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<title>FFTW 3.3.8: Guru Complex DFTs</title>

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<a name="Guru-Complex-DFTs"></a>
<div class="header">
<p>
Next: <a href="Guru-Real_002ddata-DFTs.html#Guru-Real_002ddata-DFTs" accesskey="n" rel="next">Guru Real-data DFTs</a>, Previous: <a href="Guru-vector-and-transform-sizes.html#Guru-vector-and-transform-sizes" accesskey="p" rel="prev">Guru vector and transform sizes</a>, Up: <a href="Guru-Interface.html#Guru-Interface" accesskey="u" rel="up">Guru Interface</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Guru-Complex-DFTs-1"></a>
<h4 class="subsection">4.5.3 Guru Complex DFTs</h4>

<div class="example">
<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></div>
<a name="index-fftw_005fplan_005fguru_005fdft"></a>
<a name="index-fftw_005fplan_005fguru_005fsplit_005fdft"></a>

<p>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-5"></a>
<p><code>flags</code> is a bitwise OR (&lsquo;<samp>|</samp>&rsquo;) of zero or more planner flags,
as defined in <a href="Planner-Flags.html#Planner-Flags">Planner Flags</a>.
</p>
<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 <em>-1</em> (=
<code>FFTW_FORWARD</code>) or <em>+1</em> (= <code>FFTW_BACKWARD</code>).  If the
pointers are equal, the transform is in-place.
</p>
<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:
</p>
<div class="example">
<pre class="example">ri = (double *) in;
ii = (double *) in + 1;
ro = (double *) out;
io = (double *) out + 1;
</pre></div>

<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>
<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:
</p>
<div class="example">
<pre class="example">ri = (double *) in + 1;
ii = (double *) in;
ro = (double *) out + 1;
io = (double *) out;
</pre></div>

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<div class="header">
<p>
Next: <a href="Guru-Real_002ddata-DFTs.html#Guru-Real_002ddata-DFTs" accesskey="n" rel="next">Guru Real-data DFTs</a>, Previous: <a href="Guru-vector-and-transform-sizes.html#Guru-vector-and-transform-sizes" accesskey="p" rel="prev">Guru vector and transform sizes</a>, Up: <a href="Guru-Interface.html#Guru-Interface" accesskey="u" rel="up">Guru Interface</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Mon, 02 Mar 2020 14:03:47 +0000
parents	bd3cc4d1df30
children