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<title>FFTW 3.3.8: Advanced Complex DFTs</title>
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<a name="Advanced-Complex-DFTs"></a>
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<p>
Next: <a href="Advanced-Real_002ddata-DFTs.html#Advanced-Real_002ddata-DFTs" accesskey="n" rel="next">Advanced Real-data DFTs</a>, Previous: <a href="Advanced-Interface.html#Advanced-Interface" accesskey="p" rel="prev">Advanced Interface</a>, Up: <a href="Advanced-Interface.html#Advanced-Interface" accesskey="u" rel="up">Advanced Interface</a> [<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="Advanced-Complex-DFTs-1"></a>
<h4 class="subsection">4.4.1 Advanced Complex DFTs</h4>
<div class="example">
<pre class="example">fftw_plan fftw_plan_many_dft(int rank, const int *n, int howmany,
fftw_complex *in, const int *inembed,
int istride, int idist,
fftw_complex *out, const int *onembed,
int ostride, int odist,
int sign, unsigned flags);
</pre></div>
<a name="index-fftw_005fplan_005fmany_005fdft"></a>
<p>This routine plans multiple multidimensional complex DFTs, and it
extends the <code>fftw_plan_dft</code> routine (see <a href="Complex-DFTs.html#Complex-DFTs">Complex DFTs</a>) to
compute <code>howmany</code> transforms, each having rank <code>rank</code> and size
<code>n</code>. In addition, the transform data need not be contiguous, but
it may be laid out in memory with an arbitrary stride. To account for
these possibilities, <code>fftw_plan_many_dft</code> adds the new parameters
<code>howmany</code>, {<code>i</code>,<code>o</code>}<code>nembed</code>,
{<code>i</code>,<code>o</code>}<code>stride</code>, and
{<code>i</code>,<code>o</code>}<code>dist</code>. The FFTW basic interface
(see <a href="Complex-DFTs.html#Complex-DFTs">Complex DFTs</a>) provides routines specialized for ranks 1, 2,
and 3, but the advanced interface handles only the general-rank
case.
</p>
<p><code>howmany</code> is the (nonnegative) number of transforms to compute. The resulting
plan computes <code>howmany</code> transforms, where the input of the
<code>k</code>-th transform is at location <code>in+k*idist</code> (in C pointer
arithmetic), and its output is at location <code>out+k*odist</code>. Plans
obtained in this way can often be faster than calling FFTW multiple
times for the individual transforms. The basic <code>fftw_plan_dft</code>
interface corresponds to <code>howmany=1</code> (in which case the <code>dist</code>
parameters are ignored).
<a name="index-howmany-parameter"></a>
<a name="index-dist"></a>
</p>
<p>Each of the <code>howmany</code> transforms has rank <code>rank</code> and size
<code>n</code>, as in the basic interface. In addition, the advanced
interface allows the input and output arrays of each transform to be
row-major subarrays of larger rank-<code>rank</code> arrays, described by
<code>inembed</code> and <code>onembed</code> parameters, respectively.
{<code>i</code>,<code>o</code>}<code>nembed</code> must be arrays of length <code>rank</code>,
and <code>n</code> should be elementwise less than or equal to
{<code>i</code>,<code>o</code>}<code>nembed</code>. Passing <code>NULL</code> for an
<code>nembed</code> parameter is equivalent to passing <code>n</code> (i.e. same
physical and logical dimensions, as in the basic interface.)
</p>
<p>The <code>stride</code> parameters indicate that the <code>j</code>-th element of
the input or output arrays is located at <code>j*istride</code> or
<code>j*ostride</code>, respectively. (For a multi-dimensional array,
<code>j</code> is the ordinary row-major index.) When combined with the
<code>k</code>-th transform in a <code>howmany</code> loop, from above, this means
that the (<code>j</code>,<code>k</code>)-th element is at <code>j*stride+k*dist</code>.
(The basic <code>fftw_plan_dft</code> interface corresponds to a stride of 1.)
<a name="index-stride-1"></a>
</p>
<p>For in-place transforms, the input and output <code>stride</code> and
<code>dist</code> parameters should be the same; otherwise, the planner may
return <code>NULL</code>.
</p>
<p>Arrays <code>n</code>, <code>inembed</code>, and <code>onembed</code> are not used after
this function returns. You can safely free or reuse them.
</p>
<p><strong>Examples</strong>:
One transform of one 5 by 6 array contiguous in memory:
</p><div class="example">
<pre class="example"> int rank = 2;
int n[] = {5, 6};
int howmany = 1;
int idist = odist = 0; /* unused because howmany = 1 */
int istride = ostride = 1; /* array is contiguous in memory */
int *inembed = n, *onembed = n;
</pre></div>
<p>Transform of three 5 by 6 arrays, each contiguous in memory,
stored in memory one after another:
</p><div class="example">
<pre class="example"> int rank = 2;
int n[] = {5, 6};
int howmany = 3;
int idist = odist = n[0]*n[1]; /* = 30, the distance in memory
between the first element
of the first array and the
first element of the second array */
int istride = ostride = 1; /* array is contiguous in memory */
int *inembed = n, *onembed = n;
</pre></div>
<p>Transform each column of a 2d array with 10 rows and 3 columns:
</p><div class="example">
<pre class="example"> int rank = 1; /* not 2: we are computing 1d transforms */
int n[] = {10}; /* 1d transforms of length 10 */
int howmany = 3;
int idist = odist = 1;
int istride = ostride = 3; /* distance between two elements in
the same column */
int *inembed = n, *onembed = n;
</pre></div>
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Next: <a href="Advanced-Real_002ddata-DFTs.html#Advanced-Real_002ddata-DFTs" accesskey="n" rel="next">Advanced Real-data DFTs</a>, Previous: <a href="Advanced-Interface.html#Advanced-Interface" accesskey="p" rel="prev">Advanced Interface</a>, Up: <a href="Advanced-Interface.html#Advanced-Interface" accesskey="u" rel="up">Advanced Interface</a> [<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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