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<h4 class="subsection">4.4.1 Advanced Complex DFTs</h4>

<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>
   <p><a name="index-fftw_005fplan_005fmany_005fdft-232"></a>
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&nbsp;3, but the advanced interface handles only the general-rank
case.

   <p><code>howmany</code> is the 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-233"></a><a name="index-dist-234"></a>

   <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>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-235"></a>

   <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>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><strong>Examples</strong>:
One transform of one 5 by 6 array contiguous in memory:
<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>
   <p>Transform of three 5 by 6 arrays, each contiguous in memory,
stored in memory one after another:
<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>
   <p>Transform each column of a 2d array with 10 rows and 3 columns:
<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>
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author	Chris Cannam
date	Wed, 20 Mar 2013 15:35:50 +0000
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