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<title>FFTW 3.3.5: Other Multi-dimensional Real-data MPI Transforms</title>

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<a name="Other-Multi_002ddimensional-Real_002ddata-MPI-Transforms"></a>
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Next: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" accesskey="n" rel="next">FFTW MPI Transposes</a>, Previous: <a href="Multi_002ddimensional-MPI-DFTs-of-Real-Data.html#Multi_002ddimensional-MPI-DFTs-of-Real-Data" accesskey="p" rel="prev">Multi-dimensional MPI DFTs of Real Data</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</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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<hr>
<a name="Other-multi_002ddimensional-Real_002dData-MPI-Transforms"></a>
<h3 class="section">6.6 Other multi-dimensional Real-Data MPI Transforms</h3>

<a name="index-r2r-3"></a>
<p>FFTW&rsquo;s MPI interface also supports multi-dimensional &lsquo;<samp>r2r</samp>&rsquo;
transforms of all kinds supported by the serial interface
(e.g. discrete cosine and sine transforms, discrete Hartley
transforms, etc.).  Only multi-dimensional &lsquo;<samp>r2r</samp>&rsquo; transforms, not
one-dimensional transforms, are currently parallelized.
</p>
<a name="index-fftw_005fr2r_005fkind-1"></a>
<p>These are used much like the multidimensional complex DFTs discussed
above, except that the data is real rather than complex, and one needs
to pass an r2r transform kind (<code>fftw_r2r_kind</code>) for each
dimension as in the serial FFTW (see <a href="More-DFTs-of-Real-Data.html#More-DFTs-of-Real-Data">More DFTs of Real Data</a>).
</p>
<p>For example, one might perform a two-dimensional L&nbsp;&times;&nbsp;M that is
an REDFT10 (DCT-II) in the first dimension and an RODFT10 (DST-II) in
the second dimension with code like:
</p>
<div class="example">
<pre class="example">    const ptrdiff_t L = ..., M = ...;
    fftw_plan plan;
    double *data;
    ptrdiff_t alloc_local, local_n0, local_0_start, i, j;

    /* <span class="roman">get local data size and allocate</span> */
    alloc_local = fftw_mpi_local_size_2d(L, M, MPI_COMM_WORLD,
                                         &amp;local_n0, &amp;local_0_start);
    data = fftw_alloc_real(alloc_local);

    /* <span class="roman">create plan for in-place REDFT10 x RODFT10</span> */
    plan = fftw_mpi_plan_r2r_2d(L, M, data, data, MPI_COMM_WORLD,
                                FFTW_REDFT10, FFTW_RODFT10, FFTW_MEASURE);

    /* <span class="roman">initialize data to some function</span> my_function(x,y) */
    for (i = 0; i &lt; local_n0; ++i) for (j = 0; j &lt; M; ++j)
       data[i*M + j] = my_function(local_0_start + i, j);

    /* <span class="roman">compute transforms, in-place, as many times as desired</span> */
    fftw_execute(plan);

    fftw_destroy_plan(plan);
</pre></div>

<a name="index-fftw_005falloc_005freal-3"></a>
<p>Notice that we use the same &lsquo;<samp>local_size</samp>&rsquo; functions as we did for
complex data, only now we interpret the sizes in terms of real rather
than complex values, and correspondingly use <code>fftw_alloc_real</code>.
</p>


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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Mon, 02 Mar 2020 14:03:47 +0000
parents	7867fa7e1b6b
children