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6.7 KiB
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<html>
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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approved by the Free Software Foundation. -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>Guru vector and transform sizes (FFTW 3.3.10)</title>
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<meta name="description" content="Guru vector and transform sizes (FFTW 3.3.10)">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Guru-Interface.html" rel="up" title="Guru Interface">
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<link href="Guru-Complex-DFTs.html" rel="next" title="Guru Complex DFTs">
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<link href="Interleaved-and-split-arrays.html" rel="prev" title="Interleaved and split arrays">
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</head>
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<body lang="en">
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<span id="Guru-vector-and-transform-sizes"></span><div class="header">
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<p>
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Next: <a href="Guru-Complex-DFTs.html" accesskey="n" rel="next">Guru Complex DFTs</a>, Previous: <a href="Interleaved-and-split-arrays.html" accesskey="p" rel="prev">Interleaved and split arrays</a>, Up: <a href="Guru-Interface.html" accesskey="u" rel="up">Guru Interface</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Guru-vector-and-transform-sizes-1"></span><h4 class="subsection">4.5.2 Guru vector and transform sizes</h4>
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<p>The guru interface introduces one basic new data structure,
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<code>fftw_iodim</code>, that is used to specify sizes and strides for
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multi-dimensional transforms and vectors:
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</p>
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<div class="example">
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<pre class="example">typedef struct {
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int n;
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int is;
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int os;
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} fftw_iodim;
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</pre></div>
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<span id="index-fftw_005fiodim"></span>
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<p>Here, <code>n</code> is the size of the dimension, and <code>is</code> and <code>os</code>
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are the strides of that dimension for the input and output arrays. (The
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stride is the separation of consecutive elements along this dimension.)
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</p>
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<p>The meaning of the stride parameter depends on the type of the array
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that the stride refers to. <em>If the array is interleaved complex,
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strides are expressed in units of complex numbers
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(<code>fftw_complex</code>). If the array is split complex or real, strides
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are expressed in units of real numbers (<code>double</code>).</em> This
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convention is consistent with the usual pointer arithmetic in the C
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language. An interleaved array is denoted by a pointer <code>p</code> to
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<code>fftw_complex</code>, so that <code>p+1</code> points to the next complex
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number. Split arrays are denoted by pointers to <code>double</code>, in
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which case pointer arithmetic operates in units of
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<code>sizeof(double)</code>.
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<span id="index-stride-2"></span>
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</p>
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<p>The guru planner interfaces all take a (<code>rank</code>, <code>dims[rank]</code>)
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pair describing the transform size, and a (<code>howmany_rank</code>,
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<code>howmany_dims[howmany_rank]</code>) pair describing the “vector” size (a
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multi-dimensional loop of transforms to perform), where <code>dims</code> and
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<code>howmany_dims</code> are arrays of <code>fftw_iodim</code>. Each <code>n</code> field must
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be positive for <code>dims</code> and nonnegative for <code>howmany_dims</code>, while both
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<code>rank</code> and <code>howmany_rank</code> must be nonnegative.
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</p>
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<p>For example, the <code>howmany</code> parameter in the advanced complex-DFT
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interface corresponds to <code>howmany_rank</code> = 1,
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<code>howmany_dims[0].n</code> = <code>howmany</code>, <code>howmany_dims[0].is</code> =
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<code>idist</code>, and <code>howmany_dims[0].os</code> = <code>odist</code>.
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<span id="index-howmany-loop"></span>
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<span id="index-dist-1"></span>
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(To compute a single transform, you can just use <code>howmany_rank</code> = 0.)
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</p>
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<p>A row-major multidimensional array with dimensions <code>n[rank]</code>
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(see <a href="Row_002dmajor-Format.html">Row-major Format</a>) corresponds to <code>dims[i].n</code> =
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<code>n[i]</code> and the recurrence <code>dims[i].is</code> = <code>n[i+1] *
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dims[i+1].is</code> (similarly for <code>os</code>). The stride of the last
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(<code>i=rank-1</code>) dimension is the overall stride of the array.
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e.g. to be equivalent to the advanced complex-DFT interface, you would
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have <code>dims[rank-1].is</code> = <code>istride</code> and
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<code>dims[rank-1].os</code> = <code>ostride</code>.
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<span id="index-row_002dmajor-3"></span>
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</p>
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<p>In general, we only guarantee FFTW to return a non-<code>NULL</code> plan if
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the vector and transform dimensions correspond to a set of distinct
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indices, and for in-place transforms the input/output strides should
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be the same.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Guru-Complex-DFTs.html" accesskey="n" rel="next">Guru Complex DFTs</a>, Previous: <a href="Interleaved-and-split-arrays.html" accesskey="p" rel="prev">Interleaved and split arrays</a>, Up: <a href="Guru-Interface.html" accesskey="u" rel="up">Guru Interface</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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