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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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Chris@42 25 <title>FFTW 3.3.5: Guru vector and transform sizes</title>
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Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Guru-vector-and-transform-sizes"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Next: <a href="Guru-Complex-DFTs.html#Guru-Complex-DFTs" accesskey="n" rel="next">Guru Complex DFTs</a>, Previous: <a href="Interleaved-and-split-arrays.html#Interleaved-and-split-arrays" accesskey="p" rel="prev">Interleaved and split arrays</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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Guru-vector-and-transform-sizes-1"></a>
Chris@42 79 <h4 class="subsection">4.5.2 Guru vector and transform sizes</h4>
Chris@42 80
Chris@42 81 <p>The guru interface introduces one basic new data structure,
Chris@42 82 <code>fftw_iodim</code>, that is used to specify sizes and strides for
Chris@42 83 multi-dimensional transforms and vectors:
Chris@42 84 </p>
Chris@42 85 <div class="example">
Chris@42 86 <pre class="example">typedef struct {
Chris@42 87 int n;
Chris@42 88 int is;
Chris@42 89 int os;
Chris@42 90 } fftw_iodim;
Chris@42 91 </pre></div>
Chris@42 92 <a name="index-fftw_005fiodim"></a>
Chris@42 93
Chris@42 94 <p>Here, <code>n</code> is the size of the dimension, and <code>is</code> and <code>os</code>
Chris@42 95 are the strides of that dimension for the input and output arrays. (The
Chris@42 96 stride is the separation of consecutive elements along this dimension.)
Chris@42 97 </p>
Chris@42 98 <p>The meaning of the stride parameter depends on the type of the array
Chris@42 99 that the stride refers to. <em>If the array is interleaved complex,
Chris@42 100 strides are expressed in units of complex numbers
Chris@42 101 (<code>fftw_complex</code>). If the array is split complex or real, strides
Chris@42 102 are expressed in units of real numbers (<code>double</code>).</em> This
Chris@42 103 convention is consistent with the usual pointer arithmetic in the C
Chris@42 104 language. An interleaved array is denoted by a pointer <code>p</code> to
Chris@42 105 <code>fftw_complex</code>, so that <code>p+1</code> points to the next complex
Chris@42 106 number. Split arrays are denoted by pointers to <code>double</code>, in
Chris@42 107 which case pointer arithmetic operates in units of
Chris@42 108 <code>sizeof(double)</code>.
Chris@42 109 <a name="index-stride-2"></a>
Chris@42 110 </p>
Chris@42 111
Chris@42 112 <p>The guru planner interfaces all take a (<code>rank</code>, <code>dims[rank]</code>)
Chris@42 113 pair describing the transform size, and a (<code>howmany_rank</code>,
Chris@42 114 <code>howmany_dims[howmany_rank]</code>) pair describing the &ldquo;vector&rdquo; size (a
Chris@42 115 multi-dimensional loop of transforms to perform), where <code>dims</code> and
Chris@42 116 <code>howmany_dims</code> are arrays of <code>fftw_iodim</code>.
Chris@42 117 </p>
Chris@42 118 <p>For example, the <code>howmany</code> parameter in the advanced complex-DFT
Chris@42 119 interface corresponds to <code>howmany_rank</code> = 1,
Chris@42 120 <code>howmany_dims[0].n</code> = <code>howmany</code>, <code>howmany_dims[0].is</code> =
Chris@42 121 <code>idist</code>, and <code>howmany_dims[0].os</code> = <code>odist</code>.
Chris@42 122 <a name="index-howmany-loop"></a>
Chris@42 123 <a name="index-dist-1"></a>
Chris@42 124 (To compute a single transform, you can just use <code>howmany_rank</code> = 0.)
Chris@42 125 </p>
Chris@42 126
Chris@42 127 <p>A row-major multidimensional array with dimensions <code>n[rank]</code>
Chris@42 128 (see <a href="Row_002dmajor-Format.html#Row_002dmajor-Format">Row-major Format</a>) corresponds to <code>dims[i].n</code> =
Chris@42 129 <code>n[i]</code> and the recurrence <code>dims[i].is</code> = <code>n[i+1] *
Chris@42 130 dims[i+1].is</code> (similarly for <code>os</code>). The stride of the last
Chris@42 131 (<code>i=rank-1</code>) dimension is the overall stride of the array.
Chris@42 132 e.g. to be equivalent to the advanced complex-DFT interface, you would
Chris@42 133 have <code>dims[rank-1].is</code> = <code>istride</code> and
Chris@42 134 <code>dims[rank-1].os</code> = <code>ostride</code>.
Chris@42 135 <a name="index-row_002dmajor-3"></a>
Chris@42 136 </p>
Chris@42 137
Chris@42 138 <p>In general, we only guarantee FFTW to return a non-<code>NULL</code> plan if
Chris@42 139 the vector and transform dimensions correspond to a set of distinct
Chris@42 140 indices, and for in-place transforms the input/output strides should
Chris@42 141 be the same.
Chris@42 142 </p>
Chris@42 143 <hr>
Chris@42 144 <div class="header">
Chris@42 145 <p>
Chris@42 146 Next: <a href="Guru-Complex-DFTs.html#Guru-Complex-DFTs" accesskey="n" rel="next">Guru Complex DFTs</a>, Previous: <a href="Interleaved-and-split-arrays.html#Interleaved-and-split-arrays" accesskey="p" rel="prev">Interleaved and split arrays</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>
Chris@42 147 </div>
Chris@42 148
Chris@42 149
Chris@42 150
Chris@42 151 </body>
Chris@42 152 </html>