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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents d0c2a83c1364
children
rev   line source
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Chris@82 25 <title>FFTW 3.3.8: Guru vector and transform sizes</title>
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Chris@82 36 <link href="Guru-Interface.html#Guru-Interface" rel="up" title="Guru Interface">
Chris@82 37 <link href="Guru-Complex-DFTs.html#Guru-Complex-DFTs" rel="next" title="Guru Complex DFTs">
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Chris@82 69
Chris@82 70 <body lang="en">
Chris@82 71 <a name="Guru-vector-and-transform-sizes"></a>
Chris@82 72 <div class="header">
Chris@82 73 <p>
Chris@82 74 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@82 75 </div>
Chris@82 76 <hr>
Chris@82 77 <a name="Guru-vector-and-transform-sizes-1"></a>
Chris@82 78 <h4 class="subsection">4.5.2 Guru vector and transform sizes</h4>
Chris@82 79
Chris@82 80 <p>The guru interface introduces one basic new data structure,
Chris@82 81 <code>fftw_iodim</code>, that is used to specify sizes and strides for
Chris@82 82 multi-dimensional transforms and vectors:
Chris@82 83 </p>
Chris@82 84 <div class="example">
Chris@82 85 <pre class="example">typedef struct {
Chris@82 86 int n;
Chris@82 87 int is;
Chris@82 88 int os;
Chris@82 89 } fftw_iodim;
Chris@82 90 </pre></div>
Chris@82 91 <a name="index-fftw_005fiodim"></a>
Chris@82 92
Chris@82 93 <p>Here, <code>n</code> is the size of the dimension, and <code>is</code> and <code>os</code>
Chris@82 94 are the strides of that dimension for the input and output arrays. (The
Chris@82 95 stride is the separation of consecutive elements along this dimension.)
Chris@82 96 </p>
Chris@82 97 <p>The meaning of the stride parameter depends on the type of the array
Chris@82 98 that the stride refers to. <em>If the array is interleaved complex,
Chris@82 99 strides are expressed in units of complex numbers
Chris@82 100 (<code>fftw_complex</code>). If the array is split complex or real, strides
Chris@82 101 are expressed in units of real numbers (<code>double</code>).</em> This
Chris@82 102 convention is consistent with the usual pointer arithmetic in the C
Chris@82 103 language. An interleaved array is denoted by a pointer <code>p</code> to
Chris@82 104 <code>fftw_complex</code>, so that <code>p+1</code> points to the next complex
Chris@82 105 number. Split arrays are denoted by pointers to <code>double</code>, in
Chris@82 106 which case pointer arithmetic operates in units of
Chris@82 107 <code>sizeof(double)</code>.
Chris@82 108 <a name="index-stride-2"></a>
Chris@82 109 </p>
Chris@82 110
Chris@82 111 <p>The guru planner interfaces all take a (<code>rank</code>, <code>dims[rank]</code>)
Chris@82 112 pair describing the transform size, and a (<code>howmany_rank</code>,
Chris@82 113 <code>howmany_dims[howmany_rank]</code>) pair describing the &ldquo;vector&rdquo; size (a
Chris@82 114 multi-dimensional loop of transforms to perform), where <code>dims</code> and
Chris@82 115 <code>howmany_dims</code> are arrays of <code>fftw_iodim</code>. Each <code>n</code> field must
Chris@82 116 be positive for <code>dims</code> and nonnegative for <code>howmany_dims</code>, while both
Chris@82 117 <code>rank</code> and <code>howmany_rank</code> must be nonnegative.
Chris@82 118 </p>
Chris@82 119 <p>For example, the <code>howmany</code> parameter in the advanced complex-DFT
Chris@82 120 interface corresponds to <code>howmany_rank</code> = 1,
Chris@82 121 <code>howmany_dims[0].n</code> = <code>howmany</code>, <code>howmany_dims[0].is</code> =
Chris@82 122 <code>idist</code>, and <code>howmany_dims[0].os</code> = <code>odist</code>.
Chris@82 123 <a name="index-howmany-loop"></a>
Chris@82 124 <a name="index-dist-1"></a>
Chris@82 125 (To compute a single transform, you can just use <code>howmany_rank</code> = 0.)
Chris@82 126 </p>
Chris@82 127
Chris@82 128 <p>A row-major multidimensional array with dimensions <code>n[rank]</code>
Chris@82 129 (see <a href="Row_002dmajor-Format.html#Row_002dmajor-Format">Row-major Format</a>) corresponds to <code>dims[i].n</code> =
Chris@82 130 <code>n[i]</code> and the recurrence <code>dims[i].is</code> = <code>n[i+1] *
Chris@82 131 dims[i+1].is</code> (similarly for <code>os</code>). The stride of the last
Chris@82 132 (<code>i=rank-1</code>) dimension is the overall stride of the array.
Chris@82 133 e.g. to be equivalent to the advanced complex-DFT interface, you would
Chris@82 134 have <code>dims[rank-1].is</code> = <code>istride</code> and
Chris@82 135 <code>dims[rank-1].os</code> = <code>ostride</code>.
Chris@82 136 <a name="index-row_002dmajor-3"></a>
Chris@82 137 </p>
Chris@82 138
Chris@82 139 <p>In general, we only guarantee FFTW to return a non-<code>NULL</code> plan if
Chris@82 140 the vector and transform dimensions correspond to a set of distinct
Chris@82 141 indices, and for in-place transforms the input/output strides should
Chris@82 142 be the same.
Chris@82 143 </p>
Chris@82 144 <hr>
Chris@82 145 <div class="header">
Chris@82 146 <p>
Chris@82 147 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@82 148 </div>
Chris@82 149
Chris@82 150
Chris@82 151
Chris@82 152 </body>
Chris@82 153 </html>