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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 37bf6b4a2645
children
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Chris@10 1 <html lang="en">
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Chris@10 3 <title>Guru vector and transform sizes - FFTW 3.3.3</title>
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Chris@10 48 <div class="node">
Chris@10 49 <a name="Guru-vector-and-transform-sizes"></a>
Chris@10 50 <p>
Chris@10 51 Next:&nbsp;<a rel="next" accesskey="n" href="Guru-Complex-DFTs.html#Guru-Complex-DFTs">Guru Complex DFTs</a>,
Chris@10 52 Previous:&nbsp;<a rel="previous" accesskey="p" href="Interleaved-and-split-arrays.html#Interleaved-and-split-arrays">Interleaved and split arrays</a>,
Chris@10 53 Up:&nbsp;<a rel="up" accesskey="u" href="Guru-Interface.html#Guru-Interface">Guru Interface</a>
Chris@10 54 <hr>
Chris@10 55 </div>
Chris@10 56
Chris@10 57 <h4 class="subsection">4.5.2 Guru vector and transform sizes</h4>
Chris@10 58
Chris@10 59 <p>The guru interface introduces one basic new data structure,
Chris@10 60 <code>fftw_iodim</code>, that is used to specify sizes and strides for
Chris@10 61 multi-dimensional transforms and vectors:
Chris@10 62
Chris@10 63 <pre class="example"> typedef struct {
Chris@10 64 int n;
Chris@10 65 int is;
Chris@10 66 int os;
Chris@10 67 } fftw_iodim;
Chris@10 68 </pre>
Chris@10 69 <p><a name="index-fftw_005fiodim-243"></a>
Chris@10 70 Here, <code>n</code> is the size of the dimension, and <code>is</code> and <code>os</code>
Chris@10 71 are the strides of that dimension for the input and output arrays. (The
Chris@10 72 stride is the separation of consecutive elements along this dimension.)
Chris@10 73
Chris@10 74 <p>The meaning of the stride parameter depends on the type of the array
Chris@10 75 that the stride refers to. <em>If the array is interleaved complex,
Chris@10 76 strides are expressed in units of complex numbers
Chris@10 77 (</em><code>fftw_complex</code><em>). If the array is split complex or real, strides
Chris@10 78 are expressed in units of real numbers (</em><code>double</code><em>).</em> This
Chris@10 79 convention is consistent with the usual pointer arithmetic in the C
Chris@10 80 language. An interleaved array is denoted by a pointer <code>p</code> to
Chris@10 81 <code>fftw_complex</code>, so that <code>p+1</code> points to the next complex
Chris@10 82 number. Split arrays are denoted by pointers to <code>double</code>, in
Chris@10 83 which case pointer arithmetic operates in units of
Chris@10 84 <code>sizeof(double)</code>.
Chris@10 85 <a name="index-stride-244"></a>
Chris@10 86
Chris@10 87 <p>The guru planner interfaces all take a (<code>rank</code>, <code>dims[rank]</code>)
Chris@10 88 pair describing the transform size, and a (<code>howmany_rank</code>,
Chris@10 89 <code>howmany_dims[howmany_rank]</code>) pair describing the &ldquo;vector&rdquo; size (a
Chris@10 90 multi-dimensional loop of transforms to perform), where <code>dims</code> and
Chris@10 91 <code>howmany_dims</code> are arrays of <code>fftw_iodim</code>.
Chris@10 92
Chris@10 93 <p>For example, the <code>howmany</code> parameter in the advanced complex-DFT
Chris@10 94 interface corresponds to <code>howmany_rank</code> = 1,
Chris@10 95 <code>howmany_dims[0].n</code> = <code>howmany</code>, <code>howmany_dims[0].is</code> =
Chris@10 96 <code>idist</code>, and <code>howmany_dims[0].os</code> = <code>odist</code>.
Chris@10 97 <a name="index-howmany-loop-245"></a><a name="index-dist-246"></a>(To compute a single transform, you can just use <code>howmany_rank</code> = 0.)
Chris@10 98
Chris@10 99 <p>A row-major multidimensional array with dimensions <code>n[rank]</code>
Chris@10 100 (see <a href="Row_002dmajor-Format.html#Row_002dmajor-Format">Row-major Format</a>) corresponds to <code>dims[i].n</code> =
Chris@10 101 <code>n[i]</code> and the recurrence <code>dims[i].is</code> = <code>n[i+1] *
Chris@10 102 dims[i+1].is</code> (similarly for <code>os</code>). The stride of the last
Chris@10 103 (<code>i=rank-1</code>) dimension is the overall stride of the array.
Chris@10 104 e.g. to be equivalent to the advanced complex-DFT interface, you would
Chris@10 105 have <code>dims[rank-1].is</code> = <code>istride</code> and
Chris@10 106 <code>dims[rank-1].os</code> = <code>ostride</code>.
Chris@10 107 <a name="index-row_002dmajor-247"></a>
Chris@10 108
Chris@10 109 <p>In general, we only guarantee FFTW to return a non-<code>NULL</code> plan if
Chris@10 110 the vector and transform dimensions correspond to a set of distinct
Chris@10 111 indices, and for in-place transforms the input/output strides should
Chris@10 112 be the same.
Chris@10 113
Chris@10 114 <!-- =========> -->
Chris@10 115 </body></html>
Chris@10 116