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1 <html lang="en">
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2 <head>
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3 <title>Guru vector and transform sizes - FFTW 3.3.3</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.3.3">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Guru-Interface.html#Guru-Interface" title="Guru Interface">
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9 <link rel="prev" href="Interleaved-and-split-arrays.html#Interleaved-and-split-arrays" title="Interleaved and split arrays">
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10 <link rel="next" href="Guru-Complex-DFTs.html#Guru-Complex-DFTs" title="Guru Complex DFTs">
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11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.3.3, 25 November 2012).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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19
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20 Permission is granted to make and distribute verbatim copies of
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21 this manual provided the copyright notice and this permission
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22 notice are preserved on all copies.
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23
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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28
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29 Permission is granted to copy and distribute translations of this
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30 manual into another language, under the above conditions for
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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33 -->
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45 --></style>
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46 </head>
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47 <body>
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48 <div class="node">
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49 <a name="Guru-vector-and-transform-sizes"></a>
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50 <p>
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51 Next: <a rel="next" accesskey="n" href="Guru-Complex-DFTs.html#Guru-Complex-DFTs">Guru Complex DFTs</a>,
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52 Previous: <a rel="previous" accesskey="p" href="Interleaved-and-split-arrays.html#Interleaved-and-split-arrays">Interleaved and split arrays</a>,
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53 Up: <a rel="up" accesskey="u" href="Guru-Interface.html#Guru-Interface">Guru Interface</a>
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54 <hr>
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55 </div>
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56
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57 <h4 class="subsection">4.5.2 Guru vector and transform sizes</h4>
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58
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59 <p>The guru interface introduces one basic new data structure,
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60 <code>fftw_iodim</code>, that is used to specify sizes and strides for
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61 multi-dimensional transforms and vectors:
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62
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63 <pre class="example"> typedef struct {
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64 int n;
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65 int is;
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66 int os;
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67 } fftw_iodim;
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68 </pre>
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69 <p><a name="index-fftw_005fiodim-243"></a>
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70 Here, <code>n</code> is the size of the dimension, and <code>is</code> and <code>os</code>
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71 are the strides of that dimension for the input and output arrays. (The
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72 stride is the separation of consecutive elements along this dimension.)
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73
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74 <p>The meaning of the stride parameter depends on the type of the array
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75 that the stride refers to. <em>If the array is interleaved complex,
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76 strides are expressed in units of complex numbers
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77 (</em><code>fftw_complex</code><em>). If the array is split complex or real, strides
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78 are expressed in units of real numbers (</em><code>double</code><em>).</em> This
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79 convention is consistent with the usual pointer arithmetic in the C
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80 language. An interleaved array is denoted by a pointer <code>p</code> to
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81 <code>fftw_complex</code>, so that <code>p+1</code> points to the next complex
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82 number. Split arrays are denoted by pointers to <code>double</code>, in
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83 which case pointer arithmetic operates in units of
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84 <code>sizeof(double)</code>.
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85 <a name="index-stride-244"></a>
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86
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87 <p>The guru planner interfaces all take a (<code>rank</code>, <code>dims[rank]</code>)
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88 pair describing the transform size, and a (<code>howmany_rank</code>,
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89 <code>howmany_dims[howmany_rank]</code>) pair describing the “vector” size (a
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90 multi-dimensional loop of transforms to perform), where <code>dims</code> and
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91 <code>howmany_dims</code> are arrays of <code>fftw_iodim</code>.
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92
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93 <p>For example, the <code>howmany</code> parameter in the advanced complex-DFT
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94 interface corresponds to <code>howmany_rank</code> = 1,
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95 <code>howmany_dims[0].n</code> = <code>howmany</code>, <code>howmany_dims[0].is</code> =
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96 <code>idist</code>, and <code>howmany_dims[0].os</code> = <code>odist</code>.
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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.)
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98
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99 <p>A row-major multidimensional array with dimensions <code>n[rank]</code>
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100 (see <a href="Row_002dmajor-Format.html#Row_002dmajor-Format">Row-major Format</a>) corresponds to <code>dims[i].n</code> =
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101 <code>n[i]</code> and the recurrence <code>dims[i].is</code> = <code>n[i+1] *
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102 dims[i+1].is</code> (similarly for <code>os</code>). The stride of the last
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103 (<code>i=rank-1</code>) dimension is the overall stride of the array.
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104 e.g. to be equivalent to the advanced complex-DFT interface, you would
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105 have <code>dims[rank-1].is</code> = <code>istride</code> and
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106 <code>dims[rank-1].os</code> = <code>ostride</code>.
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107 <a name="index-row_002dmajor-247"></a>
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108
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109 <p>In general, we only guarantee FFTW to return a non-<code>NULL</code> plan if
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110 the vector and transform dimensions correspond to a set of distinct
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111 indices, and for in-place transforms the input/output strides should
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112 be the same.
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113
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114 <!-- =========> -->
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115 </body></html>
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116
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