annotate fft/fftw/fftw-3.3.4/doc/html/Row_002dmajor-Format.html @ 40:223f770b5341 kissfft-double tip

Try a double-precision kissfft
author Chris Cannam
date Wed, 07 Sep 2016 10:40:32 +0100
parents 26056e866c29
children
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Chris@19 3 <title>Row-major Format - FFTW 3.3.4</title>
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Chris@19 49 <a name="Row-major-Format"></a>
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Chris@19 51 <p>
Chris@19 52 Next:&nbsp;<a rel="next" accesskey="n" href="Column_002dmajor-Format.html#Column_002dmajor-Format">Column-major Format</a>,
Chris@19 53 Previous:&nbsp;<a rel="previous" accesskey="p" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>,
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Chris@19 57
Chris@19 58 <h4 class="subsection">3.2.1 Row-major Format</h4>
Chris@19 59
Chris@19 60 <p><a name="index-row_002dmajor-115"></a>
Chris@19 61 The multi-dimensional arrays passed to <code>fftw_plan_dft</code> etcetera
Chris@19 62 are expected to be stored as a single contiguous block in
Chris@19 63 <dfn>row-major</dfn> order (sometimes called &ldquo;C order&rdquo;). Basically, this
Chris@19 64 means that as you step through adjacent memory locations, the first
Chris@19 65 dimension's index varies most slowly and the last dimension's index
Chris@19 66 varies most quickly.
Chris@19 67
Chris@19 68 <p>To be more explicit, let us consider an array of rank d whose
Chris@19 69 dimensions are n<sub>0</sub>&nbsp;&times;&nbsp;n<sub>1</sub>&nbsp;&times;&nbsp;n<sub>2</sub>&nbsp;&times;&nbsp;&hellip;&nbsp;&times;&nbsp;n<sub>d-1</sub>. Now, we specify a location in the array by a
Chris@19 70 sequence of d (zero-based) indices, one for each dimension:
Chris@19 71 (i<sub>0</sub>, i<sub>1</sub>, i<sub>2</sub>,..., i<sub>d-1</sub>). If the array is stored in row-major
Chris@19 72 order, then this element is located at the position
Chris@19 73 i<sub>d-1</sub> + n<sub>d-1</sub> * (i<sub>d-2</sub> + n<sub>d-2</sub> * (... + n<sub>1</sub> * i<sub>0</sub>)).
Chris@19 74
Chris@19 75 <p>Note that, for the ordinary complex DFT, each element of the array
Chris@19 76 must be of type <code>fftw_complex</code>; i.e. a (real, imaginary) pair of
Chris@19 77 (double-precision) numbers.
Chris@19 78
Chris@19 79 <p>In the advanced FFTW interface, the physical dimensions n from
Chris@19 80 which the indices are computed can be different from (larger than)
Chris@19 81 the logical dimensions of the transform to be computed, in order to
Chris@19 82 transform a subset of a larger array.
Chris@19 83 <a name="index-advanced-interface-116"></a>Note also that, in the advanced interface, the expression above is
Chris@19 84 multiplied by a <dfn>stride</dfn> to get the actual array index&mdash;this is
Chris@19 85 useful in situations where each element of the multi-dimensional array
Chris@19 86 is actually a data structure (or another array), and you just want to
Chris@19 87 transform a single field. In the basic interface, however, the stride
Chris@19 88 is 1.
Chris@19 89 <a name="index-stride-117"></a>
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