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Chris@82:The multi-dimensional arrays passed to fftw_plan_dft etcetera
Chris@82: are expected to be stored as a single contiguous block in
Chris@82: row-major order (sometimes called “C order”). Basically, this
Chris@82: means that as you step through adjacent memory locations, the first
Chris@82: dimension’s index varies most slowly and the last dimension’s index
Chris@82: varies most quickly.
Chris@82:
To be more explicit, let us consider an array of rank d whose Chris@82: dimensions are n0 × n1 × n2 × … × nd-1 Chris@82: . Now, we specify a location in the array by a Chris@82: sequence of d (zero-based) indices, one for each dimension: Chris@82: (i0, i1, i2,..., id-1). Chris@82: If the array is stored in row-major Chris@82: order, then this element is located at the position Chris@82: id-1 + nd-1 * (id-2 + nd-2 * (... + n1 * i0)). Chris@82:
Chris@82:Note that, for the ordinary complex DFT, each element of the array
Chris@82: must be of type fftw_complex; i.e. a (real, imaginary) pair of
Chris@82: (double-precision) numbers.
Chris@82:
In the advanced FFTW interface, the physical dimensions n from Chris@82: which the indices are computed can be different from (larger than) Chris@82: the logical dimensions of the transform to be computed, in order to Chris@82: transform a subset of a larger array. Chris@82: Chris@82: Note also that, in the advanced interface, the expression above is Chris@82: multiplied by a stride to get the actual array index—this is Chris@82: useful in situations where each element of the multi-dimensional array Chris@82: is actually a data structure (or another array), and you just want to Chris@82: transform a single field. In the basic interface, however, the stride Chris@82: is 1. Chris@82: Chris@82:
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