fftw3: Row-major Format
3.2.1 Row-major Format
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The multi-dimensional arrays passed to 'fftw_plan_dft' etcetera are
expected to be stored as a single contiguous block in "row-major" order
(sometimes called "C order"). Basically, this means that as you step
through adjacent memory locations, the first dimension's index varies
most slowly and the last dimension's index varies most quickly.
To be more explicit, let us consider an array of rank d whose
dimensions are n[0] x n[1] x n[2] x ... x n[d-1] . Now, we specify a
location in the array by a sequence of d (zero-based) indices, one for
each dimension: (i[0], i[1], ..., i[d-1]). If the array is stored in
row-major order, then this element is located at the position i[d-1] +
n[d-1] * (i[d-2] + n[d-2] * (... + n[1] * i[0])).
Note that, for the ordinary complex DFT, each element of the array
must be of type 'fftw_complex'; i.e. a (real, imaginary) pair of
(double-precision) numbers.
In the advanced FFTW interface, the physical dimensions n from which
the indices are computed can be different from (larger than) the logical
dimensions of the transform to be computed, in order to transform a
subset of a larger array. Note also that, in the advanced interface,
the expression above is multiplied by a "stride" to get the actual array
index--this is useful in situations where each element of the
multi-dimensional array is actually a data structure (or another array),
and you just want to transform a single field. In the basic interface,
however, the stride is 1.
Info Catalog
fftw3: Multi-dimensional Array Format
fftw3: Multi-dimensional Array Format
fftw3: Column-major Format