sffft3 implements a complex-to-complex Fast Fourier Transform (FFT) along an arbitrary axis.
The FFT library that Madagascar uses is KISS FFT, created by Mark Borgerding. KISS stands for Keep it simple, Stupid!KISS FFT may not be as fast as FFTW but it is lightweight and easy to include in the distribution.
The axis= parameter specifies the data axis for performing the transform. The following example from bei/ft1/plane4 (Jon Claerbout's Basic Earth Imaging) shows different forms of the Fourier transform applied to a 2-D dataset.
The algorithm in KISS FFT uses a mixed-radix algorithm, which is most efficient when the input size is N=2k 3l 5m. By default, the input data is padded to the next optimal size, additionally multiplied by 2. To disable this behavior, use opt=n pad=1.
By default, sffft3 applies no scaling in the forward transform and 1/N scaling in the inverse transform. To apply a symmetric 1/√N scaling in both cases, use sym=y.
For a real-to-complex FFT along the first axis, use sffft1.
For a real-to-real Cosine Fourier Transform, use sfcosft.
To apply FFT as a linear operator, try sffft wrapper script.