net.grelf

Class FFT

java.lang.Object

net.grelf.FFT

public class FFT
extends java.lang.Object

Fast Fourier Transform in double precision. There is no reverse() method because it can be done by swapping Re/Im inputs, doing the forward transform and then swapping back again, also dividing by n (see net.grelf.image.Image64 for an example of an inverse FFT using this class).

Constructor Summary

Constructors

Constructor and Description
FFT(int n) Construct a transformer for arrays of the given length.

Method Summary

Modifier and Type	Method and Description
void	forward(double[] xRe, double[] xIm, int offsetToStart) Compute the in-place FFT of a length n complex sequence.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

FFT

public FFT(int n)
    throws java.lang.Exception

Construct a transformer for arrays of the given length. Initialises tables of trigonometrical functions so they are not recomputed for multiple FFTs of the same length. Requires n to be a power of 2 - throws RuntimeException if it is not.

Throws:

java.lang.Exception

Method Detail

forward

public void forward(double[] xRe,
                    double[] xIm,
                    int offsetToStart)

Compute the in-place FFT of a length n complex sequence. Uses a non-recursive version of the Cooley-Tukey FFT. Runs in O (n log n) time. n must equal the value set in the constructor. The offsetToStart is the starting position for the n values in either array, so that successive ROWS of an image can be transformed in-place. To do columns of an image it must be rotated through 90 degrees first and back again after. For performance, does not use objects of class Complex.