flopscope.numpy.fft.irfft

fnp.fft.irfft(a, n=None, axis=-1, norm=None, out=None)[flopscope source][numpy source]

Computes the inverse of `rfft`.

Adapted from NumPy docs np.fft.irfft

Areafft

Typecustom

Cost

5(n/2) \cdot \lceil\log_2 n\rceil

Flopscope Context

Inverse 1-D real FFT. Cost: 5*(n//2)*ceil(log2(n)) (Cooley-Tukey radix-2; Van Loan 1992 §1.4).

Computes the inverse of rfft.

This function computes the inverse of the one-dimensional n-point discrete Fourier Transform of real input computed by rfft. In other words, irfft(rfft(a), len(a)) == a to within numerical accuracy. (See Notes below for why len(a) is necessary here.)

The input is expected to be in the form returned by rfft, i.e. the real zero-frequency term followed by the complex positive frequency terms in order of increasing frequency. Since the discrete Fourier Transform of real input is Hermitian-symmetric, the negative frequency terms are taken to be the complex conjugates of the corresponding positive frequency terms.

Parameters

a:array_like: The input array.
n:int, optional: Length of the transformed axis of the output. For n output points, n//2+1 input points are necessary. If the input is longer than this, it is cropped. If it is shorter than this, it is padded with zeros. If n is not given, it is taken to be 2*(m-1) where m is the length of the input along the axis specified by axis.
axis:int, optional: Axis over which to compute the inverse FFT. If not given, the last axis is used.
norm:{"backward", "ortho", "forward"}, optional: Normalization mode (see flops.fft). Default is "backward". Indicates which direction of the forward/backward pair of transforms is scaled and with what normalization factor.
Added in version 1.20.0.
out:ndarray, optional: If provided, the result will be placed in this array. It should be of the appropriate shape and dtype.
Added in version 2.0.0.

Returns

out:ndarray: The truncated or zero-padded input, transformed along the axis indicated by axis, or the last one if axis is not specified. The length of the transformed axis is n, or, if n is not given, 2*(m-1) where m is the length of the transformed axis of the input. To get an odd number of output points, n must be specified.

Raises

:IndexError: If axis is not a valid axis of a.

Notes

Returns the real valued n-point inverse discrete Fourier transform of a, where a contains the non-negative frequency terms of a Hermitian-symmetric sequence. n is the length of the result, not the input.

If you specify an n such that a must be zero-padded or truncated, the extra/removed values will be added/removed at high frequencies. One can thus resample a series to m points via Fourier interpolation by: a_resamp = irfft(rfft(a), m).

The correct interpretation of the hermitian input depends on the length of the original data, as given by n. This is because each input shape could correspond to either an odd or even length signal. By default, irfft assumes an even output length which puts the last entry at the Nyquist frequency; aliasing with its symmetric counterpart. By Hermitian symmetry, the value is thus treated as purely real. To avoid losing information, the correct length of the real input must be given.

Examples

>>> import flopscope.numpy as fnp
>>> flops.fft.ifft([1, -1j, -1, 1j])
array([0.+0.j,  1.+0.j,  0.+0.j,  0.+0.j]) # may vary
>>> flops.fft.irfft([1, -1j, -1])
array([0.,  1.,  0.,  0.])

Notice how the last term in the input to the ordinary ifft is the complex conjugate of the second term, and the output has zero imaginary part everywhere. When calling irfft, the negative frequencies are not specified, and the output array is purely real.