mars.tensor.fft.irfft¶

mars.tensor.fft.
irfft
(a, n=None, axis= 1, norm=None)[source]¶ Compute the inverse of the npoint DFT for real input.
This function computes the inverse of the onedimensional npoint 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 whylen(a)
is necessary here.)The input is expected to be in the form returned by rfft, i.e. the real zerofrequency term followed by the complex positive frequency terms in order of increasing frequency. Since the discrete Fourier Transform of real input is Hermitiansymmetric, the negative frequency terms are taken to be the complex conjugates of the corresponding positive frequency terms.
 Parameters
a (array_like) – The input tensor.
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 determined from 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 ({None, "ortho"}, optional) – Normalization mode (see mt.fft). Default is None.
 Returns
out – The truncated or zeropadded 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*(m1)
wherem
is the length of the transformed axis of the input. To get an odd number of output points, n must be specified. Return type
Tensor
 Raises
IndexError – If axis is larger than the last axis of a.
See also
Notes
Returns the real valued npoint inverse discrete Fourier transform of a, where a contains the nonnegative frequency terms of a Hermitiansymmetric sequence. n is the length of the result, not the input.
If you specify an n such that a must be zeropadded 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)
.Examples
>>> import mars.tenosr as mt
>>> mt.fft.ifft([1, 1j, 1, 1j]).execute() array([ 0.+0.j, 1.+0.j, 0.+0.j, 0.+0.j]) >>> mt.fft.irfft([1, 1j, 1]).execute() 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.