Module « scipy.fft »

Fonction fft - module scipy.fft

Signature de la fonction fft

def fft(x, n=None, axis=-1, norm=None, overwrite_x=False, workers=None, *, plan=None) 

Description

fft.__doc__

    Compute the 1-D discrete Fourier Transform.

    This function computes the 1-D *n*-point discrete Fourier
    Transform (DFT) with the efficient Fast Fourier Transform (FFT)
    algorithm [1]_.

    Parameters
    ----------
    x : array_like
        Input array, can be complex.
    n : int, optional
        Length of the transformed axis of the output.
        If `n` is smaller than the length of the input, the input is cropped.
        If it is larger, the input is padded with zeros. If `n` is not given,
        the length of the input along the axis specified by `axis` is used.
    axis : int, optional
        Axis over which to compute the FFT. If not given, the last axis is
        used.
    norm : {"backward", "ortho", "forward"}, optional
        Normalization mode. Default is "backward", meaning no normalization on
        the forward transforms and scaling by ``1/n`` on the `ifft`.
        "forward" instead applies the ``1/n`` factor on the forward tranform.
        For ``norm="ortho"``, both directions are scaled by ``1/sqrt(n)``.

        .. versionadded:: 1.6.0
           ``norm={"forward", "backward"}`` options were added

    overwrite_x : bool, optional
        If True, the contents of `x` can be destroyed; the default is False.
        See the notes below for more details.
    workers : int, optional
        Maximum number of workers to use for parallel computation. If negative,
        the value wraps around from ``os.cpu_count()``. See below for more
        details.
    plan : object, optional
        This argument is reserved for passing in a precomputed plan provided
        by downstream FFT vendors. It is currently not used in SciPy.

        .. versionadded:: 1.5.0

    Returns
    -------
    out : complex ndarray
        The truncated or zero-padded input, transformed along the axis
        indicated by `axis`, or the last one if `axis` is not specified.

    Raises
    ------
    IndexError
        if `axes` is larger than the last axis of `x`.

    See Also
    --------
    ifft : The inverse of `fft`.
    fft2 : The 2-D FFT.
    fftn : The N-D FFT.
    rfftn : The N-D FFT of real input.
    fftfreq : Frequency bins for given FFT parameters.
    next_fast_len : Size to pad input to for most efficient transforms

    Notes
    -----

    FFT (Fast Fourier Transform) refers to a way the discrete Fourier Transform
    (DFT) can be calculated efficiently, by using symmetries in the calculated
    terms. The symmetry is highest when `n` is a power of 2, and the transform
    is therefore most efficient for these sizes. For poorly factorizable sizes,
    `scipy.fft` uses Bluestein's algorithm [2]_ and so is never worse than
    O(`n` log `n`). Further performance improvements may be seen by zero-padding
    the input using `next_fast_len`.

    If ``x`` is a 1d array, then the `fft` is equivalent to ::

        y[k] = np.sum(x * np.exp(-2j * np.pi * k * np.arange(n)/n))

    The frequency term ``f=k/n`` is found at ``y[k]``. At ``y[n/2]`` we reach
    the Nyquist frequency and wrap around to the negative-frequency terms. So,
    for an 8-point transform, the frequencies of the result are
    [0, 1, 2, 3, -4, -3, -2, -1]. To rearrange the fft output so that the
    zero-frequency component is centered, like [-4, -3, -2, -1, 0, 1, 2, 3],
    use `fftshift`.

    Transforms can be done in single, double, or extended precision (long
    double) floating point. Half precision inputs will be converted to single
    precision and non-floating-point inputs will be converted to double
    precision.

    If the data type of ``x`` is real, a "real FFT" algorithm is automatically
    used, which roughly halves the computation time. To increase efficiency
    a little further, use `rfft`, which does the same calculation, but only
    outputs half of the symmetrical spectrum. If the data are both real and
    symmetrical, the `dct` can again double the efficiency, by generating
    half of the spectrum from half of the signal.

    When ``overwrite_x=True`` is specified, the memory referenced by ``x`` may
    be used by the implementation in any way. This may include reusing the
    memory for the result, but this is in no way guaranteed. You should not
    rely on the contents of ``x`` after the transform as this may change in
    future without warning.

    The ``workers`` argument specifies the maximum number of parallel jobs to
    split the FFT computation into. This will execute independent 1-D
    FFTs within ``x``. So, ``x`` must be at least 2-D and the
    non-transformed axes must be large enough to split into chunks. If ``x`` is
    too small, fewer jobs may be used than requested.

    References
    ----------
    .. [1] Cooley, James W., and John W. Tukey, 1965, "An algorithm for the
           machine calculation of complex Fourier series," *Math. Comput.*
           19: 297-301.
    .. [2] Bluestein, L., 1970, "A linear filtering approach to the
           computation of discrete Fourier transform". *IEEE Transactions on
           Audio and Electroacoustics.* 18 (4): 451-455.

    Examples
    --------
    >>> import scipy.fft
    >>> scipy.fft.fft(np.exp(2j * np.pi * np.arange(8) / 8))
    array([-2.33486982e-16+1.14423775e-17j,  8.00000000e+00-1.25557246e-15j,
            2.33486982e-16+2.33486982e-16j,  0.00000000e+00+1.22464680e-16j,
           -1.14423775e-17+2.33486982e-16j,  0.00000000e+00+5.20784380e-16j,
            1.14423775e-17+1.14423775e-17j,  0.00000000e+00+1.22464680e-16j])

    In this example, real input has an FFT which is Hermitian, i.e., symmetric
    in the real part and anti-symmetric in the imaginary part:

    >>> from scipy.fft import fft, fftfreq, fftshift
    >>> import matplotlib.pyplot as plt
    >>> t = np.arange(256)
    >>> sp = fftshift(fft(np.sin(t)))
    >>> freq = fftshift(fftfreq(t.shape[-1]))
    >>> plt.plot(freq, sp.real, freq, sp.imag)
    [<matplotlib.lines.Line2D object at 0x...>, <matplotlib.lines.Line2D object at 0x...>]
    >>> plt.show()