`scipy.signal._arraytools:even_ext`

Signature

def even_ext(x, n, axis = -1)

Even extension at the boundaries of an array

Generate a new ndarray by making an even extension of x along an axis.

x : ndarray: The array to be extended.
n : int: The number of elements by which to extend x at each end of the axis.
axis : int, optional: The axis along which to extend x. Default is -1.

import numpy as np
from scipy.signal._arraytools import even_ext
a = np.array([[1, 2, 3, 4, 5], [0, 1, 4, 9, 16]])
even_ext(a, 2)

✓

array([[ 3,  2,  1,  2,  3,  4,  5,  4,  3],
       [ 4,  1,  0,  1,  4,  9, 16,  9,  4]])

Even extension is a "mirror image" at the boundaries of the original array:

t = np.linspace(0, 1.5, 100)
a = 0.9 * np.sin(2 * np.pi * t**2)
b = even_ext(a, 40)
import matplotlib.pyplot as plt

✓

plt.plot(np.arange(-40, 140), b, 'b', lw=1, label='even extension')
plt.plot(np.arange(100), a, 'r', lw=2, label='original')
plt.legend(loc='best')

✗

<matplotlib.legend.Legend object at 0x7f3f43a7b4d0>

plt.show()

✓