Contents

scipy.interpolate

scipy.interpolate

This module provides general interpolation capability for data in 1, 2, and higher dimensions. This list of features is from the documentation:

  • A class representing an interpolant (interp1d) in 1-D, offering several interpolation methods.

  • Convenience function griddata offering a simple interface to interpolation in N dimensions (N = 1, 2, 3, 4, …). Object-oriented interface for the underlying routines is also available.

  • Functions for 1- and 2-dimensional (smoothed) cubic-spline interpolation, based on the FORTRAN library FITPACK. There are both procedural and object-oriented interfaces for the FITPACK library.

  • Interpolation using Radial Basis Functions.

%matplotlib inline
import matplotlib.pyplot as plt
import numpy as np

1D data

from scipy.interpolate import interp1d

x = np.linspace(0, 10, num=11, endpoint=True)
y = np.cos(-x**2/9.0)
f = interp1d(x, y, kind='linear') # default if kind=None
f2 = interp1d(x, y, kind='cubic')
f3 = interp1d(x, y, kind='nearest')

xnew = np.linspace(0, 10, num=41, endpoint=True)
plt.plot(x, y, 'o', xnew, f(xnew), '-', xnew, f2(xnew), '--', xnew, f3(xnew), '.-')
plt.legend(['data', 'linear', 'cubic', 'nearest'], loc='best')
plt.show()

nD data

There are fewer approaches to n-dimensional data, the evaluation for arbitrary dimensions is always for points on an n dimensional grid.

from scipy.interpolate import griddata

def func(x, y):
    return x*(1-x)*np.cos(4*np.pi*x) * np.sin(4*np.pi*y**2)**2

# A regular grid array of x,y coordinates

grid_x, grid_y = np.mgrid[0:1:100j, 0:1:200j] # see np.info(np.mgrid) for an explanation of the 200j !!

np.info(np.mgrid)

# A random sampling within the same area

points = np.random.rand(1000, 2)
values = func(points[:,0], points[:,1])

# Resample from the values at these points onto the regular mesh

grid_z0 = griddata(points, values, (grid_x, grid_y), method='nearest')
grid_z1 = griddata(points, values, (grid_x, grid_y), method='linear')
grid_z2 = griddata(points, values, (grid_x, grid_y), method='cubic')

plt.subplot(221)
plt.imshow(func(grid_x, grid_y).T, extent=(0,1,0,1), origin='lower', cmap='jet')
plt.plot(points[:,0], points[:,1], 'k.', ms=1)
plt.title('Original')
plt.subplot(222)
plt.imshow(grid_z0.T, extent=(0,1,0,1), origin='lower', cmap='jet')
plt.title('Nearest')
plt.subplot(223)
plt.imshow(grid_z1.T, extent=(0,1,0,1), origin='lower', cmap='jet')
plt.title('Linear')
plt.subplot(224)
plt.imshow(grid_z2.T, extent=(0,1,0,1), origin='lower', cmap='jet')
plt.title('Cubic')
plt.gcf().set_size_inches(6, 6)
plt.show()

Splines

Which have the added benefit of giving smooth derivative information

from scipy.interpolate import splrep, splev

x = np.arange(0, 2*np.pi+np.pi/4, 2*np.pi/8)
y = np.sin(x)
tck = splrep(x, y, s=0)
xnew = np.arange(0, 2*np.pi, np.pi/50)
ynew = splev(xnew, tck, der=0)
yder = splev(xnew, tck, der=1)

plt.figure()
plt.plot(x, y, 'x', xnew, ynew, xnew, np.sin(xnew), x, y, 'b')
plt.legend(['Linear', 'Cubic Spline', 'True'])
plt.axis([-0.05, 6.33, -1.05, 1.05])
plt.title('Cubic-spline interpolation')
plt.show()

plt.figure()
plt.plot(xnew, yder, xnew, np.cos(xnew),'--')
plt.legend(['Cubic Spline', 'True'])
plt.axis([-0.05, 6.33, -1.05, 1.05])
plt.title('Derivative estimation from spline')
plt.show()

2D splines are also available

from scipy.interpolate import bisplrep, bisplev

# Gridded function (at low resolution ... doesn't need to be gridded data here)

x, y = np.mgrid[-1:1:20j, -1:1:20j]
z = (x+y) * np.exp(-6.0*(x*x+y*y))

plt.figure()
plt.pcolor(x, y, z, cmap='jet')
plt.colorbar()
plt.title("Sparsely sampled function.")
plt.show()

xnew, ynew = np.mgrid[-1:1:70j, -1:1:70j]

## Create the spline-representation object tck

tck = bisplrep(x, y, z, s=0)
znew = bisplev(xnew[:,0], ynew[0,:], tck)

plt.figure()
plt.pcolor(xnew, ynew, znew, cmap='jet')
plt.colorbar()
plt.title("Interpolated function.")
plt.show()

See also

  • Radial basis function interpolation for scattered data in n dimensions (slow for large numbers of points): from scipy.interpolate import Rbf

  • scipy.ndimage for fast interpolation operations on image-like arrays

  • B-splines on regular arrays are found in the scipy.signal module

previous

scipy

next

scipy.spatial