# Improving the performance of interpolating between two arrays

The Problem

I am currently writing a script that converts images into numerical array representation and then calculates "in-between" images based on linear interpolation between the start and end array.

My code does exactly what I want but goes over many nested loops which strikes me as something that will lead to very high computation times for many interpolation steps or big images.

The Code

The code is in python

import numpy as np

# Helper function that calculates the interpolation between two points
def interpolate_points(p1, p2, n_steps=3):
# interpolate ratios between the points
ratios = np.linspace(0, 1, num=n_steps)
# linear interpolate vectors
vectors = list()
for ratio in ratios:
v = (1.0 - ratio) * p1 + ratio * p2
vectors.append(v)
return np.asarray(vectors)

# final function that interpolates arrays
def interpolate_arrays(start_array,end_array,n_steps=10):
n = 0
array_interpolation = []
while n < n_steps:
i = 0
x = []
while i < len(start_array):
e = interpolate_points(start_array[i],end_array[i],n_steps)[n]
x.append(e)
i += 1
array_interpolation += [x]
n += 1
return array_interpolation


This results in:

#Test
X1 = [1,1]
X2 = [3,3]

interpolate_arrays(X1,X2,n_steps=3)
#[[1.0, 1.0], [2.0, 2.0], [3.0, 3.0]]


## 1 Answer

There are some easy wins here. Your interpolate_points doesn't need a loop:

def interpolate_points(p1, p2, n_steps=3):
"""Helper function that calculates the interpolation between two points"""
# interpolate ratios between the points
ratios = np.linspace(0, 1, num=n_steps)
# linear interpolate vectors
vectors = (1.0 - ratios) * p1 + ratios * p2
return vectors


Also, even without further vectorization, you should be making use of range in your main function:

def interpolate_arrays(start_array, end_array, n_steps=10):
"""final function that interpolates arrays"""
array_interpolation = []
for n in range(n_steps):
x = []
for i in range(len(start_array)):
e = interpolate_points(start_array[i], end_array[i], n_steps)[n]
x.append(e)
array_interpolation += [x]
return array_interpolation


However, all of that can be replaced with a call to interp1d:

import numpy as np
from scipy.interpolate import interp1d

def interpolate_arrays(bounds, n_steps=10):
"""final function that interpolates arrays"""
bounds = np.array(bounds)

fun = interp1d(
x=[0, 1],
y=bounds.T,
)
y = fun(np.linspace(0, 1, n_steps))

return y

def test():
X1 = [1.5, 1]
X2 = [5.5, 3]

y = interpolate_arrays([X1, X2], n_steps=3)
assert y.T.tolist() == [[1.5, 1.0], [3.5, 2.0], [5.5, 3.0]]


Even easier:

def interpolate_arrays(X1, X2, n_steps=10):
"""final function that interpolates arrays"""
return np.linspace(X1, X2, n_steps)

def test():
X1 = [1.5, 1]
X2 = [5.5, 3]

y = interpolate_arrays(X1, X2, n_steps=3)
assert y.tolist() == [[1.5, 1.0], [3.5, 2.0], [5.5, 3.0]]


Notes:

• If you use interp1d, it would be better if your inputs and outputs are both two-dimensional np.ndarray; in their current form they need a transposition
• Write some unit tests such as the one shown, although it would be a better idea to call isclose since this is floating-point math
• If you want, it is trivial to make this extrapolate as well as interpolate

Basically: if there is a math thing in your head, before even thinking about what it would take to implement it yourself, do a search through scipy/numpy to see if it has already been done for you.

• Thanks a lot, this is answer is both a helpful guide for improvements as well as a good "SO just give me the final code"-style answer. Apr 27, 2020 at 15:28