Interpolation in Machine Learning

In machine learning, interpolation refers to the process of estimating unknown values that fall between known data points. This can be useful in various scenarios, such as filling in missing values in a dataset or generating new data points to smooth out a curve. In this article, we are going to explore fundamentals and implementation of different types of interpolation along with it's application in machine learning.

In machine learning, interpolation is an essential method for estimating values within a range of known data points. Forecasting values at intermediate points entails building a function that roughly mimics the behavior of the underlying data.

Interpolation in Machine Learning

The practice of guessing unknown values based on available data points is known as interpolation in the context of machine learning. In tasks like regression and classification, where the objective is to predict outcomes based on input features, it is important. Machine learning algorithms are capable of producing well-informed predictions for unknown or intermediate values by interpolating between known data points.

Interpolation Types

The intricacy and applicability of interpolation techniques varied for various kinds of data. Typical forms of interpolation include the following:

• Interpolation in Linear Form: By assuming a linear relationship between neighboring data points, linear interpolation calculates values along a straight line that connects them.
• Equation-Based Interpolation: By fitting a polynomial function to the data points, polynomial interpolation produces a more flexible approximation that is capable of capturing nonlinear relationships.
• Interpolation of Splines: By building piece wise polynomial functions that connect data points gradually, spline interpolation prevents abrupt changes in the interpolated function.
• Interpolation of Radial Basis Function: Values based on the separations between data points are interpolated using radial basis functions in radial basis function interpolation.

Interpolation in Linear Form

A straightforward but efficient technique for guessing values between two known data points is linear interpolation.

The value of y at any intermediate point x can be approximated using the following formula, given two data points: (⁽ 1, 1 ) (x 1​ ,y 1​  ) and ( 2 , 2 ) (x 2​ ,y 2​  ). i.e y=y_1+(x−x_1)⋅(y_2−y_1)/x_2−x_1

Implementation

• This code snippet illustrates linear interpolation using LinearNDInterpolator from SciPy.
• It randomly generates 10 data points in 2D space with corresponding values.
• The LinearNDInterpolator function constructs an interpolation function based on these points. It then interpolates the value at a specified point and visualizes both the data points and the interpolated point on a scatter plot.
• Finally, the interpolated value at the specified point is printed.

Output:

Interpolated value at [0.5 0.5]: [0.76124023]

Polynomial Interpolation

• Polynomial interpolation is a method of estimating values between known data points by fitting a polynomial function to the data. The goal is to find a polynomial that passes through all the given points.
• This method is useful for approximating functions that may not have a simple analytical form. One common approach to polynomial interpolation is to use the Lagrange polynomial or Newton's divided differences method to construct the interpolating polynomial.

Implementation

• This article demonstrates polynomial interpolation using the interp1d function from SciPy.
• It begins by generating sample data representing points along a sine curve. The interp1d function is then applied with a cubic spline interpolation method to approximate the curve between the data points.
• Finally, the original data points and the interpolated curve are visualized using matplotlib, showcasing the effectiveness of polynomial interpolation in approximating the underlying function from sparse data points.

import numpy as np
from scipy.interpolate import interp1d
import matplotlib.pyplot as plt

# Generate some sample data
x = np.linspace(0, 10, 10)
y = np.sin(x)

# Perform polynomial interpolation
poly_interp = interp1d(x, y, kind='cubic')

# Generate points for plotting the interpolated curve
x_interp = np.linspace(0, 10, 100)
y_interp = poly_interp(x_interp)

# Plot the original data and the interpolated curve
plt.scatter(x, y, label='Original Data')
plt.plot(x_interp, y_interp, color='red', label='Polynomial Interpolation')
plt.xlabel('X')
plt.ylabel('Y')
plt.title('Polynomial Interpolation with interp1d')
plt.legend()
plt.grid(True)
plt.show()

Output:

Spline Interpolation

Spline interpolation is a method of interpolation where the interpolating function is a piecewise-defined polynomial called a spline. Unlike polynomial interpolation, which uses a single polynomial to fit all the data points, spline interpolation divides the data into smaller segments and fits a separate polynomial to each segment. This approach results in a smoother interpolating function that can better capture the local behavior of the data. The most common type of spline interpolation is cubic spline interpolation, which uses cubic polynomials for each segment and ensures continuity of the first and second derivatives at the endpoints of each segment. Spline interpolation is particularly useful for smoothing noisy data or interpolating functions with complex shapes.

Implementation

• This code demonstrates cubic spline interpolation using `CubicSpline` from SciPy. It starts with a set of sample data points defined in arrays `x` and `y`.
• The `CubicSpline` function constructs a cubic spline interpolation function based on these points.
• Then, it generates interpolated points along the x-axis and calculates corresponding y-values.
• Finally, both the original data points and the interpolated curve are plotted using matplotlib to visualize the interpolation result.

import numpy as np
import matplotlib.pyplot as plt
from scipy.interpolate import CubicSpline

# Generate some sample data points
x = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
y = np.array([5, 6, 9, 8, 7, 4, 6, 7, 8, 5])

# Create a CubicSpline interpolation
cs = CubicSpline(x, y)

# Generate points for plotting the interpolated curve
x_interp = np.linspace(1, 10, 100)
y_interp = cs(x_interp)

# Plot original data points and interpolated curve
plt.figure(figsize=(8, 6))
plt.plot(x, y, 'o', label='Data Points')
plt.plot(x_interp, y_interp, label='Cubic Spline Interpolation')
plt.title('Cubic Spline Interpolation')
plt.xlabel('X')
plt.ylabel('Y')
plt.legend()
plt.grid(True)
plt.show()

Output:

Radial Basis Function Interpolation

Radial Basis Function (RBF) interpolation is a method of interpolation that uses radial basis functions to approximate the underlying data. Unlike polynomial interpolation, which fits a single polynomial to the entire dataset, RBF interpolation uses a combination of radial basis functions centered at each data point to construct the interpolating function.

Implementation

• This code demonstrates Radial Basis Function (RBF) interpolation using `RBFInterpolator` from SciPy.
• It generates random data points in a 2D space and calculates corresponding y-values based on a predefined function.
• A grid is then created for visualization purposes.
• The `RBFInterpolator` function constructs an interpolation function based on the random data points.
• Finally, it plots the interpolated surface and scatter plot of the original data points to visualize the interpolation result.

import numpy as np
import matplotlib.pyplot as plt
from scipy.interpolate import RBFInterpolator

# Generate random data points
rng = np.random.default_rng()
x_data = rng.uniform(-1, 1, size=(100, 2))
y_data = np.sum(x_data, axis=1) * np.exp(-6 * np.sum(x_data**2, axis=1))

# Generate a grid for visualization
x_grid = np.mgrid[-1:1:50j, -1:1:50j]
x_flat = np.column_stack((x_grid[0].flatten(), x_grid[1].flatten()))

# Perform RBF interpolation
rbf_interpolator = RBFInterpolator(x_data, y_data)
y_flat = rbf_interpolator(x_flat)
y_grid = y_flat.reshape(50, 50)

# Plot the interpolated surface and scatter plot of original points
fig, ax = plt.subplots()
ax.pcolormesh(x_grid[0], x_grid[1], y_grid)
p = ax.scatter(x_data[:,0], x_data[:,1], c=y_data, s=50, ec='k')
fig.colorbar(p)
plt.title('RBF Interpolation with Random Data')
plt.xlabel('X1')
plt.ylabel('X2')
plt.show()

Output:

Applications Of Interpolation in Machine Learning

Interpolation is a method used in various fields for estimating values between known data points. Some common applications of interpolation include:

• Image Processing: Interpolation is used to resize images by estimating the values of pixels in the resized image based on the values of neighboring pixels in the original image.
• Computer Graphics: In computer graphics, interpolation is used to generate smooth curves and surfaces, such as Bezier curves and surfaces, which are used to create shapes and animations.
• Numerical Analysis: Interpolation is used in numerical analysis to approximate the value of a function between two known data points. This is useful in areas such as finite element analysis and computational fluid dynamics.
• Signal Processing: In signal processing, interpolation is used to upsample signals, which increases the number of samples in a signal without changing its frequency content.
• Mathematical Modeling: Interpolation is used in mathematical modeling to estimate unknown values based on known data points, such as in the construction of mathematical models for physical systems.
• Geographic Information Systems (GIS): Interpolation is used in GIS to estimate values of geographical features, such as elevation or temperature, at locations where data is not available.
• Audio Processing: In audio processing, interpolation is used to resample audio signals, which allows for changing the