# SVD

SVD (Singular Value Decomposition) is a method of decomposing a matrix into a product of an orthogonal matrix and a diagonal matrix, and can be viewed as a type of dimensionality reduction algorithm. Here, we will try to use SVD to represent image data in a lower dimensional data.

import numpy as np
import matplotlib.pyplot as plt
import japanize_matplotlib
from scipy import linalg
from PIL import Image


## Data for experiment

Let’s consider the image of the word “実験” as a series of data (grayscale, so [0, 0, 1, 1, 0 …]) ) and decompose it into singular values.

img = Image.open("./sample.png").convert("L").resize((163, 372)).rotate(90, expand=True)
img


## Perform singular value decomposition

## Decompose matrix X into singular values
X = np.asarray(img)
U, Sigma, VT = linalg.svd(X, full_matrices=True)

print(f"A: {X.shape}, U: {U.shape}, Σ:{Sigma.shape}, V^T:{VT.shape}")

A: (163, 372), U: (163, 163), Σ:(163,), V^T:(372, 372)


## Approximate image with low rank

# Increasing the rank restores the original characters more precisely
for rank in [1, 2, 3, 4, 5, 10, 20, 50]:
# Extract elements up to the rank-th element
U_i = U[:, :rank]
Sigma_i = np.matrix(linalg.diagsvd(Sigma[:rank], rank, rank))
VT_i = VT[:rank, :]

# Restore image
temp_image = np.asarray(U_i * Sigma_i * VT_i)
Image.fromarray(np.uint8(temp_image))
plt.title(f"rank={rank}")
plt.imshow(temp_image, cmap="gray")
plt.show()


## Contents of matrix V

total = np.zeros((163, 372))

for rank in [1, 2, 3, 4, 5]:
# Extract elements up to the rank-th element
U_i = U[:, :rank]
Sigma_i = np.matrix(linalg.diagsvd(Sigma[:rank], rank, rank))
VT_i = VT[:rank, :]

# Set all values except the rank-th singular value to 0, leaving only the rank-th element.
if rank > 1:
for ri in range(rank - 1):
Sigma_i[ri, ri] = 0

# Restore image
temp_image = np.asarray(U_i * Sigma_i * VT_i)
Image.fromarray(np.uint8(temp_image))

# Add only the rank-th element
total += temp_image

plt.figure(figsize=(5, 5))
plt.suptitle(f"$u_{rank}$")
plt.subplot(211)
plt.imshow(temp_image, cmap="gray")
plt.subplot(212)
plt.plot(VT[0])
plt.show()

# Confirm that adding the first through the fifth elements together properly restores the original image
plt.imshow(total)