How to pixelate image using OpenCV in Python?

Question

Top answer in this link How to pixelate a square image to 256 big pixels with python? uses PIL to pixelate image. Converting image from PIL to cv2.Mat is possible but I'm not allowed to use other library, and I couldn't find any good method using opencv.

Is there any way to pixelate image using OpenCV library only in Python? Any sample image is fine. Solution with pixel size parameter that I can control for later adjustment would be very appreciated.

Answer 1

EDIT^2

With the help of himself, I moved Mark Setchell's answer, which is the above mentioned top answer, to plain OpenCV Python code. (Have a look at the revision history of my answer to see the old version using a loop.)

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import cv2

# Input image
input = cv2.imread('images/paddington.png')

# Get input size
height, width = input.shape[:2]

# Desired "pixelated" size
w, h = (16, 16)

# Resize input to "pixelated" size
temp = cv2.resize(input, (w, h), interpolation=cv2.INTER_LINEAR)

# Initialize output image
output = cv2.resize(temp, (width, height), interpolation=cv2.INTER_NEAREST)

cv2.imshow('Input', input)
cv2.imshow('Output', output)

cv2.waitKey(0)

Input (from linked question):

Output:

Disclaimer: I'm new to Python in general, and specially to the Python API of OpenCV (C++ for the win). Comments, improvements, highlighting Python no-gos are highly welcome!

Answer 2

For anyone looking for alternate solution,

import cv2
import numpy as np

img = cv2.imread('input.png')
height, width, channels = img.shape

pixel_size = 50

# Pad image
pad_x = (pixel_size - width % pixel_size) % pixel_size
pad_y = (pixel_size - height % pixel_size) % pixel_size
img = np.pad(img, ((0, pad_y), (0, pad_x), (0, 0)), mode='reflect')

# Reshape image into blocks and compute average color of each block
h, w, c = img.shape
blocks = np.mean(img.reshape(h//pixel_size, pixel_size, -1, pixel_size, c), axis=(1, 3))

# Repeat average color of each block to fill corresponding region in the image
output = np.repeat(np.repeat(blocks, pixel_size, axis=1), pixel_size, axis=0)

# Remove padding
output = output[:height, :width].astype("uint8")

cv2.imshow('Output', output)
cv2.waitKey(0)