Python color quantization / reduction to a custom degree using CV2 library. The color palette gets chosen automatically, by the most used colors.

color-quantization-cv2.py

# Credits to Stackoverflow user u1234x1234
# Source: https://stackoverflow.com/questions/73666119/open-cv-python-quantize-to-a-given-color-palette
# The example was changed to automatically pick up the primary colors

import cv2
import numpy as np

image_path = 'input.png'
image_output_path = 'output.png'
quantize_count = 16  # How many colors will be left


def quantize_to_palette(image, palette):
    x_query = image.reshape(-1, 3).astype(np.float32)
    x_index = palette.astype(np.float32)

    knn = cv2.ml.KNearest_create()
    knn.train(x_index, cv2.ml.ROW_SAMPLE, np.arange(len(palette)))
    ret, results, neighbours, dist = knn.findNearest(x_query, 1)

    quantized_image = np.array([palette[idx] for idx in neighbours.astype(int)])
    quantized_image = quantized_image.reshape(image.shape)
    return quantized_image


def get_most_relevant_colors():
    # Load image
    img = cv2.imread(image_path)

    # Convert image from BGR to RGB format
    img = cv2.cvtColor(img, cv2.COLOR_BGR2RGB)

    # Flatten image into a 2D array of pixels
    pixels = img.reshape((-1, 3))

    # Use NumPy's unique() and bincount() functions to count unique colors
    unique_colors, counts = np.unique(pixels, axis=0, return_counts=True)

    # Sort colors by count (most common first)
    sorted_indices = np.argsort(-counts)
    unique_colors = unique_colors[sorted_indices]

    # Create an array of the most common colors in RGB format
    rgb_colors = unique_colors[:quantize_count]

    # Swap R and B colors
    for i in range(len(rgb_colors)):
        tmp = rgb_colors[i][0]
        rgb_colors[i][0] = rgb_colors[i][2]
        rgb_colors[i][2] = tmp
    return rgb_colors


palette = get_most_relevant_colors()

# Quantize image with palette
input = cv2.imread(image_output_path)
output = quantize_to_palette(image=input, palette=np.array(palette))
cv2.imwrite(image_output_path, output)