7. ImageOps posterize

See: https://pillow.readthedocs.io/en/stable/reference/ImageOps.html#PIL.ImageOps.posterize

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7.1. Posterize

Use the ImageOps.posterize(image, bits) method to return an image with a reduced the number of bits for each color channel.

bits - The number of bits to keep for each channel (1-8).

from PIL import Image, ImageOps

with Image.open("test_images/cliffs.jpg") as im:
    im1 = ImageOps.posterize(im, 3)
    im1.save("imageOps/posterize.png")