Create an internal colour lookup table from a reference image, apply it to a source image and save the result.

colour-lookup.py

'''

Create an internal colour lookup table from a reference image, apply it to a
source image and save the result.

Aaron Brady, 2020

'''
from PIL import Image
from math import floor

def create_lookup_table():
    w = 256
    values = [[0]*w, [0]*w, [0]*w]
    im = Image.open('polaroid.png')

    prefixes = [w*13, w*23, w*33]
    for ii, prefix in enumerate(prefixes):
        for i in range(256):
            values[ii][i] = im.getdata()[prefix + i]
    return values

def munge(im, lut):
    w, h = im.size
    for x in range(w):
        for y in range(h):
            p = im.getpixel((x, y))
            red_values = lut[0][p[0]]
            green_values = lut[1][p[1]]
            blue_values = lut[2][p[2]]
            q = [0, 0, 0]
            q[0] = floor(sum((red_values[0], green_values[0], blue_values[0])) / 3)
            q[1] = floor(sum((red_values[1], green_values[1], blue_values[1])) / 3)
            q[2] = floor(sum((red_values[2], green_values[2], blue_values[2])) / 3)
            im.putpixel((x, y), tuple(q))
    return im

lut = create_lookup_table()

im = Image.open('input.jpg')
im = munge(im, lut)
im.save('output.jpg')