Encode and decode low-bandwidth colour information in grayscale images

colordetecttest.py

#!/usr/bin/python
import sys
import random
from PIL import Image, ImageDraw

img = Image.open(sys.argv[1])
img = img.convert(mode='YCbCr')
width = img.width

data = img.getdata()

new_data = []
prev_row = [pixel[0] for pixel in list(data)[:width]]
prev_prev_row = prev_row
prev_prev_prev_row = prev_row
this_row = []
prev_color = 128

for i, pixel in enumerate(data):
    row_number = i // width
    row_position = i % width

    is_even_row = row_number % 2

    if row_position == 0 and i > 0:
        prev_prev_prev_row = prev_prev_row
        prev_prev_row = prev_row
        prev_row = this_row
        this_row = []

    luma = pixel[0] & 243
    color = 80 + ((pixel[0] >> 2 & 3) ^ 2) * 32
    if is_even_row:
        color = 255 - color
    old_color = prev_row[row_position]

    this_row.append(color)

    #if color != prev_prev_row[row_position] or old_color != prev_prev_prev_row[row_position]:# or abs(color - prev_color) > 32:
        #color = 128
        #old_color = 128

    prev_color = color

    if is_even_row:
        b = color
        r = old_color
    else:
        b = old_color
        r = color

    # b, r = int(-0.564 * (b - luma)), int(0.713 * (r - luma))
    # b, r = int(0.8 * b + 0.2 * r), int(1.2 * r - 0.2 * b)

    new_data.append((luma, b, r))

img.putdata(new_data)

img.save(sys.argv[2])

colorenc.py

#!/usr/bin/python
import sys
import random
from PIL import Image, ImageDraw

img = Image.open(sys.argv[1])
img = img.convert(mode='YCbCr')
width = img.width

data = img.getdata()

new_data = []

for i, pixel in enumerate(data):
    row_number = i // width

    is_even_row = row_number % 2

    color = 255 - pixel[1] if is_even_row else pixel[2]

    composite = (pixel[0] & 243) | ((((min(95, max(0, color - 80))) // 32) << 2) ^ 8)
    new_data.append((composite, 128, 128))

img.putdata(new_data)
img = img.convert(mode='RGB')

img.save(sys.argv[2])