https://twitter.com/hakatashi/status/561511110461755393

main.py

from PIL import Image

omote = Image.open('omote.png', 'r')
ura = Image.open('ura.png', 'r')

dest = Image.new('RGBA', omote.size)

(width, height) = omote.size

for x in range(width):
	for y in range(height):
		omotepx = omote.getpixel((x, y))[0]
		urapx = ura.getpixel((x, y))[0]

		alpha = 255 + urapx - omotepx
		if alpha == 0:
			val = 255
		else:
			val = 255 * urapx / alpha

		dest.putpixel((x, y), (val, val, val, alpha))

dest.save('dest.png', 'PNG')