#!/usr/bin/env python3

from PIL import Image

thresh = 10

img = Image.open('nur_oben_Dokument_gross3.png')
counts = dict(map(reversed, img.getcolors(2**24)))

for y in range(img.height):
  for x in range(img.width):
    cval = 0 if counts[img.getpixel((x, y))] > thresh else 255
    img.putpixel((x, y), (cval, cval, cval))

img.save('nur_oben_Dokument_gross3a.png')