bmp_negator.py

# coding: utf-8
# Python Bitmap (bmp) Image Negation (using module struct)

import struct

with open('in.bmp', 'rb') as fd, open('out.bmp', 'wb') as out:
	s = fd.read(1)
	count = 1
	while s:
		if count < 37: # header bytes
			out.write(s)
		else: # data bytes
			b = struct.unpack('B', s)[0]
			out.write(struct.pack('B', 255 - b))
		s = fd.read(1)
		count += 1