PIL Grayscale improve

PIL_grayscale.py

# (c) yoya@awm.jp 2019/06/20-
# ref) https://pillow.readthedocs.io/en/stable/reference/Image.html

import sys
from PIL import Image

_rgb2xyz_rec709 = (
    0.412453, 0.357580, 0.180423, 0,
    0.212671, 0.715160, 0.072169, 0, # RGB mixing weight
    0.019334, 0.119193, 0.950227, 0 )

_gamma22LUT  = [pow(x/255.0 , 2.2) * 255 for x in range(256)] * 3
_gamma045LUT = [pow(x/255.0 , 1.0/2.2) * 255 for x in range(256)]

def grayscale(im):
    if im.mode != "RGB":  # mainly, care for mode=="P"
       im = im.convert("RGB")
    im = im.point(_gamma22LUT)  # sRGB to linear (approximate value 2.2)
    im_gray = im.convert("L", _rgb2xyz_rec709)
    return  im_gray.point(_gamma045LUT)  # linear to sRGB

# main
_,infile,outfile = sys.argv
im = Image.open(infile)
im_gray = grayscale(im)
im_gray.save(outfile)