forbidden the porn image

import sys, Image

img_name = sys.argv[1]
img = Image.open(img_name).convert('YCbCr')
w, h = img.size
data = img.getdata()
cnt = 0

for i, ycbcr in enumerate(data):
    y, cb, cr = ycbcr
    if 86 <= cb <= 117 and 140 <= cr <= 168:
        cnt += 1

print '%s %s a porn image.'%(img_name, 'is' if cnt > w * h * 0.3 else 'is not')