Image is a porn image?

seximage.py

#!/usr/local/bin/python

import sys, Image

if __name__ == '__main__':
    fn = sys.argv[1]
    img = Image.open(fn).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.'%(fn, 'is' if cnt > w * h * 0.3 else 'is not')