Calculate Image brightness with Python Pillow

brightness.py

import sys
from PIL import Image


def calculate_brightness(image):
    greyscale_image = image.convert('L')
    histogram = greyscale_image.histogram()
    pixels = sum(histogram)
    brightness = scale = len(histogram)

    for index in range(0, scale):
        ratio = histogram[index] / pixels
        brightness += ratio * (-scale + index)

    return 1 if brightness == 255 else brightness / scale


if __name__ == '__main__':
    for file in sys.argv[1:]:
        image = Image.open(file)
        print("%s\t%s" % (file, calculate_brightness(image)))

Does anyone know the motivation for the implementation? why does it subtract from the length/range of the histogram?

Hi, what about measure brightness over selfies?. What happen if person has black or caucasican race?. The measure of face brightness must be independent of person race..