make_alpha.py

def make_alpha(image):
    image = image.to_rgba()

    # Find the brightest pixel in the image.
    brightest = 0
    for y in range(image.bounds.height):
        for x in range(image.bounds.width):
            p = image.get(x, y)
            brightest = max(max(brightest, p.r), max(p.g, p.b))

    if not brightest:
        return image

    for y in range(image.bounds.height):
        for x in range(image.bounds.width):
            p = image.get(x, y)
            if p.a:
                # Brightest pixel has alpha of 255.
                a = max(max(p.r, p.g), p.b)
                a = (a * 255) // brightest
                if a:
                    # Un-premultiply alpha.
                    r = (p.r * 255) // a
                    g = (p.g * 255) // a
                    b = (p.b * 255) // a
                else:
                    r, g, b = 0, 0, 0
                image.set(x, y, r, g, b, a)

    return image