def clean_pixel_alpha_below(image, v=1):
    """
    clean pixels of the image which alpha channel is below the given value
    :param image: Image
    :param v: given value
    :return:
    """
    image = image.copy()
    width, height = image.size
    if image.mode != 'RGBA':
        image = image.convert('RGBA')
    pa = image.load()
    for x in range(width):
        for y in range(height):
            pixel = pa[x, y]
            if pixel[3] < v:
                pa[x, y] = 0, 0, 0, 0
    return image
