def quantize(image, bits_per_channel=None):

    '''Reduces the number of bits per channel in the given image.'''

    if bits_per_channel is None:
        bits_per_channel = 6

    assert image.dtype == np.uint8

    shift = 8-bits_per_channel
    halfbin = (1 << shift) >> 1

    return ((image.astype(int) >> shift) << shift) + halfbin
Reduces the number of bits per channel in the given image.