Pixel sorting with Numpy

sort.py

import numpy as np
from PIL import Image


def sort_pixels(image, condition, rotation=0):
    pixels = np.rot90(np.array(image), rotation)
    luminosity = np.sum(pixels, axis=2) / (255 * 3)
    mask = np.zeros_like(luminosity)
    mask[condition(luminosity)] = 1

    edges = np.apply_along_axis(lambda row: np.convolve(row, [-1, 1], 'same'), 0, mask)
    intervals = [np.flatnonzero(row) for row in edges]

    for row, key in enumerate(luminosity):
        order = np.split(key, intervals[row])
        for index, segment in enumerate(order[1:]):
            order[index + 1] = np.argsort(segment) + intervals[row][index]
        order = np.concatenate(order)

        for channel in range(3):
            pixels[row, :, channel] = pixels[row, order.astype('uint32'), channel]

    return Image.fromarray(np.rot90(pixels, -rotation))


sort_pixels(Image.open('in.jpg'), lambda lum: (lum > 2 / 6) & (lum < 4 / 6), 1).save('out.png')