drag shade

import cv2
import numpy as np

parts = 5
phase = 0 # 0 ~ parts*2 = 0
w, h = 500, 100

while True:
    img = np.zeros((h, w, 3), dtype='uint8')
    if phase >= parts:
        per = parts - (phase - parts)
    else:
        per = phase
    coord = int(per / parts * w)

    img = cv2.rectangle(img, (coord, 0), (0, h-1), (255, 255, 255), -1)
    cv2.imshow('win', img)
    cv2.waitKey(1)

    phase += 1
    phase %= parts*2