OpenCV-選定攝像頭圖像區域,顯示對比度和Lab中的L份量

import cv2
import numpy as np

rectangle_flag = 0
global ix_start, iy_start, ix_end, iy_end
ix_start = -1
iy_start = -1
ix_end = 0
iy_end = 0

global drawing
drawing =