OpenCV waitKey 无法正常捕捉方向键（上下左右），总结和解决方案，可以用waitKeyEx

在win10+python+opencv3.4.5，测试

while True:
    key = cv2.waitKey(0)
    print('key =', key)

依次按上、下、左、右方向键，输出：

key = 0
key = 0
key = 0
key = 0

测试代码：

while True:
    key = cv2.waitKeyEx(0)
    print('key =', key)

依次按上、下、左、右方向键，输出：

key = 2490368
key = 2621440
key = 2424832
key = 2555904

官方说，waitKeyEx和waitKey类似，Key code is implementation specific and depends on used backend: QT/GTK/Win32/etc