Opencv之waitKey（）

官方的解释，摘下来备忘
Waits for a pressed key.

C++: int waitKey(int delay=0)
Python: cv2.waitKey([delay]) → retval
C: int cvWaitKey(int delay=0 )
Python: cv.WaitKey(delay=0) → int

Parameters: delay – Delay in milliseconds. 0 is the special value that means “forever”.所以调用时用不带参数的方式替代0；
    The function waitKey waits for a key event infinitely (when delay 0 ) or for delay milliseconds, when it is positive. Since the OS has a minimum time between switching threads, the function will not wait exactly delay ms, it will wait at least delay ms, depending on what else is running on your computer at that time. It returns the code of the pressed key or -1 if no key was pressed before the specified time had elapsed.
    Note: This function is the only method in HighGUI that can fetch and handle events, so it needs to be called periodically for normal event processing unless HighGUI is used within an environment that takes care of event processing.
    Note: The function only works if there is at least one HighGUI window created and the window is active. If there are several HighGUI windows, any of them can be active.

    此外，在调用时会启动消息循环。这就是使用imshow（）却不调用waitKey（）就不会显示图片的原因。