关于waitKey()函数按键无反应情况

按下键的时候，焦点要落在窗口上，不能落在cmd窗口上。

另外，一般在imshow()后要使用waitKey()，给图像绘制留下时间，不然窗口会出现无响应情况，并且图像无法显示出来。

int waitKey(int delay=0)
- 延时delay毫秒，默认0则延时无限长，必须有键按下才继续执行。
- 函数返回值为按下的键的ASCII码值，没按键则返回-1。

OpenCV: waitKey

waitKey
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”.

The function waitKey waits for a key event infinitely (when \texttt{delay}\leq 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.