waitKey(x);

第一个参数： 等待x ms，如果在此期间有按键按下，则立即结束并返回按下按键的

ASCII码，否则返回-1

如果x=0，那么无限等待下去，直到有按键按下

waitKey(…)

    waitKey([, delay]) -> retval

    .   @brief Waits for a pressed key.

    .   

    .   The function waitKey waits for a key event infinitely (when \f$\texttt{delay}\leq 0\f$ ) 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.

    .   

    .   @param delay Delay in milliseconds. 0 is the special value that means “forever”.