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".
