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implementation of pollKey, companion to waitKey

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w32 backend: implemented
other backends: stubbed or fallback to waitKey
documentation: cross-linked and more precise in some places
This commit is contained in:
Christoph Rackwitz
2021-02-01 15:44:03 +01:00
parent 674ecc5581
commit 26da491053
7 changed files with 152 additions and 72 deletions
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modules/highgui/include/opencv2/highgui.hpp
+27 -15
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@@ -347,23 +347,34 @@ The function waitKey waits for a key event infinitely (when \f$\texttt{delay}\le
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.
pressed before the specified time had elapsed. To check for a key press but not wait for it, use
#pollKey.
@note
@note The functions #waitKey and #pollKey are the only methods in HighGUI that can fetch and handle
GUI events, so one of them needs to be called periodically for normal event processing unless
HighGUI is used within an environment that takes care of event processing.
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.
@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".
*/
CV_EXPORTS_W int waitKey(int delay = 0);
/** @brief Polls for a pressed key.
The function pollKey polls for a key event without waiting. It returns the code of the pressed key
or -1 if no key was pressed since the last invocation. To wait until a key was pressed, use #waitKey.
@note The functions #waitKey and #pollKey are the only methods in HighGUI that can fetch and handle
GUI events, so one of them 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.
*/
CV_EXPORTS_W int pollKey();
/** @brief Displays an image in the specified window.
The function imshow displays an image in the specified window. If the window was created with the
@@ -383,11 +394,12 @@ If the window was not created before this function, it is assumed creating a win
If you need to show an image that is bigger than the screen resolution, you will need to call namedWindow("", WINDOW_NORMAL) before the imshow.
@note This function should be followed by cv::waitKey function which displays the image for specified
milliseconds. Otherwise, it won't display the image. For example, **waitKey(0)** will display the window
infinitely until any keypress (it is suitable for image display). **waitKey(25)** will display a frame
for 25 ms, after which display will be automatically closed. (If you put it in a loop to read
videos, it will display the video frame-by-frame)
@note This function should be followed by a call to cv::waitKey or cv::pollKey to perform GUI
housekeeping tasks that are necessary to actually show the given image and make the window respond
to mouse and keyboard events. Otherwise, it won't display the image and the window might lock up.
For example, **waitKey(0)** will display the window infinitely until any keypress (it is suitable
for image display). **waitKey(25)** will display a frame and wait approximately 25 ms for a key
press (suitable for displaying a video frame-by-frame). To remove the window, use cv::destroyWindow.
@note