:param value:Optional pointer to an integer variable whose value reflects the position of the slider. Upon creation, the slider position is defined by this variable.
:param onChange:Pointer to the function to be called every time the slider changes position. This function should be prototyped as ``void Foo(int,void*);`` , where the first parameter is the trackbar position and the second parameter is the user data (see the next parameter). If the callback is the NULL pointer, no callbacks are called, but only ``value`` is updated.
:param userdata:User data that is passed as is to the callback. It can be used to handle trackbar events without using global variables.
The function ``createTrackbar`` creates a trackbar (a.k.a. slider or range control) with the specified name and range, assigns a variable ``value`` to be syncronized with the trackbar position and specifies the callback function ``onChange`` to be called on the trackbar position change. The created trackbar is displayed on top of the given window.
***winname** Name of the window that will be used as a parent for created trackbar. It can be NULL if the trackbar should be attached to the control panel.
The function ``imshow`` displays an image in the specified window. If the window was created with the ``CV_WINDOW_AUTOSIZE`` flag, the image is shown with its original size. Otherwise, the image is scaled to fit the window. The function may scale the image, depending on its depth:
:param flags:Flags of the window. Currently the only supported flag is ``CV_WINDOW_AUTOSIZE`` . If this is set, the window size is automatically adjusted to fit the displayed image (see :ref:`imshow` ), and you cannot change the window size manually.
The function ``namedWindow`` creates a window that can be used as a placeholder for images and trackbars. Created windows are referred to by their names.
You can call :cpp:func:`destroyWindow` or :cpp:func:`destroyAllWindows` to close the window and de-allocate any associated memory usage. For a simple program, you do not really have to call these functions because all the resources and windows of the application are closed automatically by the operating system upon exit.
***CV_WINDOW_NORMAL or CV_WINDOW_AUTOSIZE:**``CV_WINDOW_NORMAL`` enables you to resize the window, whereas ``CV_WINDOW_AUTOSIZE`` adjusts automatically the window size to fit the displayed image (see :ref:`imshow` ), and you cannot change the window size manually.
***CV_WINDOW_FREERATIO or CV_WINDOW_KEEPRATIO:**``CV_WINDOW_FREERATIO`` adjusts the image with no respect to its ratio, whereas ``CV_WINDOW_KEEPRATIO`` keeps the image ratio.
***CV_GUI_NORMAL or CV_GUI_EXPANDED:**``CV_GUI_NORMAL`` is the old way to draw the window without statusbar and toolbar, whereas ``CV_GUI_EXPANDED`` is a new enhanced GUI.
:math:`\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.
* 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.
* 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.
