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modules/highgui/doc/highgui.rst
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*************************************
highgui. High-level GUI and Media I/O
*************************************
While OpenCV was designed for use in full-scale
applications and can be used within functionally rich UI frameworks (such as Qt*, WinForms*, or Cocoa*) or without any UI at all, sometimes there it is required to try functionality quickly and visualize the results. This is what the HighGUI module has been designed for.
It provides easy interface to:
* Create and manipulate windows that can display images and "remember" their content (no need to handle repaint events from OS).
* Add trackbars to the windows, handle simple mouse events as well as keyboard commands.
.. toctree::
:maxdepth: 2
user_interface
qt_new_functions

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Qt New Functions
================
.. highlight:: cpp
.. image:: pics/qtgui.png
This figure explains new functionality implemented with Qt* GUI. The new GUI provides a statusbar, a toolbar, and a control panel. The control panel can have trackbars and buttonbars attached to it. If you cannot see the control panel, press Ctrl+P or right-click any Qt window and select **Display properties window**.
*
To attach a trackbar, the window name parameter must be NULL.
*
To attach a buttonbar, a button must be created.
If the last bar attached to the control panel is a buttonbar, the new button is added to the right of the last button.
If the last bar attached to the control panel is a trackbar, or the control panel is empty, a new buttonbar is created. Then, a new button is attached to it.
See below the example used to generate the figure: ::
int main(int argc, char *argv[])
int value = 50;
int value2 = 0;
cvNamedWindow("main1",CV_WINDOW_NORMAL);
cvNamedWindow("main2",CV_WINDOW_AUTOSIZE | CV_GUI_NORMAL);
cvCreateTrackbar( "track1", "main1", &value, 255, NULL);//OK tested
char* nameb1 = "button1";
char* nameb2 = "button2";
cvCreateButton(nameb1,callbackButton,nameb1,CV_CHECKBOX,1);
cvCreateButton(nameb2,callbackButton,nameb2,CV_CHECKBOX,0);
cvCreateTrackbar( "track2", NULL, &value2, 255, NULL);
cvCreateButton("button5",callbackButton1,NULL,CV_RADIOBOX,0);
cvCreateButton("button6",callbackButton2,NULL,CV_RADIOBOX,1);
cvSetMouseCallback( "main2",on_mouse,NULL );
IplImage* img1 = cvLoadImage("files/flower.jpg");
IplImage* img2 = cvCreateImage(cvGetSize(img1),8,3);
CvCapture* video = cvCaptureFromFile("files/hockey.avi");
IplImage* img3 = cvCreateImage(cvGetSize(cvQueryFrame(video)),8,3);
while(cvWaitKey(33) != 27)
{
cvAddS(img1,cvScalarAll(value),img2);
cvAddS(cvQueryFrame(video),cvScalarAll(value2),img3);
cvShowImage("main1",img2);
cvShowImage("main2",img3);
}
cvDestroyAllWindows();
cvReleaseImage(&img1);
cvReleaseImage(&img2);
cvReleaseImage(&img3);
cvReleaseCapture(&video);
return 0;
}
setWindowProperty
---------------------
Changes parameters of a window dynamically.
.. ocv:function:: void setWindowProperty( const String& winname, int prop_id, double prop_value )
.. ocv:pyfunction:: cv2.setWindowProperty(winname, prop_id, prop_value) -> None
.. ocv:cfunction:: void cvSetWindowProperty( const char* name, int prop_id, double prop_value )
:param name: Name of the window.
:param prop_id: Window property to edit. The following operation flags are available:
* **CV_WND_PROP_FULLSCREEN** Change if the window is fullscreen ( ``CV_WINDOW_NORMAL`` or ``CV_WINDOW_FULLSCREEN`` ).
* **CV_WND_PROP_AUTOSIZE** Change if the window is resizable (``CV_WINDOW_NORMAL`` or ``CV_WINDOW_AUTOSIZE`` ).
* **CV_WND_PROP_ASPECTRATIO** Change if the aspect ratio of the image is preserved ( ``CV_WINDOW_FREERATIO`` or ``CV_WINDOW_KEEPRATIO`` ).
:param prop_value: New value of the window property. The following operation flags are available:
* **CV_WINDOW_NORMAL** Change the window to normal size or make the window resizable.
* **CV_WINDOW_AUTOSIZE** Constrain the size by the displayed image. The window is not resizable.
* **CV_WINDOW_FULLSCREEN** Change the window to fullscreen.
* **CV_WINDOW_FREERATIO** Make the window resizable without any ratio constraints.
* **CV_WINDOW_KEEPRATIO** Make the window resizable, but preserve the proportions of the displayed image.
The function ``setWindowProperty`` enables changing properties of a window.
getWindowProperty
---------------------
Provides parameters of a window.
.. ocv:function:: double getWindowProperty( const String& winname, int prop_id )
.. ocv:pyfunction:: cv2.getWindowProperty(winname, prop_id) -> retval
.. ocv:cfunction:: double cvGetWindowProperty( const char* name, int prop_id )
:param name: Name of the window.
:param prop_id: Window property to retrieve. The following operation flags are available:
* **CV_WND_PROP_FULLSCREEN** Change if the window is fullscreen ( ``CV_WINDOW_NORMAL`` or ``CV_WINDOW_FULLSCREEN`` ).
* **CV_WND_PROP_AUTOSIZE** Change if the window is resizable (``CV_WINDOW_NORMAL`` or ``CV_WINDOW_AUTOSIZE`` ).
* **CV_WND_PROP_ASPECTRATIO** Change if the aspect ratio of the image is preserved (``CV_WINDOW_FREERATIO`` or ``CV_WINDOW_KEEPRATIO`` ).
See
:ocv:func:`setWindowProperty` to know the meaning of the returned values.
The function ``getWindowProperty`` returns properties of a window.
fontQt
----------
Creates the font to draw a text on an image.
.. ocv:function:: QtFont fontQt( const String& nameFont, int pointSize=-1, Scalar color=Scalar::all(0), int weight=QT_FONT_NORMAL, int style=QT_STYLE_NORMAL, int spacing=0 )
.. ocv:cfunction:: CvFont cvFontQt(const char* nameFont, int pointSize=-1, CvScalar color=cvScalarAll(0), int weight=CV_FONT_NORMAL, int style=CV_STYLE_NORMAL, int spacing=0)
:param nameFont: Name of the font. The name should match the name of a system font (such as *Times*). If the font is not found, a default one is used.
:param pointSize: Size of the font. If not specified, equal zero or negative, the point size of the font is set to a system-dependent default value. Generally, this is 12 points.
:param color: Color of the font in BGRA where A = 255 is fully transparent. Use the macro ``CV _ RGB`` for simplicity.
:param weight: Font weight. The following operation flags are available:
* **CV_FONT_LIGHT** Weight of 25
* **CV_FONT_NORMAL** Weight of 50
* **CV_FONT_DEMIBOLD** Weight of 63
* **CV_FONT_BOLD** Weight of 75
* **CV_FONT_BLACK** Weight of 87
You can also specify a positive integer for better control.
:param style: Font style. The following operation flags are available:
* **CV_STYLE_NORMAL** Normal font
* **CV_STYLE_ITALIC** Italic font
* **CV_STYLE_OBLIQUE** Oblique font
:param spacing: Spacing between characters. It can be negative or positive.
The function ``fontQt`` creates a ``CvFont`` object. This ``CvFont`` is not compatible with ``putText`` .
A basic usage of this function is the following: ::
CvFont font = fontQt(''Times'');
addText( img1, ``Hello World !'', Point(50,50), font);
addText
-----------
Creates the font to draw a text on an image.
.. ocv:function:: void addText( const Mat& img, const String& text, Point org, const QtFont& font )
.. ocv:cfunction:: void cvAddText( const CvArr* img, const char* text, CvPoint org, CvFont * arg2 )
:param img: 8-bit 3-channel image where the text should be drawn.
:param text: Text to write on an image.
:param org: Point(x,y) where the text should start on an image.
:param font: Font to use to draw a text.
The function ``addText`` draws
*text*
on an image
*img*
using a specific font
*font*
(see example :ocv:func:`fontQt` )
.. index:: displayOverlay
displayOverlay
------------------
Displays a text on a window image as an overlay for a specified duration.
.. ocv:function:: void displayOverlay( const String& winname, const String& text, int delayms=0 )
.. ocv:cfunction:: void cvDisplayOverlay(const char* name, const char* text, int delayms = 0)
:param name: Name of the window.
:param text: Overlay text to write on a window image.
:param delayms: The period (in milliseconds), during which the overlay text is displayed. If this function is called before the previous overlay text timed out, the timer is restarted and the text is updated. If this value is zero, the text never disappears.
The function ``displayOverlay`` displays useful information/tips on top of the window for a certain amount of time *delayms*. The function does not modify the image, displayed in the window, that is, after the specified delay the original content of the window is restored.
displayStatusBar
--------------------
Displays a text on the window statusbar during the specified period of time.
.. ocv:function:: void displayStatusBar( const String& winname, const String& text, int delayms=0 )
.. ocv:cfunction:: void cvDisplayStatusBar(const char* name, const char* text, int delayms = 0)
:param name: Name of the window.
:param text: Text to write on the window statusbar.
:param delayms: Duration (in milliseconds) to display the text. If this function is called before the previous text timed out, the timer is restarted and the text is updated. If this value is zero, the text never disappears.
The function ``displayOverlay`` displays useful information/tips on top of the window for a certain amount of time
*delayms*
. This information is displayed on the window statusbar (the window must be created with the ``CV_GUI_EXPANDED`` flags).
setOpenGlDrawCallback
------------------------
Sets a callback function to be called to draw on top of displayed image.
.. ocv:function:: void setOpenGlDrawCallback( const String& winname, OpenGlDrawCallback onOpenGlDraw, void* userdata=0 )
.. ocv:cfunction:: void cvSetOpenGlDrawCallback( const char* window_name, CvOpenGlDrawCallback callback, void* userdata=NULL )
:param window_name: Name of the window.
:param onOpenGlDraw: Pointer to the function to be called every frame. This function should be prototyped as ``void Foo(void*)`` .
:param userdata: Pointer passed to the callback function. *(Optional)*
The function ``setOpenGlDrawCallback`` can be used to draw 3D data on the window. See the example of callback function below: ::
void on_opengl(void* param)
{
glLoadIdentity();
glTranslated(0.0, 0.0, -1.0);
glRotatef( 55, 1, 0, 0 );
glRotatef( 45, 0, 1, 0 );
glRotatef( 0, 0, 0, 1 );
static const int coords[6][4][3] = {
{ { +1, -1, -1 }, { -1, -1, -1 }, { -1, +1, -1 }, { +1, +1, -1 } },
{ { +1, +1, -1 }, { -1, +1, -1 }, { -1, +1, +1 }, { +1, +1, +1 } },
{ { +1, -1, +1 }, { +1, -1, -1 }, { +1, +1, -1 }, { +1, +1, +1 } },
{ { -1, -1, -1 }, { -1, -1, +1 }, { -1, +1, +1 }, { -1, +1, -1 } },
{ { +1, -1, +1 }, { -1, -1, +1 }, { -1, -1, -1 }, { +1, -1, -1 } },
{ { -1, -1, +1 }, { +1, -1, +1 }, { +1, +1, +1 }, { -1, +1, +1 } }
};
for (int i = 0; i < 6; ++i) {
glColor3ub( i*20, 100+i*10, i*42 );
glBegin(GL_QUADS);
for (int j = 0; j < 4; ++j) {
glVertex3d(0.2 * coords[i][j][0], 0.2 * coords[i][j][1], 0.2 * coords[i][j][2]);
}
glEnd();
}
}
saveWindowParameters
------------------------
Saves parameters of the specified window.
.. ocv:function:: void saveWindowParameters( const String& windowName )
.. ocv:cfunction:: void cvSaveWindowParameters(const char* name)
:param name: Name of the window.
The function ``saveWindowParameters`` saves size, location, flags, trackbars value, zoom and panning location of the window
``window_name`` .
loadWindowParameters
------------------------
Loads parameters of the specified window.
.. ocv:function:: void loadWindowParameters( const String& windowName )
.. ocv:cfunction:: void cvLoadWindowParameters(const char* name)
:param name: Name of the window.
The function ``loadWindowParameters`` loads size, location, flags, trackbars value, zoom and panning location of the window
``window_name`` .
createButton
----------------
Attaches a button to the control panel.
.. ocv:function:: int createButton( const String& bar_name, ButtonCallback on_change, void* userdata=0, int type=QT_PUSH_BUTTON, bool initial_button_state=false )
.. ocv:cfunction:: int cvCreateButton( const char* button_name=NULL, CvButtonCallback on_change=NULL, void* userdata=NULL, int button_type=CV_PUSH_BUTTON, int initial_button_state=0 )
:param button_name: Name of the button.
:param on_change: Pointer to the function to be called every time the button changes its state. This function should be prototyped as ``void Foo(int state,*void);`` . *state* is the current state of the button. It could be -1 for a push button, 0 or 1 for a check/radio box button.
:param userdata: Pointer passed to the callback function.
:param button_type: Optional type of the button.
* **CV_PUSH_BUTTON** Push button
* **CV_CHECKBOX** Checkbox button
* **CV_RADIOBOX** Radiobox button. The radiobox on the same buttonbar (same line) are exclusive, that is only one can be selected at a time.
:param initial_button_state: Default state of the button. Use for checkbox and radiobox. Its value could be 0 or 1. *(Optional)*
The function ``createButton`` attaches a button to the control panel. Each button is added to a buttonbar to the right of the last button.
A new buttonbar is created if nothing was attached to the control panel before, or if the last element attached to the control panel was a trackbar.
See below various examples of the ``createButton`` function call: ::
createButton(NULL,callbackButton);//create a push button "button 0", that will call callbackButton.
createButton("button2",callbackButton,NULL,CV_CHECKBOX,0);
createButton("button3",callbackButton,&value);
createButton("button5",callbackButton1,NULL,CV_RADIOBOX);
createButton("button6",callbackButton2,NULL,CV_PUSH_BUTTON,1);
..

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User Interface
==============
.. highlight:: cpp
createTrackbar
------------------
Creates a trackbar and attaches it to the specified window.
.. ocv:function:: int createTrackbar( const String& trackbarname, const String& winname, int* value, int count, TrackbarCallback onChange=0, void* userdata=0)
.. ocv:pyfunction:: cv2.createTrackbar(trackbarName, windowName, value, count, onChange) -> None
.. ocv:cfunction:: int cvCreateTrackbar( const char* trackbar_name, const char* window_name, int* value, int count, CvTrackbarCallback on_change=NULL )
:param trackbarname: Name of the created trackbar.
:param winname: Name of the window that will be used as a parent of the created trackbar.
: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 count: Maximal position of the slider. The minimal position is always 0.
: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 slider or range control) with the specified name and range, assigns a variable ``value`` to be a position synchronized with the trackbar and specifies the callback function ``onChange`` to be called on the trackbar position change. The created trackbar is displayed in the specified window ``winname``.
.. note::
**[Qt Backend Only]** ``winname`` can be empty (or NULL) if the trackbar should be attached to the control panel.
Clicking the label of each trackbar enables editing the trackbar values manually.
.. note::
* An example of using the trackbar functionality can be found at opencv_source_code/samples/cpp/connected_components.cpp
getTrackbarPos
------------------
Returns the trackbar position.
.. ocv:function:: int getTrackbarPos( const String& trackbarname, const String& winname )
.. ocv:pyfunction:: cv2.getTrackbarPos(trackbarname, winname) -> retval
.. ocv:cfunction:: int cvGetTrackbarPos( const char* trackbar_name, const char* window_name )
:param trackbarname: Name of the trackbar.
:param winname: Name of the window that is the parent of the trackbar.
The function returns the current position of the specified trackbar.
.. note::
**[Qt Backend Only]** ``winname`` can be empty (or NULL) if the trackbar is attached to the control panel.
imshow
----------
Displays an image in the specified window.
.. ocv:function:: void imshow( const String& winname, InputArray mat )
.. ocv:pyfunction:: cv2.imshow(winname, mat) -> None
.. ocv:cfunction:: void cvShowImage( const char* name, const CvArr* image )
:param winname: Name of the window.
:param image: Image to be shown.
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:
* If the image is 8-bit unsigned, it is displayed as is.
* If the image is 16-bit unsigned or 32-bit integer, the pixels are divided by 256. That is, the value range [0,255*256] is mapped to [0,255].
* If the image is 32-bit floating-point, the pixel values are multiplied by 255. That is, the value range [0,1] is mapped to [0,255].
If window was created with OpenGL support, ``imshow`` also support :ocv:class:`ogl::Buffer` , :ocv:class:`ogl::Texture2D` and :ocv:class:`cuda::GpuMat` as input.
.. note:: This function should be followed by ``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::
[Windows Backend Only] Pressing Ctrl+C will copy the image to the clipboard.
namedWindow
---------------
Creates a window.
.. ocv:function:: void namedWindow( const String& winname, int flags=WINDOW_AUTOSIZE )
.. ocv:pyfunction:: cv2.namedWindow(winname[, flags]) -> None
.. ocv:cfunction:: int cvNamedWindow( const char* name, int flags=CV_WINDOW_AUTOSIZE )
:param name: Name of the window in the window caption that may be used as a window identifier.
:param flags: Flags of the window. The supported flags are:
* **WINDOW_NORMAL** If this is set, the user can resize the window (no constraint).
* **WINDOW_AUTOSIZE** If this is set, the window size is automatically adjusted to fit the displayed image (see :ocv:func:`imshow` ), and you cannot change the window size manually.
* **WINDOW_OPENGL** If this is set, the window will be created with OpenGL support.
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.
If a window with the same name already exists, the function does nothing.
You can call :ocv:func:`destroyWindow` or :ocv: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.
.. note::
Qt backend supports additional flags:
* **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 :ocv:func:`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.
By default, ``flags == CV_WINDOW_AUTOSIZE | CV_WINDOW_KEEPRATIO | CV_GUI_EXPANDED``
destroyWindow
-------------
Destroys a window.
.. ocv:function:: void destroyWindow( const String& winname )
.. ocv:pyfunction:: cv2.destroyWindow(winname) -> None
.. ocv:cfunction:: void cvDestroyWindow( const char* name )
:param winname: Name of the window to be destroyed.
The function ``destroyWindow`` destroys the window with the given name.
destroyAllWindows
-----------------
Destroys all of the HighGUI windows.
.. ocv:function:: void destroyAllWindows()
.. ocv:pyfunction:: cv2.destroyAllWindows() -> None
.. ocv:cfunction:: void cvDestroyAllWindows()
The function ``destroyAllWindows`` destroys all of the opened HighGUI windows.
MoveWindow
----------
Moves window to the specified position
.. ocv:function:: void moveWindow( const String& winname, int x, int y )
.. ocv:pyfunction:: cv2.moveWindow(winname, x, y) -> None
.. ocv:cfunction:: void cvMoveWindow( const char* name, int x, int y )
:param winname: Window name
:param x: The new x-coordinate of the window
:param y: The new y-coordinate of the window
ResizeWindow
------------
Resizes window to the specified size
.. ocv:function:: void resizeWindow( const String& winname, int width, int height )
.. ocv:pyfunction:: cv2.resizeWindow(winname, width, height) -> None
.. ocv:cfunction:: void cvResizeWindow( const char* name, int width, int height )
:param winname: Window name
:param width: The new window width
:param height: The new window height
.. note::
* The specified window size is for the image area. Toolbars are not counted.
* Only windows created without CV_WINDOW_AUTOSIZE flag can be resized.
SetMouseCallback
----------------
Sets mouse handler for the specified window
.. ocv:function:: void setMouseCallback( const String& winname, MouseCallback onMouse, void* userdata=0 )
.. ocv:pyfunction:: cv2.setMouseCallback(windowName, onMouse [, param]) -> None
.. ocv:cfunction:: void cvSetMouseCallback( const char* window_name, CvMouseCallback on_mouse, void* param=NULL )
:param winname: Window name
:param onMouse: Mouse callback. See OpenCV samples, such as https://github.com/Itseez/opencv/tree/master/samples/cpp/ffilldemo.cpp, on how to specify and use the callback.
:param userdata: The optional parameter passed to the callback.
getMouseWheelDelta
------------------
Gets the mouse-wheel motion delta, when handling mouse-wheel events EVENT_MOUSEWHEEL and EVENT_MOUSEHWHEEL.
.. ocv:function:: int getMouseWheelDelta(int flags)
:param flags: The mouse callback flags parameter.
For regular mice with a scroll-wheel, delta will be a multiple of 120. The value 120 corresponds to a one notch rotation of the wheel or the threshold for action to be taken and one such action should occur for each delta.
Some high-precision mice with higher-resolution freely-rotating wheels may generate smaller values.
For EVENT_MOUSEWHEEL positive and negative values mean forward and backward scrolling, respectively.
For EVENT_MOUSEHWHEEL, where available, positive and negative values mean right and left scrolling, respectively.
With the C API, the macro CV_GET_WHEEL_DELTA(flags) can be used alternatively.
.. note::
Mouse-wheel events are currently supported only on Windows.
setTrackbarPos
------------------
Sets the trackbar position.
.. ocv:function:: void setTrackbarPos( const String& trackbarname, const String& winname, int pos )
.. ocv:pyfunction:: cv2.setTrackbarPos(trackbarname, winname, pos) -> None
.. ocv:cfunction:: void cvSetTrackbarPos( const char* trackbar_name, const char* window_name, int pos )
:param trackbarname: Name of the trackbar.
:param winname: Name of the window that is the parent of trackbar.
:param pos: New position.
The function sets the position of the specified trackbar in the specified window.
.. note::
**[Qt Backend Only]** ``winname`` can be empty (or NULL) if the trackbar is attached to the control panel.
waitKey
-----------
Waits for a pressed key.
.. ocv:function:: int waitKey(int delay=0)
.. ocv:pyfunction:: cv2.waitKey([delay]) -> retval
.. ocv:cfunction:: int cvWaitKey( int delay=0 )
:param delay: Delay in milliseconds. 0 is the special value that means "forever".
The function ``waitKey`` waits for a key event infinitely (when
: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.
.. 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.
setOpenGlDrawCallback
---------------------
Set OpenGL render handler for the specified window.
.. ocv:function:: void setOpenGlDrawCallback(const string& winname, OpenGlDrawCallback onOpenGlDraw, void* userdata = 0)
:param winname: Window name
:param onOpenGlDraw: Draw callback.
:param userdata: The optional parameter passed to the callback.
setOpenGlContext
----------------
Sets the specified window as current OpenGL context.
.. ocv:function:: void setOpenGlContext(const String& winname)
:param winname: Window name
updateWindow
------------
Force window to redraw its context and call draw callback ( :ocv:func:`setOpenGlDrawCallback` ).
.. ocv:function:: void updateWindow(const String& winname)
:param winname: Window name