user_interface.rst



User Interface

createTrackbar
Creates a trackbar and attaches it to the specified window.
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.
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.
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, however it is still limited by the screen resolution. 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 the window was not created before this function, it is assumed creating a window with CV_WINDOW_AUTOSIZE.
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.
If window was created with OpenGL support, imshow also support :ocv:class:`ogl::Buffer` ,  :ocv:class:`ogl::Texture2D` and  :ocv:class:`gpu::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)

[Windows Backend Only] Pressing Ctrl+C will copy the image to the clipboard.

namedWindow
Creates a window.
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.
The function destroyWindow destroys the window with the given name.

destroyAllWindows
Destroys all of the HighGUI windows.
The function destroyAllWindows destroys all of the opened HighGUI windows.

MoveWindow
Moves window to the specified position

ResizeWindow
Resizes window to the specified size

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

setTrackbarPos
Sets the trackbar 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.
The function waitKey waits for a key event infinitely (when
\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.

setOpenGlContext
Sets the specified window as current OpenGL context.

updateWindow
Force window to redraw its context and call draw callback ( :ocv:func:`setOpenGlDrawCallback` ).