Creates a trackbar and attaches it to the specified window.

Parameters

trackbarname Name of the created trackbar.

winname Name of the window that will be used as a parent of the created trackbar.

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.

count Maximal position of the slider. The minimal position is always 0.

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.

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

Examples:

contours2.cpp, demhist.cpp, distrans.cpp, edge.cpp, ffilldemo.cpp, fitellipse.cpp, laplace.cpp, morphology2.cpp, and pca.cpp.

Destroys all of the HighGUI windows.

The function destroyAllWindows destroys all of the opened HighGUI windows.

Destroys a window.

Parameters

winname Name of the window to be destroyed.

The function destroyWindow destroys the window with the given name.

Examples:

ffilldemo.cpp, and grabcut.cpp.

Gets the mouse-wheel motion delta, when handling mouse-wheel events EVENT_MOUSEWHEEL and EVENT_MOUSEHWHEEL.

Parameters

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.

Returns the trackbar position.

Parameters

trackbarname Name of the trackbar.

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.

Provides parameters of a window.

Parameters

winname Name of the window.

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 setWindowProperty to know the meaning of the returned values.

The function getWindowProperty returns properties of a window.

Displays an image in the specified window.

Parameters

winname Name of the window.

mat 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, 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 window was created with OpenGL support, imshow also support ogl::Buffer , ogl::Texture2D and cuda::GpuMat as input.

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.

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.

Examples:

contours2.cpp, convexhull.cpp, demhist.cpp, distrans.cpp, edge.cpp, ffilldemo.cpp, fitellipse.cpp, grabcut.cpp, houghcircles.cpp, houghlines.cpp, kmeans.cpp, laplace.cpp, lsd_lines.cpp, minarea.cpp, morphology2.cpp, pca.cpp, segment_objects.cpp, and watershed.cpp.

Moves window to the specified position.

Parameters

winname Window name

x The new x-coordinate of the window

y The new y-coordinate of the window

Creates a window.

Parameters

winname Name of the window in the window caption that may be used as a window identifier.

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 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 destroyWindow or 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 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

Examples:

contours2.cpp, demhist.cpp, distrans.cpp, edge.cpp, ffilldemo.cpp, fitellipse.cpp, grabcut.cpp, laplace.cpp, morphology2.cpp, pca.cpp, segment_objects.cpp, and watershed.cpp.

Resizes window to the specified size.

Parameters

winname Window name

width The new window width

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.

Sets mouse handler for the specified window.

Parameters

winname Window name

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.

userdata The optional parameter passed to the callback.

Examples:

ffilldemo.cpp, grabcut.cpp, and watershed.cpp.

Sets the trackbar maximum position.

Parameters

trackbarname Name of the trackbar.

winname Name of the window that is the parent of trackbar.

maxval New maximum position.

The function sets the maximum 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.

Sets the trackbar position.

Parameters

trackbarname Name of the trackbar.

winname Name of the window that is the parent of trackbar.

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.

Changes parameters of a window dynamically.

Parameters

winname Name of the window.

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 ).

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.

Updates window title.

Waits for a pressed key.

Parameters

delay Delay in milliseconds. 0 is the special value that means "forever".

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.

Examples:

contours2.cpp, convexhull.cpp, demhist.cpp, distrans.cpp, edge.cpp, ffilldemo.cpp, fitellipse.cpp, grabcut.cpp, houghcircles.cpp, houghlines.cpp, kmeans.cpp, laplace.cpp, lsd_lines.cpp, minarea.cpp, morphology2.cpp, pca.cpp, segment_objects.cpp, and watershed.cpp.