User Interface


int createTrackbar(const string& trackbarname, const string& winname, int* value, int count, TrackbarCallback onChange CV_DEFAULT(0), void* userdata CV_DEFAULT(0))

Creates a trackbar and attaches it to the specified window

  • trackbarname – Name of the created trackbar.
  • winname – Name of the window which will be used as a parent of the created trackbar.
  • value – The optional pointer to an integer variable, whose value will reflect the position of the slider. Upon creation, the slider position is defined by this variable.
  • count – The 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 NULL pointer, then no callbacks is called, but only value is updated
  • userdata – The 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 trackbar position and specifies a callback function onChange to be called on the trackbar position change. The created trackbar is displayed on the top of the given window.


int getTrackbarPos(const string& trackbarname, const string& winname)

Returns the trackbar position.

  • trackbarname – Name of the trackbar.
  • winname – Name of the window which is the parent of the trackbar.

The function returns the current position of the specified trackbar.


void imshow(const string& winname, const Mat& image)

Displays the image in the specified window

  • winname – Name of the window.
  • image – Image to be shown.

The function imshow displays the image in the specified window. If the window was created with the CV_WINDOW_AUTOSIZE flag then the image is shown with its original size, otherwise the image is scaled to fit in 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].


void namedWindow(const string& winname, int flags)

Creates a window.

  • name – Name of the window in the window caption that may be used as a window identifier.
  • 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 imshow ), and the user can not change the window size manually.

The function namedWindow creates a window which 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.


void setTrackbarPos(const string& trackbarname, const string& winname, int pos)

Sets the trackbar position.

  • trackbarname – Name of the trackbar.
  • winname – Name of the window which is the parent of trackbar.
  • pos – The new position.

The function sets the position of the specified trackbar in the specified window.


int waitKey(int delay=0)

Waits for a pressed key.

Parameter:delay – Delay in milliseconds. 0 is the special value that means “forever”

The function waitKey waits for key event infinitely (when \texttt{delay}\leq 0 ) or for delay milliseconds, when it’s positive. 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 some environment that takes care of event processing.

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