createTrackbar

createTrackbar is a function in OpenCV's HighGUI module that adds a trackbar, or slider, to a named window. The slider lets users adjust a parameter in real time, either by updating a bound variable (in C++) or by triggering a callback function when the position changes.

Usage notes: The target window must exist before creating a trackbar; the usual sequence is to create the window with namedWindow and then call createTrackbar. The function takes the trackbar name, the window name, a value or pointer, the maximum value, and an optional callback. The slider range runs from 0 to count-1. In C++, the value argument is a pointer to an int that tracks the current position; in Python, the value is an initial position and the current value should be retrieved with getTrackbarPos.

Examples: In C++: createTrackbar("Brightness", "Image", &brightness, 255, onBrightnessChange); In Python: cv2.createTrackbar("Brightness", "Image", brightness, 255, on_brightness_change). The current position can be read at any time with getTrackbarPos(trackbarname, winname).

Behavior: The function itself does not render output; it creates the UI control in the existing window. If a callback is supplied, it is invoked when the slider moves; otherwise, the current position can be queried as needed. To update the slider programmatically, use setTrackbarPos. Closing the window or destroying all windows removes the trackbars.

Notes: Trackbars are commonly used for interactive parameter tuning, calibration, and demonstrations. They are associated with the window they are created in, and multiple trackbars can be added to a single window to adjust several parameters.