Trackbar in OpenCV is a slider control which helps in picking a value for the variable from a continuous range by manually sliding the tab over the bar. Position of the tab is synchronised with a value.

The createTrackbar() function creates a Trackbar object with the following command −

cv2.createTrackbar(trackbarname, winname, value, count, TrackbarCallback)

In the following example, three trackbars are provided for the user to set values of R, G and B from the grayscale range 0 to 255.

Using the track bar position values, a rectangle is drawn with the fill colour corresponding to RGB colour value.

Example

Following program is for adding a trackbar −

import numpy as np
import cv2 as cv
img = np.zeros((300,400,3), np.uint8)
cv.namedWindow(''image'')
def nothing(x):
   pass

# create trackbars for color change
cv.createTrackbar(''R'',''image'',0,255,nothing)
cv.createTrackbar(''G'',''image'',0,255,nothing)
cv.createTrackbar(''B'',''image'',0,255,nothing)

while(1):
   cv.imshow(''image'',img)
   k = cv.waitKey(1) & 0xFF
   if k == 27:
      break
   # get current positions of four trackbars
   r = cv.getTrackbarPos(''R'',''image'')
   g = cv.getTrackbarPos(''G'',''image'')
   b = cv.getTrackbarPos(''B'',''image'')

   #s = cv.getTrackbarPos(switch,''image'')
   #img[:] = [b,g,r]
   cv.rectangle(img, (100,100),(200,200), (b,g,r),-1)
   cv.destroyAllWindows()

Output