example2: change colored image to gray

import numpy as np
import cv2 as cv

def nothing(x):
    print(x)

cv.namedWindow('image')

cv.createTrackbar('CP', 'image', 10, 400, nothing)

#adding a switch(on/Off)
switch = 'color/gray'
cv.createTrackbar(switch, 'image', 0, 1, nothing)

while(1):
    img = cv.imread('lena.jpg')
    pos = cv.getTrackbarPos('CP', 'image')
    font = cv.FONT_HERSHEY_SIMPLEX
    cv.putText(img, str(pos), (50, 150), font, 6,(0, 0, 255), 10)

    k=cv.waitKey(1)
    if k==27:
        break

    s = cv.getTrackbarPos(switch, 'image')

    if s==0:
        pass
    else:
        img = cv.cvtColor(img, cv.COLOR_BGR2GRAY)

    img = cv.imshow('image', img)
cv.destroyAllWindows()