Opencv Create Trackbar

Question

When I run this code :

def nothing(x):
    pass

cv.createTrackbar(‘R’, ‘image’, 0,255,nothing)

I get this error: Using ‘value’ pointer is unsafe and deprecated. Use NULL as value pointer. To fetch trackbar value setup callback.

I couldn’t find what to do.

Answer 1

You didn't show minimal working code which makes problem so I can only guess.

You have to create window "image" before you use createTrackbar()

cv2.namedWindow('image')
cv2.createTrackbar('R', 'image', 0, 255, function)

---

Minimal working example:

import cv2

# --- functions ---

def function(value):
    print(value)

    new_img = img.copy()
    new_img[:,:,2] = value

    cv2.imshow('image', new_img)

# --- main ---
    
img = cv2.imread('lenna.png')

cv2.namedWindow('image')
cv2.createTrackbar('R', 'image', 0, 255, function)

cv2.imshow('image', img)
cv2.waitKey(0)

cv2.destroyAllWindows()

---

EDIT:

More complex example with three trackbars which use the same function but with different index.

import cv2

# --- functions ---

def function(index, value):
    percent = (value/100)
    #print(index, value, percent)

    img[:,:,index] = original_img[:,:,index] * percent

    cv2.imshow('image', img)
   
# --- main ---
 
img = cv2.imread('lenna.png')

original_img = img.copy()

cv2.namedWindow('image')

cv2.createTrackbar('R (%)', 'image', 100, 100, lambda value:function(2, value))
cv2.createTrackbar('G (%)', 'image', 100, 100, lambda value:function(1, value))
cv2.createTrackbar('B (%)', 'image', 100, 100, lambda value:function(0, value))

cv2.imshow('image', img)
cv2.waitKey(0)

cv2.destroyAllWindows()

---

Image Lenna from Wikipedia

---

EDIT:

Your problem is that you use two names for window - img and image - but you should use the same name in namedWindow(), createTrackbar(), getTrackbarPos(), imshow()

BTW: If you create trackbar with '0 : OFF \n1 : ON' then you have to use it aslo to get value s = cv.getTrackbarPos(switch, 'image')

EDIT:

It seems you have the same code as in demo in documentation and demo uses "image" in all commands. demo also uses cv.getTrackbarPos(switch, ...)

import cv2 as cv
import numpy as np

def nothing(x): 
    pass

img = np.array ((512,512,3), np.uint8) 
cv.namedWindow("image") 

cv.createTrackbar('R', 'image', 0, 255, nothing)
cv.createTrackbar('G', 'image', 0, 255, nothing)
cv.createTrackbar('B', 'image', 0, 255, nothing)

switch = '0 : OFF \n1 : ON'
cv.createTrackbar(switch, 'image', 1, 1, nothing)

while True:
    cv.imshow('image', img)
    
    key = cv.waitKey(1) & 0xFF
    
    if key == 27:
        break
    
    r = cv.getTrackbarPos('R', 'image')
    g = cv.getTrackbarPos('G', 'image')
    b = cv.getTrackbarPos('B', 'image')
    s = cv.getTrackbarPos(switch, 'image')
    
    if s == 0:
        img[:] = 0 
        # reset trackbars
        #cv.setTrackbarPos('R', 'image', 0)
        #cv.setTrackbarPos('G', 'image', 0)
        #cv.setTrackbarPos('B', 'image', 0)
    else:
        img[:] = [b,g,r]
        
cv.destroyAllWindows()

The same with code in nothing so it is executed only when you change value in any trackbar.

import cv2 as cv
import numpy as np

# --- functions ---

def nothing(value): 
    r = cv.getTrackbarPos('R', 'image')
    g = cv.getTrackbarPos('G', 'image')
    b = cv.getTrackbarPos('B', 'image')
    s = cv.getTrackbarPos(switch, 'image')

    if s == 0:
        img[:] = 0
        # reset trackbars
        #cv.setTrackbarPos('R', 'image', 0)
        #cv.setTrackbarPos('G', 'image', 0)
        #cv.setTrackbarPos('B', 'image', 0)
    else:
        img[:] = [b,g,r]
        
# --- main ---

img = np.array ((512,512,3), np.uint8)

cv.namedWindow("image") 

cv.createTrackbar('R', 'image', 0, 255, nothing)
cv.createTrackbar('G', 'image', 0, 255, nothing)
cv.createTrackbar('B', 'image', 0, 255, nothing)

switch = '0 : OFF \n1 : ON'
cv.createTrackbar(switch, 'image', 1, 1, nothing)

while True:
    cv.imshow('image', img)
    
    key = cv.waitKey(1) & 0xFF
    
    if key == 27:
        break
    
cv.destroyAllWindows()