Black screen in many codes

Python
4

As @berak already said you have mistakes in code.

Here full working code for all users

import cv2
import numpy as np

# --- functions ---

def showcolour(value):
    print(value)

# --- main ---

# create black image at start
img = np.zeros((300,512,3), np.uint8)  

# create window with name `img`
cv2.namedWindow('img')  

# add trackbars to window `img` - and assign function `showcolour` to trackbars
cv2.createTrackbar('b', 'img', 0, 255, showcolour)
cv2.createTrackbar('g', 'img', 0, 255, showcolour)
cv2.createTrackbar('r', 'img', 0, 255, showcolour)

# run main loop
while True:
    # get values from trackbars in window `img`
    b = cv2.getTrackbarPos('b', 'img')
    g = cv2.getTrackbarPos('g', 'img')
    r = cv2.getTrackbarPos('r', 'img')
    
    # replace all pixels with new value
    img[:] = [b, g, r]

    # display new image in window `img`
    cv2.imshow('img', img)

    # check if pressed key (wait for key 1ms)
    k = cv2.waitKey(1) & 0xFF
    
    # exit when pressed `ESC
    if k == 27:
         break

# destroy window `img`
cv2.destroyWindow('img')

# OR: destroy all windows
#cv2.destroyAllWindows()

Screenshot: Linux Mint 20 (based on Ubuntu 20.04)

Zrzut ekranu z 2021-07-10 00-35-31