Black screen in many codes

Python
1

Hello. I’ve weird problem with many codes, for example, I wrote simple program to bitwise, and program to show figures, trackbar and i had dark screen like this in photo. I reinstalled opencv and numpy, but they didn’t helped. I also tried python reinstall.
I’ll be very thankful for any help.
Here is a code for trackbar program:

import cv2
import numpy as np

def showcolour(x):
        print(x)

img = np.zeros((300,512,3), np.uint8)
cv2.namedWindow('img')

cv2.createTrackbar('b', 'blue', 0, 255, showcolour)
cv2.createTrackbar('g', 'green', 0, 255, showcolour)
cv2.createTrackbar('r', 'red', 0, 255, showcolour)

while(1):
        cv2.imshow('image', img)
        k = cv2.waitKey(1) & 0xFF
        if k == 27:
                break
b = cv2.getTrackbarPos('b', 'blue')
g = cv2.getTrackbarPos('g', 'green')
r = cv2.getTrackbarPos('r', 'red')
img[:] = [b,g,r]

cv2.imshow('img', img)
cv2.destroyAllWindows()

2021-07-07-122310_1920x1080_scrot
2021-07-07-122310_1920x1080_scrot1094×413 230 KB

2

but it does not do anything (apart from printing numbers)

this should go inside the loop !
(and your final imshow() is useless without a waitKey(0))

problem is your code, not the install

1 Like
3

Thank you very much. I added code to loop, wrote waitKey, make variable with window name, and used it, and code worked. I think I’ll should learn from documentation, not from other sources.

1 Like
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