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 import numpy as np
import cv2

cap = cv2.VideoCapture(0)

while(True):
    # Capture frame-by-frame
    ret, frame = cap.read()

    # Our operations on the frame come here
    gray = cv2.cvtColor('frame', cv2.COLOR_BGR2GRAY)

    # Display the resulting frame
    cv2.imshow('frame',frame)
    cv2.imshow('gray',gray)
    if cv2.waitKey(20) amp; 0xFF == ord('q'):
        break

# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()`

Traceback (most recent call last):
  File "C:UsersKouissiDesktopcamera-testcamera_test.py", line 11, in <module>
    gray = cv2.cvtColor('frame', cv2.COLOR_BGR2GRAY)
TypeError: Expected cv::UMat for argument 'src'
  

Вам нужно изменить:

 gray = cv2.cvtColor('frame', cv2.COLOR_BGR2GRAY)
  

Для:

 gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
  

В данном случае переменная frame представляет собой массив numpy.