Using Videostream causes Flickering

webcam_imutils.py

from imutils.video import VideoStream
import cv2
# using VideoStream

vs = VideoStream(0).start()
# time.sleep(2.0)

lastFrame = None


while True:
    # grab the frame from the threaded video stream and resize it
    frame = vs.read()

    # resize
    # frame = imutils.resize(frame, width=800)
    frame = cv2.resize(frame, (800, 600), fx=0.5, fy=0.5, interpolation=cv2.INTER_LINEAR)
    # greyscale
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # blur
    gray = cv2.GaussianBlur(gray, (21, 21), 0)

    if lastFrame is None:
        lastFrame = gray
        continue

    # diff
    frameDelta = cv2.absdiff(lastFrame, gray)
    lastFrame = gray

    thresh = cv2.threshold(frameDelta, 20, 255, cv2.THRESH_BINARY)[1]

    # show the output frame
    cv2.imshow("thresh", thresh)
    key = cv2.waitKey(1) & 0xFF

    # if the `q` key was pressed, break from the loop
    if key == ord("q"):
        break

cv2.destroyAllWindows()