如何使用opencv在视频上绘制拖尾线

import cv2
import numpy as np

video = cv2.VideoCapture('video.mp4')

canvas = np.zeros((480, 640, 3), dtype=np.uint8)

line_color = (0, 255, 0)  # 绿色
line_thickness = 2

while True:
    ret, frame = video.read()
    if not ret:
        break

    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    gray = cv2.GaussianBlur(gray, (21, 21), 0)

    if previous_frame is None:
        previous_frame = gray
        continue

    frame_delta = cv2.absdiff(previous_frame, gray)
    thresh = cv2.threshold(frame_delta, 30, 255, cv2.THRESH_BINARY)[1]
    thresh = cv2.dilate(thresh, None, iterations=2)

    contours, _ = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

    for contour in contours:
        if cv2.contourArea(contour) < 500:
            continue

        (x, y, w, h) = cv2.boundingRect(contour)
        cv2.rectangle(frame, (x, y), (x + w, y + h), (0, 255, 0), 2)

    canvas = cv2.addWeighted(canvas, 0.9, 0)
    canvas = cv2.line(canvas, (x, y), (x + w, y + h), line_color, line_thickness)

    output = cv2.add(frame, canvas)
    cv2.imshow("Video", output)

    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

video.release()
cv2.destroyAllWindows()