Pi 4 Camera with Picamera2 and OpenCV: Motion-Triggered Recording

#!/usr/bin/env python3
# motion_cam.py - motion-triggered snapshots on a Raspberry Pi 4
import os
import time
from datetime import datetime

import cv2
from picamera2 import Picamera2

MIN_AREA = 1500          # pixels of change needed to count as motion
COOLDOWN = 5             # seconds between saved snapshots
SAVE_DIR = os.path.expanduser('~/motion')
os.makedirs(SAVE_DIR, exist_ok=True)

# Configure a 640x480 stream. In Picamera2, 'RGB888' gives pixels in
# B, G, R order in memory - exactly what OpenCV expects.
picam2 = Picamera2()
config = picam2.create_video_configuration(
    main={'size': (640, 480), 'format': 'RGB888'})
picam2.configure(config)
picam2.start()
time.sleep(2)            # let auto-exposure and white balance settle

background = None        # running average of the scene (float image)
last_saved = 0.0

try:
    while True:
        frame = picam2.capture_array()          # numpy array, 480x640x3
        gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
        gray = cv2.GaussianBlur(gray, (21, 21), 0)   # remove sensor noise

        if background is None:
            background = gray.astype('float')
            continue

        # Update the background slowly so lighting changes are absorbed
        cv2.accumulateWeighted(gray, background, 0.05)
        diff = cv2.absdiff(gray, cv2.convertScaleAbs(background))

        # Pixels that changed by more than 25 grey levels become white
        _, mask = cv2.threshold(diff, 25, 255, cv2.THRESH_BINARY)
        mask = cv2.dilate(mask, None, iterations=2)  # join nearby blobs

        contours, _ = cv2.findContours(mask, cv2.RETR_EXTERNAL,
                                       cv2.CHAIN_APPROX_SIMPLE)
        moving = [c for c in contours if cv2.contourArea(c) > MIN_AREA]

        for c in moving:                         # draw boxes on the frame
            x, y, w, h = cv2.boundingRect(c)
            cv2.rectangle(frame, (x, y), (x + w, y + h), (0, 0, 255), 2)

        now = time.time()
        if moving and now - last_saved > COOLDOWN:
            stamp = datetime.now().strftime('%Y%m%d_%H%M%S')
            path = os.path.join(SAVE_DIR, f'motion_{stamp}.jpg')
            cv2.putText(frame, stamp, (10, 25), cv2.FONT_HERSHEY_SIMPLEX,
                        0.7, (255, 255, 255), 2)
            cv2.imwrite(path, frame)
            print('Motion detected, saved', path)
            last_saved = now
except KeyboardInterrupt:
    print('Stopped')
finally:
    picam2.stop()