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What is this code do: "gray = cv2.cvtColor(prev_frame, cv2.COLOR_BGR2GRAY)
prev_gray = cv2.GaussianBlur(prev_gray, (19, 19), 0)


# Find keypoints and descriptors in the first frame
prev_keypoints, prev_descriptors = detector.detectAndCompute(prev_gray, None)

# Merge close keypoints in the first frame
prev_keypoints = merge_close_points(prev_keypoints)

# Assign ID to each keypoint in the first frame
for kp in prev_keypoints:
    tracked_points.append((kp, assign_id()))

while cap.isOpened():
    ret, frame = cap.read()
    if not ret:
        break
    
    # Convert frame to grayscale
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    
    # Find keypoints in the current frame
    keypoints = detector.detect(gray, None)
    
    # Merge close keypoints in the current frame
    keypoints = merge_close_points(keypoints)
    
    # Match keypoints between frames
    matches = []
    for kp in keypoints:
        for prev_kp, prev_id in tracked_points:
            if distance(kp.pt, prev_kp.pt) < DIST_THRE
SHOLD:
                matches.append((kp, prev_id))
                break
    
    # Update tracked points with matches and assign IDs to unmatched keypoints
    new_tracked_points = []
    for kp, prev_id in matches:
        new_tracked_points.append((kp, prev_id))
    for kp in keypoints:
        if kp not in [match[0] for match in matches]:
            new_tracked_points.append((kp, assign_id()))
    
    # Limit the number of tracked points to 200
    tracked_points = new_tracked_points[-MAX_POINTS:]
    
    # Draw tracked points and IDs on the frame
    frame_with_points = frame.copy()
    for kp, kp_id in tracked_points:
        # Draw the point
        cv2.circle(frame_with_points, (int(kp.pt[0]), int(kp.pt[1])), 5, (0, 255, 0), -1)
        # Draw the ID number next to the point
        cv2.putText(frame_with_points, str(kp_id), (int(kp.pt[0]) + 10, int(kp.pt[1]) + 10), cv2.FONT_HERSHEY_SIMPLEX, 0.5, (0, 0, 255), 1, cv2.LINE_AA)
    
    # Display the result
    cv2.imshow('Fr
ame', frame_with_points)
    if cv2.waitKey(25) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()"