Python opens the camera, uses the left mouse button to select a rectangular area, supports selecting multiple rectangular areas at one time, and clears the last drawn rectangle by right-clicking the mouse.

Option One

import cv2

# Global variables
rectangles = []
current_rectangle = []
drawing = False

# Mouse callback function
def mouse_callback(event, x, y, flags, param):
    global rectangles, current_rectangle, drawing

    if event == cv2.EVENT_LBUTTONDOWN:
        drawing = True
        current_rectangle = [(x, y)]

    elif event == cv2.EVENT_LBUTTONUP:
        drawing = False
        current_rectangle.append((x, y))
        rectangles.append(tuple(current_rectangle))
        current_rectangle = []

    elif event == cv2.EVENT_RBUTTONDOWN:
        rectangles.pop()

# Open camera
cap = cv2.VideoCapture(0)

# Create a window and set mouse callback
cv2.namedWindow('Camera')
cv2.setMouseCallback('Camera', mouse_callback)

while True:
    # Read frame from camera
    ret, frame = cap.read()

    # Draw rectangles on the frame
    for rect in rectangles:
        cv2.rectangle(frame, rect[0], rect[1], (0, 255, 0), 2)

    # Display the frame
    cv2.imshow('Camera', frame)

    # Check for key press
    key = cv2.waitKey(1)
    if key == 27:  # Press 'Esc' to exit
        break

# Release the camera and destroy all windows
cap.release()
cv2.destroyAllWindows()

Option II

import cv2

# Global variables
rectangles = []
current_rectangle = []
drawing = False
rect_x = 0
rect_y = 0

# Mouse callback function
def mouse_callback(event, x, y, flags, param):
    global rectangles, current_rectangle, drawing

    if event == cv2.EVENT_LBUTTONDOWN:
        drawing = True
        current_rectangle = [(x, y)]
        global rect_x, rect_y
        rect_x = x
        rect_y = y

    elif event == cv2.EVENT_LBUTTONUP:
        drawing = False
        current_rectangle.append((x, y))
        rectangles.append(tuple(current_rectangle))
        current_rectangle = []


    elif event == cv2.EVENT_RBUTTONDOWN:
        rectangles.pop()

# Open camera
cap = cv2.VideoCapture(0)

# Create a window and set mouse callback
cv2.namedWindow('Camera')
cv2.setMouseCallback('Camera', mouse_callback)

while True:
    # Read frame from camera
    ret, frame = cap.read()

    # Draw rectangles on the frame
    for rect in rectangles:
        cv2.rectangle(frame, rect[0], rect[1], (0, 255, 0), 2)

    # Draw current rectangle in real-time
    if drawing:
        cv2.rectangle(frame, current_rectangle[0], (rect_x, rect_y), (0, 255, 0), 2)

    # Display the frame
    cv2.imshow('Camera', frame)

    # Check for key press
    key = cv2.waitKey(1)
    if key == 27:  # Press 'Esc' to exit
        break

# Release the camera and destroy all windows
cap.release()
cv2.destroyAllWindows()

Next step to modify and optimize

a Displays the crosshairs at the mouse location in the image area in real time

b Display the process of drawing the frame in real time