OpenCV 3 Computer Vision with Python Cookbook

How to do it...

1. Import the modules:

import cv2import numpy as np

2. Open an image and define the mouse callback function to draw a rectangle on the image:

img = cv2.imread('../data/Lena.png', cv2.IMREAD_COLOR)show_img = np.copy(img)mouse_pressed = Falsey = x = w = h = 0def mouse_callback(event, _x, _y, flags, param):    global show_img, x, y, w, h, mouse_pressed    if event == cv2.EVENT_LBUTTONDOWN:        mouse_pressed = True        x, y = _x, _y        show_img = np.copy(img)    elif event == cv2.EVENT_MOUSEMOVE:        if mouse_pressed:            show_img = np.copy(img)            cv2.rectangle(show_img, (x, y),                          (_x, _y), (0, 255, 0), 3)    elif event == cv2.EVENT_LBUTTONUP:        mouse_pressed = False        w, h = _x - x, _y - y

3. Display the image, and, after the rectangle has been completed and the A button ...