window_basic.py

import cv2
import numpy as np

# Step 1. Define callback function
drawing = False
ix = -1
iy = -1

def draw_rectangle(event, x, y, flags, params):

    global ix, iy, drawing

    if event == cv2.EVENT_LBUTTONDOWN:
        drawing = True
        ix, iy = x, y
        
    elif event == cv2.EVENT_MOUSEMOVE:
        if drawing == True:
            cv2.rectangle(img, pt1 = (ix, iy), pt2 = (x, y), 
                          color = (87, 184, 237), thickness = -1)
            
    elif event == cv2.EVENT_LBUTTONUP:
        drawing = False
        cv2.rectangle(img, pt1 = (ix, iy), pt2 = (x, y), 
                      color = (87, 184, 237), thickness = -1)
        
    
# Step 2. Call the window
img = cv2.imread('map.png')

cv2.namedWindow(winname = 'my_drawing')
cv2.setMouseCallback('my_drawing', draw_rectangle)


# Step 3. Execution 
while True: 
    cv2.imshow('my_drawing', img)
    if cv2.waitKey(10) & 0xFF == 27:
        break

cv2.destroyAllWindows()