draw.py

import numpy as np
import cv2
import test

def draw_rectangle(event, x, y, flags, params):
    global x_init, y_init, drawing, top_left_pt, bottom_right_pt

    if event == cv2.EVENT_LBUTTONDOWN:
        drawing = True
        x_init, y_init = x, y

    elif event == cv2.EVENT_MOUSEMOVE:
        if drawing:
            top_left_pt = (min(x_init, x), min(y_init, y))
            bottom_right_pt = (max(x_init, x), max(y_init, y))
            img[y_init:y, x_init:x] = 255 - img[y_init:y, x_init:x]

    elif event == cv2.EVENT_LBUTTONUP:
        drawing = False
        top_left_pt = (min(x_init, x), min(y_init, y))
        bottom_right_pt = (max(x_init, x), max(y_init, y))
        img[y_init:y, x_init:x] = 255 - img[y_init:y, x_init:x]
if __name__=='__main__':
    drawing= False
    top_left_pt, bottom_right_pt=(-1,-1), (-1,-1)

cap = cv2.VideoCapture(0)
cv2.namedWindow('Webcam')
cv2.setMouseCallback('Webcam',draw_rectangle)

while True:
	ret, frame =cap.read()
	img=cv2.resize(frame, None, fx=0.5, fy=0.5, interpolation=cv2.INTER_AREA)
	img[y0:y1, x0:x1] =255-img[y0:y1, x0:x1]
	cv2.imshow('Webcam', img)
	if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()