Interacting with a live video stream

Let's see how we can use the mouse to interact with live video stream from the webcam. We can use the mouse to select a region and then apply the "negative film" effect on that region, as shown next:

In the following program, we will capture the video stream from the webcam, select a region of interest with the mouse, and then apply the effect:

import cv2
import numpy as np

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...