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 = (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)
# Check if the webcam is opened correctly
if not cap.isOpened():
raise IOError("Cannot open webcam")
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)
(x0,y0), (x1,y1) = top_left_pt, bottom_right_pt
img[y0:y1, x0:x1] = 255 - img[y0:y1, x0:x1]
cv2.imshow('Webcam', img)
c = cv2.waitKey(1)
if c == 27:
break
cap.release()
cv2.destroyAllWindows()If you run the preceding program, you will see a window displaying the video stream. You can just draw a rectangle on the window using your mouse and you will see that region being converted to its "negative".
As we can see in the main function of the program, we initialize a video capture object. We then bind the function draw_rectangle with the mouse callback in the following line:
cv2.setMouseCallback('Webcam', draw_rectangle)We then start an infinite loop and start capturing the video stream. Let's see what is happening in the function draw_rectangle. Whenever we draw a rectangle using the mouse, we basically have to detect three types of mouse events: mouse click, mouse movement, and mouse button release. This is exactly what we do in this function. Whenever we detect a mouse click event, we initialize the top left point of the rectangle. As we move the mouse, we select the region of interest by keeping the current position as the bottom right point of the rectangle.
Once we have the region of interest, we just invert the pixels to apply the "negative film" effect. We subtract the current pixel value from 255 and this gives us the desired effect. When the mouse movement stops and button-up event is detected, we stop updating the bottom right position of the rectangle. We just keep displaying this image until another mouse click event is detected.