Efficient video saving for stationary cameras

Our stationary IP camera's image has most of time no action (I did not activate motion detection trigger). Even when there is movement it covers about 5% of the image:

To conserve space, develop an algorithm that only saves the changing portions of the video frames and plays them back smoothly. Write a web application that at prespecified time periods (e.g. every 0.25 seconds) grabs frames from camera's ip address and saves filtered data to a cloud storage.