TIL: the optical mouse works by taking a picture of the desk surface and does Optical Flow with the previous capture to get relative position.

The result reminded me of the old Logitech Scanman handheld scanner. except that had a 1d light sensor (like a flatbed scanner) and a radial encoder attached a to a roller to advance the scan lines down and the user pulled it down. (kind of like a 1d mechanical mouse.) I had a ton of fun with that growing up.