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Optical flow

An optical flow sensor is a vision sensor capable of measuring optical flow or visual motion and outputting a measurement based on optical flow. Various configurations of optical flow sensors exist. One configuration is an image sensor chip connected to a processor programmed to run an optical flow algorithm. Another configuration uses a vision chip, which is an integrated circuit having both the image sensor and the processor on the same die, allowing for a compact implementation. An example of this is a generic optical mouse sensor used in an optical mouse. In some cases the processing circuitry may be implemented using analog or mixed-signal circuits to enable fast optical flow computation using minimal current consumption.

One area of contemporary research is the use of neuromorphic engineering techniques to implement circuits that respond to optical flow, and thus may be appropriate for use in an optical flow sensor. Such circuits may draw inspiration from biological neural circuitry that similarly responds to optical flow.

Optical flow sensors are used extensively in computer optical mice, as the main sensing component for measuring the motion of the mouse across a surface.

Optical flow sensors are also being used in robotics applications, primarily where there is a need to measure visual motion or relative motion between the robot and other objects in the vicinity of the robot. The use of optical flow sensors in unmanned aerial vehicles (UAVs), for stability and obstacle avoidance, is also an area of current research.


Optical flow based people counting algorithm

Town’s undersea mailbox lures divers


Fan mail from some flounder?: Divers mail water-resistant postcards in a postbox 10 meters down off the town of Susami, Wakayama Prefecture, in June 2012. | CLUB NOAH SUSAMI/KYODO

Susami, a fishing town in Wakayama Prefecture with a population of around 5,000, is in Guinness World Records for having the deepest underwater postbox, at a depth of 10 meters off its coast.

Officially recognized as a mail collection point of the local postal system, 1,000 to 1,500 pieces of mail are dropped into the box each year. Two hours by express train from the city of Wakayama, Susami has “an incomparably beautiful sea,” Hiroaki Yamatani, 37, manager of a diving shop in the town, said proudly.

The postbox is for use by divers who buy water-resistant postcards at Yamatani’s store and write messages on them with an oil-based paint marker. An employee of the shop collects the cards once every few days and takes them to the local post office. The items are then delivered to recipients within a week of being posted.

“I was touched when I saw the box,” said Kahori Kato, a 33-year-old veterinarian who visited Susami for diving from Nishinomiya, Hyogo Prefecture, to drop off postcards for family members, including in China.

The postbox was created as part of a fair in 1999 to promote the Kumano Kodo pilgrimage trail and surrounding areas in the southern part of Wakayama’s Kii Peninsula. With Susami having no special attraction, Toshihiko Matsumoto, 70, then-postmaster of the town, proposed an undersea postbox.

An old, red postbox was altered for the underwater collection of cards and installed when the fair began in April 1999.

Some 32,000 items of mail have since been posted in the box, which was recognized by Guinness World Records as the “Deepest Underwater Postbox” in 2002.

As seawater severely corrodes the cast-iron box, two of them are used in rotation every six months, allowing each to be cleaned and repainted. “We have no replacement if they become unusable,” Yamatani said.

Gallery of post boxes from around the world