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Optic flow is the perceived visual motion of objects as the observer moves relative to them. To an observer driving a car, a sign on the side of the road would move from the center of his vision to the side, growing as he approached. If he had 360 degree vision, this sign would proceed to move quickly past his side to his back, where it would shrink. This motion of the sign is its optic flow.
This allows a person to judge how close he is to certain objects, and how quickly he is approaching them. It is also useful for avoiding obstacles: if an object in front of an observer appears to be expanding but not moving, he is probably headed straight for it, but if it is expanding but moving slowly to the side, he will probably pass by it. Since optic flow relies only on relative motion, it remains the same when he is moving and the world remains still, and when he is standing still but everything he can see is moving past him. These properties have made the concept useful for robot designers writing visual navigation routines. It also appears to be used by certain insects, especially flying ones, where a large optic flow (indicating a quickly approaching obstacle) triggers muscles to move away.
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