Optical flow equation
Q:  how many unknowns and equations per pixel?
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Intuitively, what does this constraint mean?
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•The component of the flow in the gradient direction is determined
•The component of the flow parallel to an edge is unknown
This explains the Barber Pole illusion
http://www.sandlotscience.com/Ambiguous/barberpole.htm
A:  u and v are unknown, 1 equation