Solving the aperture problem
Basic idea:  assume motion field is smooth
Horn & Schunk:  add smoothness term
Lukas & Kanade:  assume locally constant motion
•pretend the pixel’s neighbors have the same (u,v)
–If we use a 5x5 window, that gives us 25 equations per pixel!
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•works better in practice than Horn & Schunk
Many other methods exist.  Here’s an overview:
•Barron, J.L., Fleet, D.J., and Beauchemin, S, Performance of optical flow techniques, International Journal of Computer Vision, 12(1):43-77, 1994.
•http://www.cs.washington.edu/education/courses/576/03sp/readings/barron92performance.pdf 
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