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\LARGE
http://www.acm.org/sigmm/MM97/papers/kobla/koblaACM97.html \\
V. Kobla, D. Doermann, and C. Faloutsos, ``VideoTrails: Representing and 
Visualizing Structure in Video Sequences," {\em Proceedings of ACM Multimedia
Conference,} Seattle, WA, 1997. \\
use motion vectors encoded in MPEG format: to determine type of camera motion \\
MPEG: Moving Picture Experts Group \\
camera motion: zoom-in, zoom-out, pan left, pan right, tilt up, tilt down, ...\\
pan/tilt analysis: to see if majority of motion vectors aligned in direction\\
each valid motion vector: compared to unit vector in one of eight directions \\
number of motion vectors falling along each of those directions: counted \\
if direction receiving highest number of vectors receives more than twice 
as many vectors as the second highest does, then the frame is declared to
have motion along that direction \\
FOE: Focus of Expansion \\
FOC: Focus of Contraction \\
zoom feature detector: tests for existence of FOE or FOC in each frame  \\
zoom feature detector: uses motion vector of each macroblock as flow data \\
2D array of bins: correspnds to array of macroblocks \\
for each motion vector: vote cast for each bin lying along path of 
line segment along motion vector \\
bins in vicinity of FOC or FOE: receive many votes \\
zoom feature detector: checks that motion vectors near FOE are small and average
magnitudes over constant radius around FOE roughly increases with increasing
radius 




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