LSH-based
Motion Estimation
Alex Giladi
Motion Estimation
Motivation
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Utilize temporal redundancy for better video compression
Improve video quality in MPEG-1 / MPEG-2 / MPEG-4 / H.264 AVC
Definition
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For each block bs  I(t), find closest br  I(t-1)
Objective: minimize the residue, bs – br
Search ranges: ± 64 is common for NTSC broadcast (720x480)
Assume: 16x16 blocks, done in MSE sense
Algorithms
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Full search (brute force):
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Fast algorithms exist
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7/26/2016
400K MSE computations for ± 16 on CIF (352x288)
132M MSE computations for ± 64 on 1080i (1920x1080)
Real time: only 33.36ms per picture
Speedups to full search
Variants of logarithmic search
Hierarchical motion estimation
LSH-based motion estimation
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Motion estimation and LSH
Re-definition:
16x16 block is represented as a vector in 256.
 Similarity measure: L2 norm
 Hash functions: dot product with a random vector in
256
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Algorithm
Hash similar blocks to same buckets.
 Pick blocks that hashed to the same bucket
 Find best match among these
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7/26/2016
LSH-based motion estimation
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Motion estimation and LSH
Why LSH?
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We don’t need exact answer
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We can allow longer pre-processing time
We don’t need an answer where blocks are dissimilar
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An approximation is enough;
These would not be coded using temporal prediction
Very large search ranges can be supported
Problems
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Search radius
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7/26/2016
Large radius – too many candidate blocks to be considered
Small radius – too many blocks have no pairs
Requires a large amount of additional memory (vs. none in the
regular algorithms)
Requires several dot product computations per pixel
LSH-based motion estimation
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Results
Answers are sufficiently close to the true values
 Complexity is reduced
 No answer for several blocks
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7/26/2016
L2 distance
Blocks found
Additional MSE
computations
128
87%
1075
64
80%
562
LSH-based motion estimation
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Conclusion
Using LSH
 Needs
tuning and speedups
 Can potentially reduce ME complexity, when
large search range is required.
Extensions:
 Prefer
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closer vectors closer pictures
represent x,y,t vector components in 259
 Multiple
references
 Fast candidate elimination techniques
7/26/2016
LSH-based motion estimation
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