1. LSH-based Motion Estimation
Alex Giladi

2. LSH-based motion estimation
Motivation
Utilize temporal redundancy for better video compression
Improve video quality in MPEG-1 / MPEG-2 / MPEG-4 / H.264 AVC
Definition
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
Full search (brute force):
400K MSE computations for ± 16 on CIF (352x288)
132M MSE computations for ± 64 on 1080i (1920x1080)
Real time: only 33.36ms per picture
Fast algorithms exist
Speedups to full search
Variants of logarithmic search
Hierarchical motion estimation
5/21/2019
LSH-based motion estimation

3. 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
Algorithm
Hash similar blocks to same buckets.
Pick blocks that hashed to the same bucket
Find best match among these
5/21/2019
LSH-based motion estimation

4. Motion estimation and LSH
Why LSH?
We don’t need exact answer
An approximation is enough;
We can allow longer pre-processing time
We don’t need an answer where blocks are dissimilar
These would not be coded using temporal prediction
Very large search ranges can be supported
Problems
Search radius
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
5/21/2019
LSH-based motion estimation

5. Results
Answers are sufficiently close to the true values
Complexity is reduced
No answer for several blocks
L2 distance
Blocks found
Additional MSE computations
128
87%
1075 64
80%
562
5/21/2019
LSH-based motion estimation

6. LSH-based motion estimation
Conclusion
Using LSH
Needs tuning and speedups
Can potentially reduce ME complexity, when large search range is required.
Extensions:
Prefer closer vectors closer pictures
represent x,y,t vector components in 259
Multiple references
Fast candidate elimination techniques
5/21/2019
LSH-based motion estimation