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Matrix Extensions to Sparse Recovery
Yi Ma1,2 Allen Yang3 John Wright1
CVPR Tutorial, June 20, 2009
1Microsoft Research Asia
3University of California Berkeley
2University of Illinois at Urbana-Champaign
FINAL TOPIC – Generalizations: sparsity to degeneracy
The tools and phenomena underlying sparse recovery generalize very nicely to low-rank matrix recovery
???
FINAL TOPIC – Generalizations: sparsity to degeneracy
The tools and phenomena underlying sparse recovery generalize very nicely to low-rank matrix recovery
Matrix completion: Given an incomplete subset of the entries of a low-rank matrix, fill in the missing values.
Robust PCA: Given a low-rank matrix which has been grossly corrupted, recover the original matrix.
???
Face images
Degeneracy: illumination models
Errors: occlusion, corruption
Relevancy data
Degeneracy: user preferences co-predict
Errors: Missing rankings, manipulation
Video
Degeneracy: temporal, dynamic structures
Errors: anomalous events, mismatches…
Examples of degenerate data:
THIS TALK – From sparse recovery to low-rank recovery
KEY ANALOGY – Connections between rank and sparsity
Sparse recovery Rank minimization
Unknown Vector x Matrix A
Observations y = Ax y = L[A] (linear map)
Combinatorial objective
Convex relaxation
Algorithmic tools
Linear programming Semidefinite programming
KEY ANALOGY – Connections between rank and sparsity
This talk: exploiting this connection for matrix completion and RPCA
Sparse recovery Rank minimization
Unknown Vector x Matrix A
Observations y = Ax y = L[A] (linear map)
Combinatorial objective
Convex relaxation
Algorithmic tools
Linear programming Semidefinite programming
CLASSICAL PCA – Fitting degenerate data
If degenerate observations are stacked as columns of a matrix
then
CLASSICAL PCA – Fitting degenerate data
If degenerate observations are stacked as columns of a matrix
then
Principal Component Analysis via singular value decomposition:
• Stable, efficient computation
• Optimal estimate of under iid Gaussian noise
• Fundamental statistical tool, huge impact in vision, search, bioinformatics
CLASSICAL PCA – Fitting degenerate data
If degenerate observations are stacked as columns of a matrix
then
But… PCA breaks down under even a single corrupted observation.
Principal Component Analysis via singular value decomposition:
• Stable, efficient computation
• Optimal estimate of under iid Gaussian noise
• Fundamental statistical tool, huge impact in vision, search, bioinformatics
ROBUST PCA – Problem formulation
……
D - observation A – low-rank E – sparse error
…
Properties of the errors:
• Each multivariate data sample (column) may be corrupted in some entries
• Corruption can be arbitrarily large in magnitude (not Gaussian!)
Problem: Given recover .A0
Low-rank structure Sparse errors
ROBUST PCA – Problem formulation
Problem: Given recover .A0
Low-rank structure Sparse errors
Numerous heuristic methods in the literature:
• Random sampling [Fischler and Bolles ‘81] • Multivariate trimming [Gnanadesikan and Kettering ‘72] • Alternating minimization [Ke and Kanade ‘03] • Influence functions [de la Torre and Black ‘03]
•
No polynomial-time algorithm with strong performance guarantees!
……
D - observation A – low-rank E – sparse error
…
ROBUST PCA – Semidefinite programming formulation
Seek the lowest-rank that agrees with the data up to some sparse error:A
ROBUST PCA – Semidefinite programming formulation
Seek the lowest-rank that agrees with the data up to some sparse error:
Not directly tractable, relax:
A
ROBUST PCA – Semidefinite programming formulation
Seek the lowest-rank that agrees with the data up to some sparse error:
Not directly tractable, relax:
A
Semidefinite program, solvable in polynomial time
Convex envelope over
MATRIX COMPLETION – Motivation for the nuclear norm
Related problem: we observe only a small known subset
of entries of a rank- matrix . Can we exactly recover ?
MATRIX COMPLETION – Motivation for the nuclear norm
Related problem: recover a rank matrix from a known subset of entries
Convex optimization heuristic [Candes and Recht] :
Spectral trimming also succeeds with for
For incoherent , exact recovery with
[Keshavan, Montanari and Oh]
[Candes and Tao]
ROBUST PCA – Exact recovery?
CONJECTURE: If with sufficiently low-rank and
exactly recovers .
Sparsity of error
sufficiently sparse, then solving
Empirical evidence: probability of correct recovery vs rank and sparsity
Perfect recovery
Rank
Decompose as or ?
ROBUST PCA – Which matrices and which errors?
Fundamental ambiguity – very sparse matrices are also low-rank:
rank-1 rank-00-sparse 1-sparse
Obviously we can only hope to uniquely recover that are incoherent with the standard basis.
Can we recover almost all low-rank matrices from almost all sparse errors?
ROBUST PCA – Which matrices and which errors?
Random orthogonal model (of rank r) [Candes & Recht ‘08]:
independent samples from invariant measure on Steifel manifold of orthobases of rank r.
arbitrary.
ROBUST PCA – Which matrices and which errors?
Random orthogonal model (of rank r) [Candes & Recht ‘08]:
independent samples from invariant measure on Steifel manifold of orthobases of rank r.
arbitrary.
Bernoulli error signs-and-support (with parameter ):
Magnitude of is arbitrary.
MAIN RESULT – Exact Solution of Robust PCA
“Convex optimization recovers almost any matrix of rank from errors affecting of the observations!”
BONUS RESULT – Matrix completion in proportional growth
“Convex optimization exactly recovers matrices of rank , even with entries missing!”
MATRIX COMPLETION – Contrast with literature
• [Candes and Tao 2009]:
Correct completion whp for
Does not apply to the large-rank case
• This work:
Correct completion whp for even with
Proof exploits rich regularity and independence in random orthogonal model.
Caveats:
- [C-T ‘09] tighter for small r. - [C-T ‘09] generalizes better to other matrix ensembles.
MAIN RESULT – Exact Solution of Robust PCA
“Convex optimization recovers almost any matrix of rank from errors affecting of the observations!”
ROBUST PCA – Solving the convex program
Semidefinite program in millions of unknowns.
Scalable solution: apply a first-order method with convergence to
Sequence of quadratic approximations [Nesterov, Beck & Teboulle]:
Solved via soft thresholding (E), and singular value thresholding (A).
ROBUST PCA – Solving the convex program
• Iteration complexity for suboptimal solution.
• Dramatic practical gains from continuation
SIMULATION – Recovery in various growth scenarios
Correct recovery with and fixed, increasing.
Empirically, almost constant number of iterations:
Provably robust PCA at only a constant factor more computation than conventional PCA.
SIMULATION – Phase Transition in Rank and Sparsity
Fraction of successes with , varying (10 trials)
Fraction of successes with , varying (65 trials)
[0,.5] x [0,.5][0,1] x [0,1]
[0,1] x [0,1] [0,.4] x [0,.4]
EXAMPLE – Background modeling from video
Video Low-rank appx. Sparse errorStatic camera surveillance video
200 frames, 72 x 88 pixels,
Significant foregroundmotion
EXAMPLE – Background modeling from video
Video Low-rank appx. Sparse errorStatic camera surveillance video
550 frames, 64 x 80 pixels,
significant illuminationvariation
Background variation
Anomalous activity
EXAMPLE – Faces under varying illumination
…
…RPCA
29 images of one person under varying lighting:
EXAMPLE – Faces under varying illumination
…
…RPCA
29 images of one person under varying lighting:
Self- shadowing
Specularity
EXAMPLE – Face tracking and alignment
Initial alignment, inappropriate for recognition:
EXAMPLE – Face tracking and alignment
EXAMPLE – Face tracking and alignment
EXAMPLE – Face tracking and alignment
EXAMPLE – Face tracking and alignment
EXAMPLE – Face tracking and alignment
EXAMPLE – Face tracking and alignment
EXAMPLE – Face tracking and alignment
EXAMPLE – Face tracking and alignment
EXAMPLE – Face tracking and alignment
EXAMPLE – Face tracking and alignment
Final result: per-pixel alignment
EXAMPLE – Face tracking and alignment
Final result: per-pixel alignment
SIMULATION – Phase Transition in Rank and Sparsity
Fraction of successes with , varying (10 trials)
Fraction of successes with , varying (65 trials)
[0,.5] x [0,.5][0,1] x [0,1]
[0,1] x [0,1] [0,.4] x [0,.4]
CONJECTURES – Phase Transition in Rank and Sparsity
1
100
Hypothesized breakdown behavior as m ∞
CONJECTURES – Phase Transition in Rank and Sparsity
1
100
What we know so far:
This work
Classical PCA
CONJECTURES – Phase Transition in Rank and Sparsity
1
100
CONJECTURE I: convex programming succeeds in proportional growth
CONJECTURES – Phase Transition in Rank and Sparsity
1
100
CONJECTURE II: for small ranks ,
any fraction of errors can eventually be corrected.
Similar to Dense Error Correction via L1 Minimization, Wright and Ma ‘08
CONJECTURES – Phase Transition in Rank and Sparsity
1
100
CONJECTURE III: for any rank fraction, ,
there exists a nonzero fraction of errors that can eventually be
corrected with high probability.
CONJECTURES – Phase Transition in Rank and Sparsity
1
100
CONJECTURE IV: there is an asymptotically sharp phase transition
between correct recovery with overwhelming probability, and
failure with overwhelming probability.
CONJECTURES – Connections to Matrix Completion
Our results also suggest the possibility of a proportional growth phase transition for matrix completion.
1
100
• How do the two breakdown points compare?
• How much is gained by knowing the location of the corruption?
Robust PCA
Matrix Completion
Similar to Recht, Xu and Hassibi ‘08
Matrix CompletionRobust PCA
FUTURE WORK – Stronger results on RPCA?
• RPCA with noise and errors:
Tradeoff between estimation error and robustness to corruption?
• Deterministic conditions on the matrix
• Simultaneous error correction and matrix completion:
bounded noise(e.g., Gaussian)
Conjecture: stable recovery with
we observe
• Faster algorithms:
Smarter continuation strategies
Parallel implementations, GPU, multi-machine
• Further applications:
Computer vision: photometric stereo, tracking, video repair
Relevancy data: search, ranking and collaborative filtering
Bioinformatics
System Identification
FUTURE WORK – Algorithms and Applications
• Reference: Robust Principal Component Analysis: Exact Recovery of Corrupted Low-Rank Matrices by Convex Optimization submitted to the Journal of the ACM
• Collaborators: Prof. Yi Ma (UIUC, MSRA)
Dr. Zhouchen Lin (MSRA)
Dr. Shankar Rao (UIUC)
Arvind Ganesh (UIUC)
Yigang Peng (MSRA)
• Funding:
Microsoft Research Fellowship (sponsored by Live Labs)Grants NSF CRS-EHS-0509151, NSF CCF-TF-0514955, ONR YIP N00014-04-1-0633, NSF IIS 07-03756
REFERENCES + ACKNOWLEDGEMENT
Questions, please?
THANK YOU!
John Wright Robust PCA: Exact Recovery of Corrupted Low-Rank Matrices
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