Approximate Nearest Subspace Search with applications to pattern recognition Ronen Basri Tal Hassner Lihi Zelnik-Manor Weizmann Institute Caltech
Subspaces in Computer Vision Zelnik-Manor & Irani, PAMI’06 Basri & Jacobs, PAMI’03 Nayar et al., IUW’96 Illumination Faces Objects Viewpoint, Motion Dynamic textures …
Query Nearest Subspace Search Which is the Nearest Subspace?
Sequential Search Sequential search: O(ndk) Too slow!! Is there a sublinear solution? Database d dimensions n subspaces k subspace dimension
A Related Problem: Nearest Neighbor Search d dimensions n points Sequential search: O(nd) There is a sublinear solution! Database
Approximate NN (1+ )r Tree search (KD-trees) Locality Sensitive Hashing Fast!! Query: Logarithmic Preprocessing: O(dn) r
Is it possible to speed-up Nearest Subspace Search? Existing point-based methods cannot be applied Tree searchLSH
Our Suggested Approach Reduction to points Works for both linear and affine spaces Run time Sequential Our Database size
Problem Definition Find Mapping Apply standard point ANN to u,v A linear function of original distance Monotonic in distance Independent mappings
Finding a Reduction Constants? Depends on query Feeling lucky? We are lucky !!
Basic Reduction Want: minimize /
Geometry of Basic Reduction Database Lies on a sphere and on a hyper-plane Query Lies on a cone
Improving the Reduction
Final Reduction = constants
Can We Do Better? If =0 Trivial mappingAdditive Constant is Inherent
Final Mapping Geometry
ANS Complexities Preprocessing: O(nkd 2 ) Linear in n Log in n Query: O(d 2 )+T ANN (n,d 2 )
Dimensionality May be Large Embedding in d 2 Might need to use small ε Current solution: –Use random projections (use Johnson- Lindenstrauss Lemma) –Repeat several times and select the nearest
Synthetic Data Varying database size d=60, k=4 Run time Sequential Our Database size Varying dimension n=5000, k=4 Run time Sequential Our dimension
Face Recognition (YaleB) Database 64 illuminations k=9 subspaces Query: New illumination
Face Recognition Result Wrong Match Wrong Person True NS Approx NS
Retiling with Patches Patch databaseQueryApprox Image Wanted
Retiling with Subspaces Subspace database QueryApprox Image Wanted
Patches + ANN ~0.6sec
Subspaces + ANS ~1.2 sec
Patches + ANN ~0.6sec
Subspaces + ANS ~1.2 sec
Summary Fast, approximate nearest subspace search Reduction to point ANN Useful applications in computer vision Disadvantages: –Embedding in d 2 –Additive constant Other methods? Additional applications? A lot more to be done…..
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