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300 nips-2013-Solving the multi-way matching problem by permutation synchronization


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Author: Deepti Pachauri, Risi Kondor, Vikas Singh

Abstract: The problem of matching not just two, but m different sets of objects to each other arises in many contexts, including finding the correspondence between feature points across multiple images in computer vision. At present it is usually solved by matching the sets pairwise, in series. In contrast, we propose a new method, Permutation Synchronization, which finds all the matchings jointly, in one shot, via a relaxation to eigenvector decomposition. The resulting algorithm is both computationally efficient, and, as we demonstrate with theoretical arguments as well as experimental results, much more stable to noise than previous methods. 1

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sentIndex sentText sentNum sentScore

1 Solving the multi-way matching problem by permutation synchronization Deepti Pachauri,† Risi Kondor§ and Vikas Singh‡† Dept. [sent-1, score-0.925]

2 edu † Abstract The problem of matching not just two, but m different sets of objects to each other arises in many contexts, including finding the correspondence between feature points across multiple images in computer vision. [sent-10, score-0.409]

3 At present it is usually solved by matching the sets pairwise, in series. [sent-11, score-0.217]

4 In contrast, we propose a new method, Permutation Synchronization, which finds all the matchings jointly, in one shot, via a relaxation to eigenvector decomposition. [sent-12, score-0.291]

5 1 Introduction Finding the correct bijection between two sets of objects X = {x1 , x2 , . [sent-14, score-0.119]

6 In this paper, we consider its generalization to matching not just two, but m different sets X1 , X2 , . [sent-21, score-0.217]

7 However, our approach is fully general and equally applicable to problems such as matching multiple graphs [10, 11]. [sent-26, score-0.217]

8 Presently, multi-matching is usually solved sequentially, by first finding a putative permutation τ12 matching X1 to X2 , then a permutation τ23 matching X2 to X3 , and so on, up to τm−1,m . [sent-27, score-1.186]

9 While one can conceive of various strategies for optimizing this process, the fact remains that when the data are noisy, a single error in the sequence will typically create a large number of erroneous pairwise matches [12, 13, 14]. [sent-28, score-0.19]

10 For consistency, the recovered matchings must satisfy τkj τji = τki . [sent-30, score-0.205]

11 While finding an optimal matrix of permutations satisfying these relations is, in general, combinatorially hard, we show that for the most natural choice of loss function the problem has a natural relaxation to just finding the n leading eigenvectors of the cost matrix. [sent-31, score-0.425]

12 In addition to vastly reducing the computational cost, using recent results from random ( ) theory, we show that the eigenvectors are very effective at aggregating matrix information from all m pairwise matches, and therefore make the algorithm surprisingly robust to 2 noise. [sent-32, score-0.302]

13 Our experiments show that in landmark matching problems Permutation Synchronization can recover the correct correspondence between landmarks across a large number of images with small error, even when a significant fraction of the pairwise matches are incorrect. [sent-33, score-0.943]

14 on a similar problem involving finding the right rotations (rather than matchings) between electron microscopic 1 images [15][16][17]. [sent-35, score-0.088]

15 However, independently of, and concurrently with the present work, Huang and Guibas [18] have recently proposed a semidefinite programming based solution, which parallels our approach, and in problems involving occlusion might perform even better. [sent-37, score-0.085]

16 2 Synchronizing permutations Consider a collection of m sets X1 , X2 , . [sent-38, score-0.171]

17 , xi }, such n 1 2 i that for each pair (Xi , Xj ), each xp in Xi has a natural counterpart xj in Xj . [sent-44, score-0.233]

18 For example, in q computer vision, given m images of the same scene taken from different viewpoints, xi , xi , . [sent-45, score-0.196]

19 , xi n 1 2 might be n visual landmarks detected in image i, while xj , xj , . [sent-48, score-0.467]

20 , xj are n landmarks detected in n 1 2 i i image j, in which case xp ∼ xj signifies that xp and xj correspond to the same physical feature. [sent-51, score-0.735]

21 q q i Since the correspondence between Xi and Xj is a bijection, one can write it as xp ∼ xjτji(p) for some permutation τji : {1, 2, . [sent-52, score-0.521]

22 , Xm is consistent if xp ∼ xj and q j k i k xq ∼ x r together imply that xp ∼ x r . [sent-67, score-0.306]

23 In terms of permutations this is equivalent to requiring that the array (τij )m satisfy i,j=1 τkj τji = τki ∀i, j, k. [sent-68, score-0.222]

24 , xn , we can think of each Xi as realizing its own permutation σi (in the sense of xℓ ∼ xi i(ℓ) ), and then τji becomes σ −1 τji = σj σi . [sent-72, score-0.414]

25 Rather, in a typical application we have some tentative (noisy) τji matchings which we ˜ must synchronize into the form (2) by finding the underlying σ1 , . [sent-79, score-0.205]

26 In this paper we limit ourselves to the simplest choice d(σ, τ ) = n − ⟨P (σ), P (τ )⟩ , where P (σ) ∈ R (4) n×n are the usual permutation matrices { 1 if σ(p) = q [P (σ)]q,p := 0 otherwise, ∑n and ⟨A, B⟩ is the matrix inner product ⟨A, B⟩ := tr(A⊤ B) = p,q=1 Ap,q Bp,q . [sent-88, score-0.45]

27 Intuitively, each Tji is an objective i matrix, the (q, p) element of which captures the utility of matching xp in Xi to xj in Xj . [sent-97, score-0.412]

28 This q i generalization is very useful when the assignments of the different xp ’s have different confidences. [sent-98, score-0.111]

29 For example, in the landmark matching case, if, due to occlusion or for some other reason, the i counterpart of xp is not present in Xj , then we can simply set [Tji ]q,p = 0 for all q. [sent-99, score-0.562]

30 1 Representations and eigenvectors The generalized Permutation Synchronization problem (5) can also be written as maximize ⟨P, T ⟩ , (6) σ1 ,σ2 ,. [sent-101, score-0.143]

31 Tmm (7) A matrix valued function ρ : Sn → Cd×d is said to be a representation of the symmetric group if ρ(σ2 ) ρ(σ1 ) = ρ(σ2 σ1 ) for any pair of permutations σ1 , σ2 ∈ Sn . [sent-135, score-0.26]

32 The synchronization matrix P is of rank n and is of the form P = U · U ⊤ , where   P (σ1 )  . [sent-140, score-0.376]

33 m [P (σm )]ℓ are mutually orthogonal unit eigenvectors of P with the same eigenvalue m, and together span the row/column space of P. [sent-170, score-0.235]

34 The columns of U are orthogonal because the columns of each constituent P (σi ) are orthogonal. [sent-172, score-0.122]

35 However, Proposition 1 and its corollary suggest relaxing it to maximize ⟨P, T ⟩ , n P∈Mm (10) where Mm is the set of mn–dimensional rank n symmetric matrices whose non-zero eigenvalues n are m. [sent-177, score-0.127]

36 , vℓ are the n leading normalized eigenvectors of T . [sent-181, score-0.209]

37 | Thus, in contrast to the original combinatorial problem, (10) can be solved by just finding the m leading eigenvectors of T . [sent-192, score-0.179]

38 Of course, from P we must still recover the inAlgorithm 1 Permutation Synchronization dividual permutations σ1 , σ2 , . [sent-193, score-0.204]

39 However, as long as P is relatively close in form Input: the objective matrix T Compute the n leading eigenvectors (v1 , v2 , . [sent-197, score-0.223]

40 , vn ) (7), this is quite a simple and stable process. [sent-200, score-0.105]

41 , when Tji = P (˜ji ) for some array (˜ji )j,i of permutations that alτ τ ready satisfy the consistency relations (1), T will have precisely the same structure as described by Proposition 1 for P. [sent-209, score-0.261]

42 In particular, it will have n mutually orthogonal eigenvectors   [P (˜1 )]ℓ σ 1   . [sent-210, score-0.191]

43 Due to the n–fold degeneracy, however, the matrix of eigenvectors (12) is only defined up to multiplication by an arbitrary rotation matrix O on the right, which means that instead of the “correct” U (whose columns are (13)), the eigenvector decomposition of T may return any U ′ = U O. [sent-217, score-0.358]

44 Fortunately, when forming the product P = U′ · U′ ⊤ = U O O⊤ U ⊤ = U · U ⊤ this rotation cancels, confirming that our algorithm recovers P = T , and hence the matchings τji = τji , with no error. [sent-218, score-0.24]

45 ˜ Of course, rather than the case when the solution is handed to us from the start, we are more interested in how the algorithm performs in situations when either the Tji blocks are not permutation matrices, or they are not synchronized. [sent-219, score-0.376]

46 To this end, we set T = T0 + N , (14) where T0 is the correct “ground truth” synchronization matrix, while N is a symmetric perturbation matrix with entries drawn independently from a zero-mean normal distribution with variance η 2 . [sent-220, score-0.508]

47 In general, to find the permutation best aligned with a given n × n matrix T , the Kuhn–Munkres algorithm solves for τ = arg maxτ ∈Sn ⟨P (τ ), T ⟩ = arg maxτ ∈Sn (vec(P (τ )) · vec(T )). [sent-221, score-0.42]

48 85} is replaced by a random permutation (m = 100, n = 30). [sent-226, score-0.376]

49 writing T = P (τ0 ) + ϵ, where P (τ0 ) is the “ground truth”, while ϵ is an error term, it is guaranteed to return the correct permutation as long as ∥ vec(ϵ) ∥ < ′ min ∥ vec(τ0 ) − vec(τ ′ ) ∥ /2. [sent-229, score-0.431]

50 , the permutation that swaps 1 with 2 and leaves 3, 4, . [sent-236, score-0.376]

51 The corresponding permutation matrix differs from the idenity in exactly 4 entries, therefore a sufficient condition for correct reconstruction is that √ ∥ϵ∥Frob = ⟨ϵ, ϵ⟩1/2 = ∥vec(ϵ)∥ < 1 4 = 1. [sent-240, score-0.475]

52 As n grows, ∥ϵ∥Frob becomes tightly concentrated 2 around ηn, so the condition for recovering the correct permutation is η < 1/n. [sent-241, score-0.431]

53 Permutation Synchronization can achieve a lower error, especially in the large m regime, because the eigenvectors aggregate information from all the Tji matrices, and tend to be very stable to perturbations. [sent-242, score-0.143]

54 As long as the largest eigenvalue of the random matrix N falls below a given multiple of the smallest non-zero eigenvalue of T0 , adding N will have very little effect on the eigenvectors of T . [sent-244, score-0.275]

55 If N is a symmetric matrix with independent N (0, η 2 ) entries, as nm → ∞, its spectrum will tend to Wigner’s famous semicircle distribution supported on the interval (−2η(nm)1/2 , 2η(nm)1/2 ), and with probability one the largest eigenvalue will approach 2η(nm)1/2 [20, 21]. [sent-246, score-0.218]

56 In contrast, the nonzero eigenvalues of T0 scale with m, which guarantees that for large enough m the two spectra will be nicely separated and Permutation Synchronization will have very low error. [sent-247, score-0.086]

57 max min Below this limit, to quantify the actual expected error, we write each leading normalized eigenvector ∗ ⊥ ∗ v1 , v2 , . [sent-251, score-0.121]

58 , vn of T as vi = vi + vi , where vi is the projection of vi to the space U0 spanned by the 0 0 0 non-zero eigenvectors v1 , v2 , . [sent-254, score-0.693]

59 ⊥ ⊥ ∗ ∗ ⊥ ⊥ It is easy to see that ⟨vi , vj ⟩ − → 0, which implies ⟨vi , vj ⟩ = ⟨vi , vj ⟩ − ⟨vi , vj ⟩ − → 0, − − ∗ ∗ so, setting λ = (1 − η 2 n/m)−1/2 , the normalized vectors λv1 , . [sent-268, score-0.194]

60 In terms of the individual Pji blocks of P = U U ⊤ , neglecting second order terms, 0 0 ⊤ Pji = (Uj + λEj )(Ui0 + λEi )⊤ ≈ P (τji ) + λUj Ei + λEj Ui0⊤ , where τji is the ground truth matching and Ui0 and Ei denote the appropriate n × n submatrices of U 0 and E. [sent-287, score-0.301]

61 1 + 4(m/n)−1 This is much better than our η < 1/n result for the naive algorithm, and remarkably only slightly stricter than the condition η < (m/n)1/2 for recovering the eigenvectors with any accuracy at all. [sent-290, score-0.143]

62 The baseline method is to compute (˜ji )m τ i,j=1 by solving m inde2 pendent linear assignment problems based on matching landmarks by their shape context features [23]. [sent-297, score-0.562]

63 Our method takes the same pairwise matches and synchronizes them with the eigenvector based procedure. [sent-298, score-0.245]

64 Figure 3 shows that this clearly outperforms the baseline, which tends to degrade progressively as the number of images increases. [sent-299, score-0.088]

65 This is due to the fact that the appearance (or descriptors) of keypoints differ considerably for large offset pairs (which is likely when the image set is large), leading to many false matches. [sent-300, score-0.193]

66 While simple, this experiment demonstrates the utility of Permutation Synchronization for multi-view stereo matching, showing that instead of heuristically propagating local pairwise matches, it can find a much more accurate globally consistent matching at little additional cost. [sent-302, score-0.475]

67 (Green circles) landmark points, (green lines) ground truth matchings, (red lines) found matches. [sent-311, score-0.233]

68 Next, we considered a dataset with severe geometric ambiguities due to repetitive structures. [sent-315, score-0.099]

69 There is some consensus in the community that even sophisticated features (like SIFT) yield unsatisfactory results in this scenario, and deriving a good initial matching for structure from motion is problematic (see [24] and references therein). [sent-316, score-0.217]

70 Our evaluations included 16 images from the Building dataset [24]. [sent-317, score-0.088]

71 We identified 25 “similar looking” landmark points in the scene, and hand annotated them across all images. [sent-318, score-0.186]

72 Many landmarks were occluded due to the camera angle. [sent-319, score-0.23]

73 Qualitative results for pairwise matching and Permutation Synchronization are shown in Fig 4 (top). [sent-320, score-0.332]

74 First, our method resolved geometrical ambiguities by enforcing mutual consistency efficiently. [sent-322, score-0.13]

75 In contrast, pairwise matching struggles with occlusion in the presence of similar looking landmarks (and feature descriptors). [sent-324, score-0.594]

76 The Books dataset (Fig 4, bottom) contains m = 20 images of multiple books on a “L” shaped study table [24], and suffers geometrical ambiguities similar to the above with severe occlusion. [sent-329, score-0.226]

77 Here we identified n = 34 landmark points, many of which were occluded in most images. [sent-330, score-0.202]

78 Our final experiment deals with matching problems where keypoints in each image preserve a common structure. [sent-335, score-0.374]

79 In the literature, this is usually tackled as a graph matching problem, with the keypoints defining the vertices, and their structural relationships being encoded by the edges of the graph. [sent-336, score-0.33]

80 Ideally, one wants to solve the problem for all images at once but most practical solutions operate on image (or graph) pairs. [sent-337, score-0.172]

81 In contrast, in keypoint matching, the background is not controlled and even sophisticated descriptors may go wrong. [sent-340, score-0.099]

82 Recent solutions often leverage supervision to make the problem tractable [25, 26]. [sent-341, score-0.142]

83 Instead of learning parameters [25, 27], we utilize supervision directly to provide the correct matches on a small subset of randomly picked image pairs (e. [sent-342, score-0.356]

84 For our experiments, we used the baseline method output to set up our objective matrix T but with a fixed “supervision probability”, we replaced the Tji block by the correct permutation matrix, and ran Permutation Synchronization. [sent-346, score-0.524]

85 We considered the “Bikes” sub-class from the Caltech 256 dataset, which contains multiple images of common objects with varying backdrops, and chose to match images in the “touring bike” class. [sent-347, score-0.209]

86 Our analysis included 28 out of 110 images in this dataset that were taken “side-on”. [sent-348, score-0.088]

87 SUSAN corner detector was used to identify landmarks in each image. [sent-349, score-0.177]

88 Further, we identified 6 interest points in each image that correspond to the frame of the bicycle. [sent-350, score-0.156]

89 We modeled the matching cost for an image pair as the shape distance between interest points in the pair. [sent-351, score-0.392]

90 For a fixed degree of supervision, we randomly selected image pairs for supervision and estimated matchings for the rest of the image pairs. [sent-353, score-0.515]

91 Mean error and standard deviation is shown in Fig 5 as supervision increases. [sent-355, score-0.142]

92 Fig 6 demonstrates qualitative results by our Figure 5: Normalized error as the degree of supervision varies. [sent-356, score-0.142]

93 5 line method PLA (red) and Permutation Synchronization (blue) Conclusions Estimating the correct matching between two sets from noisy similarity data, such as the visual feature based similarity matrices that arise in computer vision is an error-prone process. [sent-358, score-0.302]

94 However, ( ) when we have not just two, but m different sets, the consistency conditions between the m pair2 wise matchings severely constrain the solution. [sent-359, score-0.244]

95 Our eigenvector decomposition based algorithm, Permutation Synchronization, exploits this fact and pools information from all pairwise similarity matrices to jointly estimate a globally consistent array of matchings in a single shot. [sent-360, score-0.487]

96 (Green lines) Ground truth matching for image pairs (left-center) and (center-right). [sent-368, score-0.345]

97 Shape matching and object recognition using low distortion correspondences. [sent-415, score-0.252]

98 Three-dimensional structure determination from common lines in cryo-EM by eigenvectors and semidefinite programming. [sent-481, score-0.184]

99 The eigenvalues and eigenvectors of finite, low rank perturbations of large random matrices. [sent-516, score-0.195]

100 An integer projected fixed point method for graph matching and map inference. [sent-547, score-0.257]


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