nips nips2006 nips2006-174 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Oren Boiman, Michal Irani
Abstract: We propose a new approach for measuring similarity between two signals, which is applicable to many machine learning tasks, and to many signal types. We say that a signal S1 is “similar” to a signal S2 if it is “easy” to compose S1 from few large contiguous chunks of S2 . Obviously, if we use small enough pieces, then any signal can be composed of any other. Therefore, the larger those pieces are, the more similar S1 is to S2 . This induces a local similarity score at every point in the signal, based on the size of its supported surrounding region. These local scores can in turn be accumulated in a principled information-theoretic way into a global similarity score of the entire S1 to S2 . “Similarity by Composition” can be applied between pairs of signals, between groups of signals, and also between different portions of the same signal. It can therefore be employed in a wide variety of machine learning problems (clustering, classification, retrieval, segmentation, attention, saliency, labelling, etc.), and can be applied to a wide range of signal types (images, video, audio, biological data, etc.) We show a few such examples. 1
Reference: text
sentIndex sentText sentNum sentScore
1 of Computer Science and Applied Math The Weizmann Institute of Science 76100 Rehovot, Israel Abstract We propose a new approach for measuring similarity between two signals, which is applicable to many machine learning tasks, and to many signal types. [sent-2, score-0.197]
2 We say that a signal S1 is “similar” to a signal S2 if it is “easy” to compose S1 from few large contiguous chunks of S2 . [sent-3, score-0.314]
3 This induces a local similarity score at every point in the signal, based on the size of its supported surrounding region. [sent-6, score-0.274]
4 These local scores can in turn be accumulated in a principled information-theoretic way into a global similarity score of the entire S1 to S2 . [sent-7, score-0.408]
5 1 Introduction A good measure for similarity between signals is necessary in many machine learning problems. [sent-12, score-0.185]
6 In this paper we present a new notion of similarity between signals, and demonstrate its applicability to several machine learning problems and to several signal types. [sent-30, score-0.212]
7 We would like to employ this idea to indicate high similarity of Image-B to Image-A, and lower similarity of Image-C to Image-A. [sent-34, score-0.217]
8 In other words, regions in one signal (the “query” signal) which can be composed using large contiguous chunks of data from the other signal (the “reference” signal) are considered to have high local similarity. [sent-35, score-0.465]
9 On the other hand, regions in the query signal which can be composed only by using small fragmented pieces are considered locally dissimilar. [sent-36, score-0.395]
10 This induces a similarity score at every point in the signal based on the size of its largest surrounding region which can be found in the other signal (allowing for some distortions). [sent-37, score-0.496]
11 The large shared regions between B and A (indicated by colors) provide high evidence to their similarity. [sent-42, score-0.287]
12 Note that the shared regions between similar signals are typically irregularly shaped, and therefore cannot be restricted to predefined regularly shaped partitioning of the signal. [sent-46, score-0.231]
13 Other attempts to maintain the benefits of local similarity while maintaining global structural information have recently been proposed [8]. [sent-53, score-0.226]
14 In our previous work [5] we presented an approach for detecting irregularities in images/video as regions that cannot be composed from large pieces of data from other images/video. [sent-55, score-0.315]
15 In this paper we extend this approach to a general principled theory of “Similarity by Composition”, from which we derive local and global similarity and dissimilarity measures between signals. [sent-57, score-0.295]
16 Using this model we derive information-theoretic measures for local and global similarities induced by shared regions. [sent-62, score-0.216]
17 The local similarities of shared regions (“local evidence scores”) are accumulated into a global similarity score (“global evidence score”) of the entire query signal relative to the reference signal. [sent-63, score-1.064]
18 We further prove upper and lower bounds on the global evidence score, which are computationally tractable. [sent-64, score-0.231]
19 It can also be applied to compute similarity of a signal to a group of signals (i. [sent-67, score-0.281]
20 , compose a query signal from pieces extracted from multiple reference signals). [sent-69, score-0.361]
21 The importance of large shared regions between signals have been recognized by biologists for determining similarities between DNA sequences, amino acid chains, etc. [sent-74, score-0.234]
22 In principle, results of such tools can be fed into our theoretical framework, to obtain similarity scores between biological data sequences in a principled information theoretic way. [sent-78, score-0.193]
23 2 we derive information-theoretic measures for local and global “evidence” (similarity) induced by shared regions. [sent-80, score-0.195]
24 4 demonstrates the applicability of the derived local and global similarity measures for various machine learning tasks and several types of signal. [sent-84, score-0.27]
25 2 Similarity by Composition – Theoretical Framework We derive principled information-theoretic measures for local and global similarity between a “query” Q (one or more signals) and a “reference” ref (one or more signals). [sent-85, score-0.637]
26 Large shared regions between Q and ref provide high statistical evidence to their similarity. [sent-86, score-0.651]
27 We then show how these pieces of local evidence can be integrated to provide “global evidence” for the entire query Q (Sec. [sent-91, score-0.383]
28 To do so, we will compare the likelihood that R was generated by ref , versus the likelihood that it was generated by some random process. [sent-98, score-0.446]
29 More formally, we denote by Href the hypothesis that R was “generated” by ref , and by H0 the hypothesis that R was generated by a random process, or by any other application-dependent PDF (referred to as the “null hypothesis”). [sent-99, score-0.442]
30 Href assumes the following model for the “generation” of R: a region was taken from somewhere in ref , was globally transformed by some global transformation T , followed by some small possible local distortions, and then put into Q to generate R. [sent-100, score-0.586]
31 In the simplest case (only shifts), T is the corresponding location in ref . [sent-102, score-0.364]
32 If there are multiple corresponding regions in ref , (i. [sent-106, score-0.467]
33 LES is referred to as a “local evidence score”, because the higher LES is, the smaller the probability that R was generated by random (H0 ). [sent-110, score-0.18]
34 , the probability of getting a score LES(R) > l for a randomly generated region R is smaller than 2−l (this is due to LES being a log-likelihood ratio [3]). [sent-113, score-0.208]
35 High LES therefore provides higher statistical evidence that R was generated from ref . [sent-114, score-0.544]
36 Note that the larger the region R ⊆ Q is, the higher its evidence score LES(R|Href ) (and therefore it will also provide higher statistical evidence to the hypothesis that Q was composed from ref ). [sent-115, score-0.932]
37 For example, assume for simplicity that R has a single identical copy in ref , and that T is restricted to shifts with uniform probability (i. [sent-116, score-0.419]
38 Therefore, the likelihood ratio of R increases, and so does its evidence score LES. [sent-120, score-0.266]
39 LES can also be interpreted as the number of bits saved by describing the region R using ref , instead of describing it using H0 : Recall that the optimal average code length of a random variable y with probability function P (y) is length(y) = −log(P (y)). [sent-121, score-0.486]
40 Therefore we can write the evidence score as LES(R|Href ) = length(R|H0 ) − length(R|Href ). [sent-122, score-0.247]
41 LES(R|H ) ref (where A region R induces “average savings per point” for every point q ∈ R, namely, |R| |R| is the number of points in R). [sent-124, score-0.477]
42 However, a point q ∈ R may also be contained in other regions generated by ref , each with its own local evidence score. [sent-125, score-0.714]
43 q∈R |R| For any point q ∈ Q we define R[q] to be the region which provides this maximal score for q. [sent-128, score-0.254]
44 1 shows such maximal regions found in Image-B (the query Q) given Image-A (the reference ref ). [sent-130, score-0.67]
45 , Rk ⊆ Q be k disjoint regions in Q, which have been generated independently from the examples in ref . [sent-138, score-0.489]
46 The statistical significance of such an accumulated evidence is k expressed by: P ( GES(Q|Href , S) > l | H0 ) = P ( i=1 LES(Ri |Href ) > l | H0 ) < 2−l . [sent-148, score-0.194]
47 Consequently, we can accumulate local evidence of non-overlapping regions within Q which have similar regions in ref for obtaining global evidence on the hypothesis that Q was generated from ref . [sent-149, score-1.447]
48 Thus, for example, if we found 5 regions within Q with similar copies in ref , each resulting with probability less than 10% of being generated by random, then the probability that Q was generated by random is less than (10%)5 = 0. [sent-150, score-0.511]
49 (4) ) is achieved by the segmentation of Q with the best accumulated evidence score, Ri ∈S LES(Ri |Href ) = GES(Q|Href , S), penalized by the length of the segmentation description logP (S|Href ) = −length(S). [sent-168, score-0.371]
50 Obviously, every segmentation provides such a lower (albeit less tight) bound on the total evidence score. [sent-169, score-0.265]
51 Thus, if we find large enough contiguous regions in Q, with supporting regions in ref (i. [sent-170, score-0.599]
52 , high enough local evidence scores), and define R0 to be the remainder of Q, then S = R0 , R1 , . [sent-172, score-0.225]
53 As to the upper bound on GES(Q|Href ), this can be done by summing up the maximal point-wise evidence scores P ES(q|Href ) (see Eq. [sent-176, score-0.281]
54 Note that the upper bound is computed by finding the maximal evidence regions that pass through every point in the query, regardless of the region complexity length(R). [sent-179, score-0.442]
55 3 Algorithmic Framework The local and global evidence scores presented in Sec. [sent-182, score-0.322]
56 2 provide new local and global similarity measures for signal data, which can be used for various learning and inference problems (see Sec. [sent-183, score-0.366]
57 In this section we briefly describe the algorithmic framework used for computing P ES, LES, and GES to obtain the local and global compositional similarity measures. [sent-185, score-0.251]
58 Assume we are given a large region R ⊂ Q and would like to estimate its evidence score LES(R|Href ). [sent-186, score-0.344]
59 We would like to find similar regions to R in ref , that would provide large local evidence for R. [sent-187, score-0.692]
60 The search for a similar ensemble in ref is done using efficient inference on a star graphical model, while allowing for small local displacement of each local patch [5]. [sent-191, score-0.584]
61 For example, in images these would be small spatial patches around each pixel contained in the larger image region R, and the displacements would be small shifts in x and y. [sent-192, score-0.262]
62 In video data the region R would be a space-time volumetric region, and it would be broken into lots of small overlapping space-time volumetric patches. [sent-193, score-0.204]
63 In general, for any n-dimensional signal representation, the region R would be a large n-dimensional region within the signal, and the patches would be small n-dimensional overlapping regions within R. [sent-196, score-0.434]
64 The local patch descriptors are signal and application dependent, but can be very simple. [sent-197, score-0.256]
65 It is the simultaneous matching of all these simple local patch descriptors with their relative positions that provides the strong overall evidence score for the entire region R. [sent-201, score-0.52]
66 , location in ref ) and local patch displacements ∆li for each patch i in R (i = 1, 2, . [sent-204, score-0.612]
67 These patches restrict the possible locations of R in ref , i. [sent-219, score-0.405]
68 Each new patch further reduces this list of candidate positions of R in ref . [sent-223, score-0.435]
69 For each point q ∈ Q we want to estimate its maximal region R[q] and its corresponding evidence score LES(R|Href ) (Sec. [sent-228, score-0.412]
70 Using the single image (a) as a “reference” of good quality grapefruits, we can detect defects (irregularities) in an image (b) of different grapefruits at different arrangements. [sent-232, score-0.213]
71 Right side of (a) and (b) show detected defects in red (points with small intraimage evidence LES). [sent-236, score-0.306]
72 order to perform this step efficiently, we start with a small surrounding region around q, break it into patches, and search only for that region in ref (using the same efficient search method described above). [sent-238, score-0.575]
73 Locations in ref where good initial matches were found are treated as candidates, and are gradually ‘grown’ to their maximal possible matching regions (allowing for local distortions in patch position and descriptor, as before). [sent-239, score-0.715]
74 The evidence score LES of each such maximally grown region is computed. [sent-240, score-0.369]
75 In practice, a region found maximal for one point is likely to be the maximal region for many other points in Q. [sent-242, score-0.346]
76 Thus the number of different maximal regions in Q will tend to be significantly smaller than the number of points in Q. [sent-243, score-0.187]
77 , Rk , we can use these to induce a reasonable (although not optimal) segmentation using the following heuristics: We choose the first segment ˜ to be the maximal region with the largest evidence score: R1 = argmaxRi LES(Ri |Href ). [sent-251, score-0.399]
78 Re˜i |Href ) of these regions, we can obtain a reasonable lower evaluating the evidence scores LES(R bound on GES(Q|Href ) using the left-hand side of Eq. [sent-257, score-0.251]
79 4 Applications and Results The global similarity measure GES(Q|Href ) can be applied between individual signals, and/or between groups of signals (by setting Q and ref accordingly). [sent-267, score-0.607]
80 The local similarity measure LES(R|Href ) can be used for local inference problems, such as local classification, saliency, segmentation, etc. [sent-269, score-0.317]
81 , points with low intra-image evidence scores LES (when measured relative to the rest of the image). [sent-274, score-0.229]
82 Each maximal region R[q] provides high evidence (translated to high affinity scores) that all the points within it should be grouped together (see text for more details). [sent-276, score-0.339]
83 , by setting Q to be one part of the signal, and ref to be the rest of the signal). [sent-279, score-0.364]
84 Such intra-signal evidence can be used for inference tasks like segmentation, while the absence of intra-signal evidence (local dissimilarity) can be used for detecting saliency/irregularities. [sent-280, score-0.363]
85 Detection of Saliency/Irregularities (in Images): Using our statistical framework, we define a point q ∈ Q to be irregular if its best local evidence score LES(R[q] |Href ) is below some threshold. [sent-288, score-0.314]
86 a, used as ref ), we can detect defects (irregularities) in an image of different grapefruits at different arrangements (Fig. [sent-294, score-0.552]
87 The algorithm tried to compose Q from as large as possible pieces of ref . [sent-297, score-0.494]
88 , points whose maximal regions were small) were determined as irregular. [sent-300, score-0.187]
89 Alternatively, local saliency within a query signal Q can also be measured relative to other portions of Q, e. [sent-304, score-0.337]
90 , by trying to compose each region in Q using pieces from the rest of Q. [sent-306, score-0.227]
91 For each point q ∈ Q we compute its intra-signal evidence score LES(R[q] ) relative to the other (non-neighboring) parts of the image. [sent-307, score-0.263]
92 Points with low intra-signal evidence are detected as salient. [sent-308, score-0.181]
93 Examples of using intra-signal saliency to detect defects in fabric can be found in Fig. [sent-309, score-0.186]
94 We used a SIFT-like [9] patch descriptor, but computed densely for all local patches in the image. [sent-314, score-0.179]
95 Signal Segmentation (Images): For each point q ∈ Q we compute its maximal evidence region R[q] . [sent-319, score-0.323]
96 Every maximal region provides evidence to the fact that all points within the region should be clustered/segmented together. [sent-321, score-0.436]
97 Thus, large regions which appear also in ref (in the case of a single image – other regions in Q) are likely to be clustered together in Q. [sent-324, score-0.595]
98 Note that we have not used explicitly low level similarity in neighboring point, as is customary in most image segmentation algorithms. [sent-329, score-0.202]
99 In our implementation, 3D space-time video regions were broken into small spatio-temporal video patches (7 × 7 × 4). [sent-359, score-0.232]
100 These approaches are useful for recognition, but are not applicable to non-class based inference problems (such as similarity between pairs of signals with no prior data, clustering, etc. [sent-366, score-0.2]
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