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343 nips-2013-Unsupervised Structure Learning of Stochastic And-Or Grammars


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Author: Kewei Tu, Maria Pavlovskaia, Song-Chun Zhu

Abstract: Stochastic And-Or grammars compactly represent both compositionality and reconfigurability and have been used to model different types of data such as images and events. We present a unified formalization of stochastic And-Or grammars that is agnostic to the type of the data being modeled, and propose an unsupervised approach to learning the structures as well as the parameters of such grammars. Starting from a trivial initial grammar, our approach iteratively induces compositions and reconfigurations in a unified manner and optimizes the posterior probability of the grammar. In our empirical evaluation, we applied our approach to learning event grammars and image grammars and achieved comparable or better performance than previous approaches. 1

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1 edu Abstract Stochastic And-Or grammars compactly represent both compositionality and reconfigurability and have been used to model different types of data such as images and events. [sent-2, score-0.551]

2 We present a unified formalization of stochastic And-Or grammars that is agnostic to the type of the data being modeled, and propose an unsupervised approach to learning the structures as well as the parameters of such grammars. [sent-3, score-0.582]

3 In our empirical evaluation, we applied our approach to learning event grammars and image grammars and achieved comparable or better performance than previous approaches. [sent-5, score-1.013]

4 1 Introduction Stochastic grammars are traditionally used to represent natural language syntax and semantics, but they have also been extended to model other types of data like images [1, 2, 3] and events [4, 5, 6, 7]. [sent-6, score-0.603]

5 It has been shown that stochastic grammars are powerful models of patterns that combine compositionality (i. [sent-7, score-0.554]

6 Stochastic grammars can be used to parse data samples into their compositional structures, which help solve tasks like classification, annotation and segmentation in a unified way. [sent-12, score-0.495]

7 We study stochastic grammars in the form of stochastic And-Or grammars [1], which are an extension of stochastic grammars in natural language processing [8, 9] and are closely related to sum-product networks [10]. [sent-13, score-1.537]

8 Stochastic And-Or grammars have been used to model spatial structures of objects and scenes [1, 3] as well as temporal structures of actions and events [7]. [sent-14, score-0.53]

9 Manual specification of a stochastic grammar is typically very difficult and therefore machine learning approaches are often employed to automatically induce unknown stochastic grammars from data. [sent-15, score-1.289]

10 In this paper we study unsupervised learning of stochastic And-Or grammars in which the training data are unannotated (e. [sent-16, score-0.601]

11 The learning of a stochastic grammar involves two parts: learning the grammar rules (i. [sent-19, score-1.573]

12 One strategy in unsupervised learning of stochastic grammars is to manually specify a fixed grammar structure (in most cases, the full set of valid grammar rules) and try to optimize the parameters of the grammar. [sent-24, score-2.029]

13 , [11, 12]) as well as some approaches of learning image grammars [10, 13] adopt this strategy. [sent-27, score-0.498]

14 The main problem of this strategy is that in some scenarios the full set of valid grammar rules is too large for practical learning and inference, while manual specification of a compact grammar structure is challenging. [sent-28, score-1.548]

15 For example, in an image grammar the number of possible grammar rules to decompose an image patch is exponential in the size of the patch; previous approaches restrict the valid 1 ways of decomposing an image patch (e. [sent-29, score-1.69]

16 Our approach extends the previous work on structure learning of natural language grammars [14, 15, 16], while improves upon the recent work on structure learning of AndOr grammars of images [17] and events [18]. [sent-33, score-1.034]

17 Starting from a trivial initial grammar, our approach iteratively inserts new fragments into the grammar to optimize its posterior probability. [sent-34, score-0.951]

18 • We present a formalization of stochastic And-Or grammars that is agnostic to the types of atomic patterns and their compositions. [sent-37, score-0.741]

19 • We learn compositions and reconfigurations modeled in the grammar in a unified manner that is more efficient and robust to data scarcity and ambiguity than previous approaches. [sent-43, score-0.862]

20 • We empirically evaluated our approach in learning event grammars and image grammars and it achieved comparable or better performance than previous approaches. [sent-44, score-1.013]

21 2 Stochastic And-Or Grammars Stochastic And-Or grammars are first proposed to model images [1] and later adapted to model events [7]. [sent-45, score-0.541]

22 Here we provide a unified definition of stochastic And-Or grammars that is agnostic to the type of the data being modeled. [sent-46, score-0.523]

23 We restrict ourselves to the context-free subclass of stochastic And-Or grammars, which can be seen as an extension of stochastic context-free grammars in formal language theory [8] as well as an extension of decomposable sum-product networks [10]. [sent-47, score-0.594]

24 A stochastic context-free And-Or grammar is defined as a 5-tuple Σ, N, S, R, P . [sent-48, score-0.788]

25 The set of grammar rules R is divided into two disjoint sets: And-rules and Or-rules. [sent-50, score-0.785]

26 an , where A ∈ N AND is a nonterminal And-node and a1 a2 . [sent-55, score-0.173]

27 an is a set of terminal or nonterminal nodes representing the sub-patterns. [sent-58, score-0.338]

28 A set of relations are specified between the sub-patterns and between the nonterminal node A and the sub-patterns, which configure how these sub-patterns form the composite pattern represented by A. [sent-59, score-0.302]

29 Note that one can specify different types of relations in different And-rules, which allows multiple types of compositions to be modeled in the same grammar. [sent-61, score-0.207]

30 It takes the form of O → a, where O ∈ N OR is a nonterminal Or-node, and a is either a terminal or a nonterminal node representing a possible configuration. [sent-63, score-0.537]

31 A stochastic And-Or grammar defines generative processes of valid entities, i. [sent-69, score-0.788]

32 (b) Iteration 1:…… learning a a a X X …… a1 Y Y a6 Y Y X X grammar Xfragment X a6 at N1 . [sent-81, score-0.746]

33 (c)XIteration 2: learning a grammar fragment rooted at N2 . [sent-82, score-0.938]

34 1 Unsupervised Structure Learning Problem Definition In unsupervised learning of stochastic And-Or grammars, we aim to learn a grammar from a set of unannotated i. [sent-84, score-0.842]

35 By adopting a sparsity prior that penalizes the size of the grammar, we hope to learn a compact grammar with good generalizability. [sent-91, score-0.746]

36 Viterbi likelihood has been empirically shown to lead to better grammar learning results [20, 10] and can be interpreted as combining the standard likelihood with an unambiguity bias [21]. [sent-93, score-0.829]

37 2 Algorithm Framework We first define an initial grammar that generates the exact set of training samples. [sent-95, score-0.792]

38 Specifically, for each training sample xi ∈ X, there is an Or-rule S → Ai in the initial grammar where S is the start 1 symbol and Ai is an And-node, and the probability of the rule is X where X is the number of training samples; for each xi there is also an And-rule Ai → ai1 ai2 . [sent-96, score-0.855]

39 n) are the terminal nodes representing the set of atomic patterns contained in sample xi , and a set of relations are specified between these terminal nodes such that they compose sample xi . [sent-102, score-0.551]

40 This initial grammar leads to the maximal likelihood on the training data but has a very small prior probability because of its large size. [sent-104, score-0.817]

41 3 Y X a4 a5 X a3 a4 nonterminal nodes until the entity contains only terminal nodes (atomic patterns). [sent-105, score-0.409]

42 Table 1 gives a a3 a4 few examples of stochastic context-free And-Or grammars that model different types of data. [sent-106, score-0.529]

43 a2 a5 3 …… Y a6 Y a2 a5 …… Starting from the initial grammar, we introduce new intermediate nonterminal nodes between the terminal nodes and the top-level nonterminal nodes in an iterative bottom-up fashion to generalize the grammar and increase its posterior probability. [sent-107, score-1.397]

44 There are typically multiple candidate grammar fragments that can be added at each iteration, and we employ greedy search or beam search to explore the search space and maximize the posterior probability of the grammar. [sent-110, score-1.072]

45 We also restrict the types of grammar fragments that can be added in order to reduce the number of candidate grammar fragments, which will be discussed in the next subsection. [sent-111, score-1.683]

46 The algorithm terminates when no more grammar fragment can be found that increases the posterior probability of the grammar. [sent-112, score-0.961]

47 3 And-Or Fragments In each iteration of our learning algorithm framework, we search for a new grammar fragment and add it into the grammar. [sent-114, score-0.949]

48 There are many different types of grammar fragments, the choice of which greatly influences the efficiency and accuracy of the learning algorithm. [sent-115, score-0.774]

49 Two simplest types of grammar fragments are And-fragments and Or-fragments. [sent-116, score-0.937]

50 While these two types of fragments are simple and intuitive, they both have important disadvantages if they are searched for separately in the learning algorithm. [sent-128, score-0.191]

51 Instead of And-fragments and Or-fragments, we propose to search for And-Or fragments in the learning algorithm. [sent-131, score-0.193]

52 An And-Or fragment contains a new And-node A, a set of new Or-nodes O1 , O2 , . [sent-132, score-0.173]

53 Such an And-Or n fragment can generate i=1 mi number of configurations of existing nodes. [sent-145, score-0.189]

54 To perform greedy search or beam search, in each iteration of our learning algorithm we need to find the And-Or fragments that lead to the highest gain in the posterior probability of the grammar. [sent-149, score-0.305]

55 Computing the posterior gain by re-parsing the training samples can be very time-consuming if the training set or the grammar is large. [sent-150, score-0.937]

56 Fortunately, we show that by assuming grammar unambiguity the posterior gain of adding an And-Or fragment can be formulated based on a set of sufficient statistics of the training data and is efficient to compute. [sent-151, score-1.079]

57 Since the posterior probability is proportional to the product of the likelihood and the prior probability, the posterior gain is equal to the product of the likelihood gain and the prior gain, which we formulate separately below. [sent-152, score-0.212]

58 (b) The n-gram tensor of the And-Or fragment based 2: (a) context2 context3 … … a11a21athe training data (here n = 3). [sent-155, score-0.219]

59 (c) The context matrix of the And-Or fragment based on the 1 0 0 … on 31 a12a21a31 5 1 training data. [sent-156, score-0.234]

60 …n) is an existing terminal or nonterminal node that (i . [sent-164, score-0.364]

61 First, since the grammar now generates the And-node A first, which then generates a1j1 a2j2 . [sent-167, score-0.746]

62 Putting these two types of changes to the likelihood together, we can formulate the likelihood gain of adding the And-Or fragment as follows (see the supplementary material for the full derivation). [sent-177, score-0.29]

63 The first is the coherence of the n-gram tensor of the And-Or fragment (which tabulates the number of times each configuration covered by the And-Or fragment appears in the training samples, as illustrated in Figure 2(b)). [sent-180, score-0.427]

64 These two factors provide a surrogate measure of how much the training data support the context-freeness within the And-Or fragment and the context-freeness of the And-Or fragment against its context respectively. [sent-182, score-0.407]

65 The prior probability of the grammar is determined by the grammar size. [sent-186, score-1.492]

66 It can be seen that the prior gain penalizes And-Or fragments that have a large size but only cover a small number of configurations in the training data. [sent-195, score-0.248]

67 See the supplementary material for the pseudocode of the complete algorithm of grammar learning. [sent-199, score-0.746]

68 The algorithm runs reasonably fast: our prototype implementation can finish running within a few minutes on a desktop with 5000 training samples each containing more than 10 atomic patterns. [sent-200, score-0.195]

69 1 Experiments Learning Event Grammars We applied our approach to learn event grammars from human activity data. [sent-202, score-0.515]

70 The atomic actions and their start/end time are annotated in each video, as shown in Figure 4. [sent-206, score-0.185]

71 We employed three different methods to apply our grammar learning approach on these two datasets. [sent-209, score-0.746]

72 For each frame of a video in the dataset, we construct a binary vector that indicates which of the atomic actions are observed in this frame. [sent-211, score-0.198]

73 Our learning approach is applied on the ID sequences, where each terminal node represents an ID and each And-node specifies the temporal “followed-by” relation between its child nodes. [sent-227, score-0.226]

74 Each terminal node now represents an occurrence of an atomic action. [sent-229, score-0.341]

75 In this way, the resulting And-Or grammar is directly grounded to the observed atomic actions and is therefore more flexible and expressive than the grammar learned from IDs as in the first method. [sent-231, score-1.693]

76 We slightly modified our algorithm to accommodate this issue: right before the algorithm terminates, we change the top-level And-nodes in the grammar to Or-nodes, which removes any temporal relation between the learned events in each training sample and renders them independent of each other. [sent-235, score-0.896]

77 We used 55 samples of each dataset as the training set and evaluated the learned grammars on the remaining 6 samples. [sent-237, score-0.58]

78 On each testing sample, the events identified by the learned grammars were compared against manual annotations. [sent-238, score-0.545]

79 The third method leads to the best overall performance, which implies the advantage of grounding the grammar to atomic actions and simultaneously learning different relations. [sent-244, score-0.928]

80 2 Learning Image Grammars We first tested our approach in learning image grammars from a synthetic dataset of animal face sketches [24]. [sent-247, score-0.521]

81 4 nodes to represent the atomic sketches in the images and set the relations in And-rules to represent relative positions between image patches. [sent-256, score-0.32]

82 We evaluated the learned grammars against the true grammar. [sent-259, score-0.493]

83 We estimated the precision and recall of the sets of images generated from the learned grammars versus the true grammar, from which we computed the F-measure. [sent-260, score-0.54]

84 We also estimated the KL-divergence of the probability distributions defined by the learned grammars from that of the true grammar. [sent-261, score-0.493]

85 We compared our approach with the image grammar learning approach proposed in [17]. [sent-262, score-0.785]

86 We followed the method described in [17] to quantize the images and learn the atomic patterns, which become the terminal nodes of the grammar. [sent-267, score-0.343]

87 Figure 8 shows some images from the dataset, the quantization examples and the atomic patterns learned. [sent-268, score-0.214]

88 Since the true grammar is unknown, we evaluated the learned grammars by measuring their perplexity (the reciprocal of the geometric mean probability of a sample from a testing set). [sent-270, score-1.273]

89 We ran 10-fold cross-validation on the dataset: learning an image grammar from each training set and then evaluating its perplexity on the testing set. [sent-271, score-0.865]

90 Before estimating the perplexity, the probability distribution represented by each learned grammar was smoothed to avoid zero probability on the testing images. [sent-272, score-0.78]

91 5 Conclusion We have presented a unified formalization of stochastic And-Or grammars that is agnostic to the type of the data being modeled, and have proposed an unsupervised approach to learning the structures as well as the parameters of such grammars. [sent-275, score-0.582]

92 Our approach optimizes the posterior probability of the grammar and induces compositions and reconfigurations in a unified manner. [sent-276, score-0.867]

93 Our experiments in learning event grammars and image grammars show satisfactory performance of our approach. [sent-277, score-1.013]

94 Aggarwal, “Recognition of composite human activities through context-free grammar based representation,” in CVPR, 2006. [sent-310, score-0.762]

95 Huang, “An extended grammar system for learning and recognizing complex visual events,” IEEE Trans. [sent-314, score-0.746]

96 Klein, “Probabilistic grammars and hierarchical dirichlet processes,” The handbook of applied Bayesian analysis, 2009. [sent-337, score-0.459]

97 Baker, “Trainable grammars for speech recognition,” in Speech Communication Papers for the 97th Meeting of the Acoustical Society of America, 1979. [sent-343, score-0.459]

98 Omohundro, “Inducing probabilistic grammars by Bayesian model merging,” in ICGI, 1994, pp. [sent-358, score-0.459]

99 Honavar, “Unsupervised learning of probabilistic context-free grammar using iterative biclustering,” in Proceedings of 9th International Colloquium on Grammatical Inference (ICGI 2008), ser. [sent-374, score-0.746]

100 Zhu, “Unsupervised learning of event and-or grammar and semantics from video,” in ICCV, 2011. [sent-384, score-0.802]


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