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169 nips-2011-Maximum Margin Multi-Label Structured Prediction


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Author: Christoph H. Lampert

Abstract: We study multi-label prediction for structured output sets, a problem that occurs, for example, in object detection in images, secondary structure prediction in computational biology, and graph matching with symmetries. Conventional multilabel classification techniques are typically not applicable in this situation, because they require explicit enumeration of the label set, which is infeasible in case of structured outputs. Relying on techniques originally designed for single-label structured prediction, in particular structured support vector machines, results in reduced prediction accuracy, or leads to infeasible optimization problems. In this work we derive a maximum-margin training formulation for multi-label structured prediction that remains computationally tractable while achieving high prediction accuracy. It also shares most beneficial properties with single-label maximum-margin approaches, in particular formulation as a convex optimization problem, efficient working set training, and PAC-Bayesian generalization bounds. 1

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 at Abstract We study multi-label prediction for structured output sets, a problem that occurs, for example, in object detection in images, secondary structure prediction in computational biology, and graph matching with symmetries. [sent-7, score-0.964]

2 Conventional multilabel classification techniques are typically not applicable in this situation, because they require explicit enumeration of the label set, which is infeasible in case of structured outputs. [sent-8, score-0.577]

3 Relying on techniques originally designed for single-label structured prediction, in particular structured support vector machines, results in reduced prediction accuracy, or leads to infeasible optimization problems. [sent-9, score-0.847]

4 In this work we derive a maximum-margin training formulation for multi-label structured prediction that remains computationally tractable while achieving high prediction accuracy. [sent-10, score-0.862]

5 1 Introduction The recent development of conditional random fields (CRFs) [1], max-margin Markov networks (M3Ns) [2], and structured support vector machines (SSVMs) [3] has triggered a wave of interest in the prediction of complex outputs. [sent-12, score-0.512]

6 In this paper, we study multi-label structured prediction, defining the task and introducing the necessary notation in Section 2. [sent-15, score-0.265]

7 Once trained it allows the prediction of multiple structured outputs from a single input, as well as abstaining from a decision. [sent-17, score-0.561]

8 In Section 4 we discuss MLSP’s relation to existing methods for multi-label prediction with simple label sets, and to single-label structured prediction. [sent-22, score-0.581]

9 We furthermore compare MLSP to a multi-label structured prediction methods within the SSVM framework in Section 4. [sent-23, score-0.491]

10 1 2 Multi-label structured prediction We first recall some background and establish the notation necessary to discuss multi-label classification and structured prediction in a maximum margin framework. [sent-26, score-1.042]

11 Our overall task is predicting outputs y ∈ Y for inputs x ∈ X in a supervised learning setting. [sent-27, score-0.181]

12 In ordinary (single-label) multi-class prediction we use a prediction function, g : X → Y, for this, which we learn from i. [sent-28, score-0.545]

13 We measure the quality of predictions by a task-dependent loss function ∆ : Y × Y → R+ , where ∆(y, y ) specifies what cost occurs if we predict an output y while the ¯ ¯ correct prediction is y. [sent-38, score-0.414]

14 Structured output prediction can be seen as a generalization of the above setting, where one wants to make not only one, but several dependent decisions at the same time, for example, deciding for each pixel of an image to which out of several semantic classes it belongs. [sent-39, score-0.401]

15 Consequently, structured output prediction requires specialized techniques that avoid enumerating all possible outputs, and that can generalize between labels in the output set. [sent-44, score-0.778]

16 A popular technique for this task is the structured (output) support vector machine (SSVM) [3]. [sent-45, score-0.307]

17 a technique for identifying the currently most violated linear constraints, working set training, in particular cutting plane [4] or bundle methods [5] allow SSVM training to arbitrary precision in polynomial time. [sent-49, score-0.276]

18 Multi-label prediction is a generalization of single-label prediction that gives up the condition of a functional relation between inputs and outputs. [sent-50, score-0.579]

19 We say that v ∈ {±1}Y represents the subset Y ∈ P(Y) if vy = +1 for y ∈ Y and vy = −1 otherwise. [sent-57, score-0.244]

20 , we write either Y i or v i for a label set in the training data. [sent-60, score-0.188]

21 Note that multi-label prediction can also be interpreted as ordinary single-output prediction with P(Y) taking the place of the original output set Y. [sent-62, score-0.627]

22 Multi-label structured prediction combines the aspects of multi-label prediction and structured output sets: we are given a training set {(xi , Y i )}i=1,. [sent-65, score-1.162]

23 ,n ⊂ X × P(Y), where Y is a structured output set of potentially very large size, and we would like to learn a prediction function: G : X → P(Y) with the ability to generalize also in the output set. [sent-68, score-0.675]

24 In the following, we will take the view of structured prediction point of view, deriving expressions for predicting multiple structured outputs instead of single ones. [sent-69, score-0.852]

25 Alternatively, the same conclusions could be reached by interpreting the task as performing multi-label predicting with binary output vectors that are too large to store or enumerate explicitly, but that have an internal structure allowing generalization between the elements. [sent-70, score-0.202]

26 3 Maximum margin multi-label structured prediction In this section we propose a learning technique designed for multi-label structure prediction that we call MLSP. [sent-71, score-0.824]

27 It makes set-valued prediction by1 , G(x) := {y ∈ Y : f (x, y) > 0} for f (x, y) := w, ψ(x, y) . [sent-72, score-0.252]

28 (2) 1 More complex prediction rules exist in the multi-label literature, see, e. [sent-73, score-0.252]

29 We restrict ourselves to perlabel thresholding, because more advanced rules complicate the learning and prediction problem even further. [sent-76, score-0.252]

30 2 Note that the compatibility function, f (x, y), acts on individual inputs and outputs, as in single-label prediction (1), but the prediction step consists of collecting all outputs of positive scores instead of finding the outputs of maximal score. [sent-77, score-0.761]

31 By including a constant entry into the joint feature map ψ(x, y) we can model a bias term, thereby avoiding the need of a threshold during prediction (2). [sent-78, score-0.305]

32 Note that our setup differs from SSVMs training in this regard. [sent-80, score-0.145]

33 There, a bias term, or a constant entry of the feature map, would have no influence, because during training only pairwise differences of function values are considered, and during prediction a bias does not affect the argmax-decision in Equation (1). [sent-81, score-0.35]

34 As the risk depends on the loss function chosen, we first study the possibilities we have for the set loss ∆ML : P(Y) × P(Y) → R+ . [sent-83, score-0.187]

35 There are no established functions for this in the structured prediction setting, but it turns out that two canonical ¯ set losses are consistent with the following first principles. [sent-84, score-0.519]

36 In the special case of λ ≡ 1 the sum loss is known as ¯ ¯ symmetric difference loss, and it coincides with the Hamming loss of the binary indicator vector representation. [sent-89, score-0.196]

37 The max loss becomes the 0/1-loss between sets in this case. [sent-90, score-0.147]

38 In a general case, λ typically expresses partial correctness, generalizing the single-label structured loss ∆(y, y ). [sent-91, score-0.319]

39 While in the small-scale multi-label situation, the sum loss is more common, we argue in this work that that the max loss has advantages in the structured prediction situation. [sent-96, score-0.729]

40 1 Maximum margin multi-label structured prediction (MLSP) To learn the parameters w of the compatibility function f (x, y) we follow a regularized risk minimization framework: given i. [sent-105, score-0.636]

41 Using the definition of ∆max this is equivalent to minimizing 2 w n 1 2 i + C i ξ i , subject to ξ i ≥ λ(Y i , y) for all y ∈ Y with vy f (xi , y) ≤ 0. [sent-112, score-0.145]

42 3 Besides this slack rescaled variant, one can also form margin rescaled training using the constraints i ξ i ≥ λ(Y i , y) − vy f (xi , y), for all y ∈ Y. [sent-123, score-0.395]

43 The main difference between slack and margin rescaled training is how they treat the case of λ(Y i , y) = 0 for some y ∈ Y. [sent-125, score-0.215]

44 In slack rescaling, the corresponding outputs have no effect on the training at all, whereas for margin rescaling, no margin is enforced for such examples, but a penalization still occurs whenever f (xi , y) > 0 for y ∈ Y i , or if f (xi , y) < 0 for y ∈ Y i . [sent-126, score-0.359]

45 2 Generalization Properties Maximum margin structured learning has become successful not only because it provides a powerful framework for solving practical prediction problems, but also because it comes with certain theoretical guarantees, in particular generalization bounds. [sent-128, score-0.593]

46 L(Qw ,D) ≤ 1 n n (xi , Y i , f ) + i=1 s2 ||w||2 ln(rn/||w||2 ) + ln n ||w||2 σ + n 2(n − 1) 1/2 (9) i for (xi , Y i , f ) := max λ(Y i , y) vy f (xi , y) < 1 , where v i is the binary indicator vector of Y i . [sent-137, score-0.164]

47 This is the same complexity as for single-label prediction using SSVMs, despite the fact that multi-label prediction formally maps into P(Y), i. [sent-145, score-0.504]

48 For the margin-rescaled variant we obtain the dual max − αi ∈R+ y 1 2 i ı i ı ı vy v¯ αy α¯ k (xi , y), (x¯, y ) + ¯ y ¯ y ¯ (i,y),(¯,¯) ıy subject to y i αy ≤ i λi αy y (10) (i,y) C , n for i = 1, . [sent-155, score-0.187]

49 (11) For slack-rescaled MLSP, the dual is computed analogously as max − αi ∈R+ y 1 2 subject to ı i ı i ı vy v¯ αy α¯ k (xi , y), (x¯, y ) + ¯ y ¯ y ¯ (i,y),(¯,¯) ıy y i αy (12) (i,y) i αy C ≤ , λi n y for i = 1, . [sent-159, score-0.209]

50 The crucial step in working set training is the identification of violated constraints. [sent-177, score-0.211]

51 This allows us to reuse existing methods for loss augmented single label inference. [sent-179, score-0.17]

52 Identifying violated “negative” constraint requires loss-augmented prediction over Y \Y i . [sent-182, score-0.321]

53 Note that K-best versions of most standard MAP prediction methods have been developed, including max-flow [9], loopy BP [10], LP-relaxations [11], and Sampling [12]. [sent-184, score-0.273]

54 In contrast to single-label SSVM prediction this requires not only a maximization over all elements of Y, but the collection of all elements y ∈ Y of positive score. [sent-187, score-0.252]

55 Also, it is often possible to establish an upper bound on the number of desired outputs, and then, K-best prediction techniques can again be applied. [sent-192, score-0.278]

56 This makes MLSP of potential use for several classical tasks, such as parsing and chunking in natural language processing, secondary structured prediction in computational biology, or human pose estimation in computer vision. [sent-193, score-0.526]

57 In general situations, evaluating (2) might require approximate structured prediction techniques, e. [sent-194, score-0.491]

58 Note that the use of approximation algorithms is little problematic here, because, in contrast to training, the prediction step is not performed in an iterative manner, so errors do not accumulate. [sent-197, score-0.252]

59 2) Per-label decomposition [16] trains one classifier for each output label and makes independent decision for each of those. [sent-199, score-0.172]

60 Given the size of Y, 1) is not a promising direction for multi-label structured prediction. [sent-201, score-0.239]

61 However, MLSP resembles both approaches by sharing their prediction rule (2). [sent-203, score-0.274]

62 MLSP can be seen as a way to make a combination of approaches applicable to the situation of structured prediction by incorporating the ability to generalize in the label set. [sent-204, score-0.661]

63 They use a structured prediction framework to encode dependencies between the individual output labels, of which there are relatively few. [sent-209, score-0.573]

64 MLSP, on the other hand, aims at predicting multiple structured object, i. [sent-210, score-0.291]

65 the structured prediction framework is not just a tool to improve multi-class classification with multiple output labels, but it is required as a core component for predicting even a single output. [sent-212, score-0.625]

66 Some previous methods targeting multi-label prediction with large output sets, in particular using label compression [25] or a label hierarchy [26]. [sent-213, score-0.514]

67 This allows handling thousands of potential output classes, but a direct application to the structured prediction situation is not possible, because the methods still require explicit handling of the output label vectors, or cannot predict labels that were not part of the training set. [sent-214, score-0.933]

68 The actual task of predicting multiple structured outputs has so far not appeared explicitly in the literature. [sent-215, score-0.387]

69 The situation of multiple inputs during training has, however, received some attention: [27] introduces a one-class SVM based training technique for learning with ambiguous ground truth data. [sent-216, score-0.287]

70 The compatibility functions learned by the maximum-margin techniques [13, 27] have the same functional form as f (x, y) in MLSP, so they can, in principle, be used to predict multiple outputs using Equation (2). [sent-220, score-0.154]

71 However, our experiments of Section 5 show that this leads to low multilabel prediction accuracy, because the training setup is not designed for this evaluation procedure. [sent-221, score-0.505]

72 1 Structured Multilabel Prediction in the SSVM Framework At first sight, it appears unnecessary to go beyond the standard structured prediction framework at all in trying to predict subsets of Y. [sent-223, score-0.491]

73 As mentioned in Section 3, multi-label prediction into Y can be interpreted as single-label prediction into P(Y), so a straight-forward approach to multi-label structured prediction would be to use an ordinary SSVM with output set P(Y). [sent-224, score-1.118]

74 Unfortunately, as we will show in this section, the P-SSVM setup is not well suited to the structured prediction situation. [sent-227, score-0.538]

75 A P-SSVM learns a prediction function, G(x) := argmaxY ∈P(Y) F (x, Y ), with linearly parameterized compatibility function, F (x, Y ) := w, ψ(x, Y ) , by solving the optimization problem 1 w w∈H,ξ 1 ,. [sent-228, score-0.31]

76 This turns the argmax-evaluation for G(x) exactly into the prediction rule (2), and the constraint set in (15) simplifies to ξ i ≥ ∆ML (Y i , Y ) − y∈Y Yi i vy f (xi , y), for i = 1, . [sent-243, score-0.374]

77 Because w, and thereby f , are learned iteratively, they typically go through phases of low prediction quali i ity, i. [sent-250, score-0.252]

78 Consequently, we presume that P-SSVM training is intractable for structured prediction problems, except for the case of a small label set. [sent-257, score-0.679]

79 On the one hand, it is straight-forward to model as a structured prediction task, see e. [sent-265, score-0.491]

80 On the other hand, its output set is small enough such that we can compare MLSP also against other approaches that cannot handle very large output sets, in particular P-SSVM and independent per-label training. [sent-268, score-0.164]

81 As the label set is small, we use exhaustive search over Y to identify violated constraints during training and to perform the final predictions. [sent-285, score-0.287]

82 As there is no single established multi-label error measure, and because it illustrates the effect of training with different loss function, we report several common measures. [sent-287, score-0.206]

83 The results show nicely how the assumptions made during training influence the prediction characteristics. [sent-288, score-0.35]

84 Qualitatively, MLSP achieves best prediction accuracy in the max loss, P-SSVM is better if we judge by the sum loss. [sent-289, score-0.33]

85 This is also plausible, as SSVM training uses a ranking-like loss: all potential labels for each input are enforced to be in the right order (correct labels have higher score than incorrect ones), but nothing in the objective encourages a cut-off point at 0. [sent-292, score-0.225]

86 We take this as an indication that both, training with sum loss and training with max loss, make sense conceptually. [sent-298, score-0.354]

87 The problem in multi-label structured prediction is solely that |Y| is too large, and training data too scarce, to use either of these setups. [sent-301, score-0.589]

88 7 Figure 1: Multi-label structured prediction results. [sent-302, score-0.491]

89 Methods printed in italics are infeasible for general structured output sets. [sent-304, score-0.391]

90 the five methods, only MLSP, JKSE and SSVM generalize to more general structured prediction setting, as they do not require exhaustive enumeration of the label set. [sent-354, score-0.622]

91 2 Object class detection in natural images Object detection can be solved as a structured prediction problem where natural images are the inputs and coordinate tuples of bounding boxes are the outputs. [sent-357, score-0.664]

92 Following the experimental setup of [27] we use the multiscale part of the dataset for training and the singlescale part for testing. [sent-362, score-0.145]

93 For each method we train models on the training set and choose the C or ν value that maximizes the F1 score over the validation set of precropped object and background images. [sent-369, score-0.157]

94 SSVM as well as JKSE suffer particularly from low recall, and their predictions also have higher sum loss as well as max loss. [sent-373, score-0.158]

95 6 Summary and Discussion We have studied multi-label classification for structured output sets. [sent-374, score-0.321]

96 Existing multi-label techniques cannot directly be applied to this task because of the large size of the output set, and our analysis showed that formulating multi-label structured prediction set a set-valued structured support vector machine framework also leads to infeasible training problems. [sent-375, score-1.053]

97 Our experiments showed that MLSP has higher prediction accuracy than baseline methods that remain applied in structured output settings. [sent-377, score-0.573]

98 Besides these promising initial results, we believe that there are still several aspects of multi-label structured prediction that need to be better understood, in particular the prediction problem at test time. [sent-379, score-0.743]

99 Large margin methods for structured and interdependent output variables. [sent-402, score-0.381]

100 On structured output training: Hard cases and an efficient alternative. [sent-504, score-0.321]


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