nips nips2011 nips2011-119 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Sungwoong Kim, Sebastian Nowozin, Pushmeet Kohli, Chang D. Yoo
Abstract: For many of the state-of-the-art computer vision algorithms, image segmentation is an important preprocessing step. As such, several image segmentation algorithms have been proposed, however, with certain reservation due to high computational load and many hand-tuning parameters. Correlation clustering, a graphpartitioning algorithm often used in natural language processing and document clustering, has the potential to perform better than previously proposed image segmentation algorithms. We improve the basic correlation clustering formulation by taking into account higher-order cluster relationships. This improves clustering in the presence of local boundary ambiguities. We first apply the pairwise correlation clustering to image segmentation over a pairwise superpixel graph and then develop higher-order correlation clustering over a hypergraph that considers higher-order relations among superpixels. Fast inference is possible by linear programming relaxation, and also effective parameter learning framework by structured support vector machine is possible. Experimental results on various datasets show that the proposed higher-order correlation clustering outperforms other state-of-the-art image segmentation algorithms.
Reference: text
sentIndex sentText sentNum sentScore
1 kr Abstract For many of the state-of-the-art computer vision algorithms, image segmentation is an important preprocessing step. [sent-10, score-0.503]
2 As such, several image segmentation algorithms have been proposed, however, with certain reservation due to high computational load and many hand-tuning parameters. [sent-11, score-0.503]
3 Correlation clustering, a graphpartitioning algorithm often used in natural language processing and document clustering, has the potential to perform better than previously proposed image segmentation algorithms. [sent-12, score-0.534]
4 We improve the basic correlation clustering formulation by taking into account higher-order cluster relationships. [sent-13, score-0.459]
5 This improves clustering in the presence of local boundary ambiguities. [sent-14, score-0.26]
6 We first apply the pairwise correlation clustering to image segmentation over a pairwise superpixel graph and then develop higher-order correlation clustering over a hypergraph that considers higher-order relations among superpixels. [sent-15, score-2.435]
7 Experimental results on various datasets show that the proposed higher-order correlation clustering outperforms other state-of-the-art image segmentation algorithms. [sent-17, score-0.993]
8 1 Introduction Image segmentation, a partitioning of an image into disjoint regions such that each region is homogeneous, is an important preprocessing step for many of the state-of-the-art algorithms for high-level image/scene understanding for three reasons. [sent-18, score-0.371]
9 Image segmentation algorithms can be categorized into either non-graph-based or graph-based algorithms. [sent-22, score-0.358]
10 In comparison to non-graph-based segmentations, graph-based segmentations have been shown to produce consistent segmentations by adaptively balancing local 1 judgements of similarity [7]. [sent-24, score-0.234]
11 However, in contrast to the min-cuts and normalized cuts which are node-labeling algorithms, the FH algorithm benefits from the edge-labeling in that it leads to faster inference and does not require a pre-specified number of segmentations in each image [7]. [sent-26, score-0.345]
12 Correlation clustering is a graph-partitioning algorithm [8] that simultaneously maximizes intracluster similarity and inter-cluster dissimilarity by solving the global objective (discriminant) function. [sent-27, score-0.226]
13 In comparison with the previous image segmentation algorithms, correlation clustering is a graph-based, global-objective, and edge-labeling algorithm and therefore, has the potential to perform better for image segmentation. [sent-28, score-1.107]
14 Furthermore, correlation clustering leads to the linear discriminant function which allows for approximate polynomial-time inference by linear programming (LP) and large margin training based on structured support vector machine (S-SVM) [9]. [sent-29, score-0.706]
15 A framework that uses S-SVM for training the parameters in correlation clustering has been considered previously by Finley et al. [sent-30, score-0.499]
16 Taskar derived a max-margin formulation for learning the edge scores for correlation clustering [11]. [sent-32, score-0.57]
17 Even though the previous (pairwise) correlation clustering can consider global aspects of an image using the discriminatively-trained discriminant function, it is restricted in resolving the segment boundary ambiguities caused by neighboring pairwise relations presented by the pairwise graph. [sent-35, score-1.291]
18 Therefore, to capture long-range dependencies of distant nodes in a global context, this paper proposes a novel higher-order correlation clustering to incorporate higher-order relations. [sent-36, score-0.499]
19 We first apply the pairwise correlation clustering to image segmentation over a pairwise superpixel graph and then develop higher-order correlation clustering over a hypergraph that considers higher-order relations among superpixels. [sent-37, score-2.435]
20 The proposed higher-order correlation clustering is defined over a hypergraph in which an edge can connect to two or more nodes [12]. [sent-38, score-0.937]
21 Hypergraphs have been previously used to lift certain limitations of conventional pairwise graphs [13, 14, 15]. [sent-39, score-0.214]
22 However, previously proposed hypergraphs for image segmentation are restricted to partitioning based on the generalization of normalized cut framework, which suffer from a number of problems. [sent-40, score-0.635]
23 A number of algorithms to approximate the inference process have been introduced based on the coarsening algorithm [14] and the hypergraph Laplacian matrices [13]; these are heuristic approaches and therefore are sub-optimal. [sent-42, score-0.335]
24 Second, incorporating a supervised learning algorithm for parameter estimation under the spectral hypergraph partitioning framework is difficult. [sent-43, score-0.41]
25 Almost all previous hypergraph-based image segmentation algorithms are restricted to use only color variances as region features. [sent-47, score-0.576]
26 The proposed higher-order correlation clustering overcomes all of these problems due to the generalization of the pairwise correlation clustering and enables to take advantages of using a hypergraph. [sent-48, score-1.163]
27 The proposed higher-order correlation clustering algorithm uses as its input a hypergraph and leads to a linear discriminant function. [sent-49, score-0.882]
28 For fast inference, the LP relaxation is used to approximately solve the higher-order correlation clustering problem, and for supervised training of the parameter vector by S-SVM, we apply a decomposable structured loss function to handle unbalanced classes. [sent-51, score-0.656]
29 Experimental results on various datasets show that the proposed higher-order correlation clustering outperforms other state-of-the-art image segmentation algorithms. [sent-53, score-0.993]
30 Section 2 presents the higher-order correlation clustering for image segmentation. [sent-55, score-0.604]
31 Section 3 describes large margin training for supervised image segmentation based on the S-SVM and the cutting plane algorithm. [sent-56, score-0.727]
32 2 i k yik k yijk j yij y jk j (a) yj k (b) Figure 1: Illustrations of a part of (a) the pairwise graph (b) and the triplet graph built on superpixels. [sent-58, score-0.439]
33 The proposed correlation clustering merges superpixels into disjoint homogeneous regions over a superpixel graph. [sent-60, score-1.105]
34 1 Pairwise correlation clustering over pairwise superpixel graph Define a pairwise undirected graph G = (V, E) where a node corresponds to a superpixel and a link between adjacent superpixels corresponds to an edge (see Figure 1. [sent-62, score-1.757]
35 A binary label yjk for an edge (j, k) ∈ E between nodes j and k is defined such that { 1, if nodes j and k belong to the same region, yjk = (1) 0, otherwise. [sent-64, score-0.406]
36 Note that the discriminant function F (x, y; w) is assumed to be linear in both the parameter vector w and the joint feature map Φ(x, y), and ϕjk (x) is a pairwise feature vector which reflects the correspondence between the jth and the kth ˆ superpixels. [sent-66, score-0.386]
37 An image segmentation is to infer the edge label, y, over the pairwise superpixel graph G by maximizing F such that ˆ y = argmax F (x, y; w) (3) y∈Y E where Y is the set of {0, 1} that corresponds to a valid segmentation, the so called multicut polytope. [sent-67, score-1.086]
38 When producing the segmentation results based on the approximated LP solutions, we take the floor of a fractionally-predicted label of each edge independently for simply obtaining valid integer solutions that may be sub-optimal. [sent-70, score-0.506]
39 Therefore, to incorporate higher-order relations, we develop higher-order correlation clustering by generalizing the correlation clustering over a hypergraph. [sent-72, score-0.918]
40 2 Higher-order correlation clustering over hypergraph The proposed higher-order correlation clustering is defined over a hypergraph in which an edge called hyperedge can connect to two or more nodes. [sent-74, score-1.83]
41 (b), one 3 (a) (b) (c) (d) Figure 2: Example of segmentation result by pairwise correlation clustering. [sent-76, score-0.805]
42 can introduce binary labels for each adjacent vertices forming a triplet such that yijk = 1 if all vertices in the triplet ({i, j, k}) are in the same cluster; otherwise, yijk = 0. [sent-81, score-0.245]
43 Define a hypergraph HG ∪ (V, E) where V is a set of nodes (superpixels) and E is a set of hyperedges (subsets of V) such = that e∈E = V. [sent-82, score-0.443]
44 Therefore, the hyperedge set E can be divided into two disjoint subsets: pairwise edge set Ep = {e ∈ E | |e| = 2} and higher∪ order edge set Eh = {e ∈ E | |e| > 2} such that Ep Eh = E. [sent-86, score-0.647]
45 Note that in the proposed hypergraph for higher-order correlation clustering all hyperedges containing just two nodes (∀ep ∈ Ep ) are linked between adjacent superpixels. [sent-87, score-0.97]
46 The pairwise superpixel graph is a special hypergraph where all hyperedges contain just two (neighboring) superpixels: Ep = E. [sent-88, score-0.844]
47 A binary label ye for a hyperedge e ∈ E is defined such that { 1, if all nodes in e belong to the same region, (4) ye = 0, otherwise. [sent-89, score-0.361]
48 Note that the proposed discriminant function for higher-order correlation clustering is decomposed into two terms by assigning different parameter vectors to the pairwise edge set Ep and T T the higher-order edge set Eh such that w = [wp , wh ]T . [sent-91, score-1.062]
49 Thus, in addition to the pairwise similarity between neighboring superpixels, the proposed higher-order correlation clustering considers a broad homogeneous region reflecting higher-order relations among superpixels. [sent-92, score-0.936]
50 Now the problem is how to build our hypergraph from a given image. [sent-93, score-0.296]
51 We obtain unsupervised multiple partitionings by merging not pixels but superpixels with different image quantizations using the ultrametric contour maps [19]. [sent-95, score-0.528]
52 These higher-order inequalities can be formulated as yeh ≤ yep , ∀ep ∈ Ep |ep ⊂ eh , ∑ (1 − yeh ) ≤ (1 − yep ). [sent-107, score-0.862]
53 (7) ep ∈Ep |ep ⊂eh Indeed, the LP relaxation to approximately solve (6) is formulated as ∑ ∑ argmax ⟨wp , ϕep (x)⟩yep + ⟨wh , ϕeh (x)⟩yeh y s. [sent-108, score-0.258]
54 ep ∈Ep ∀ e ∈ E(= Ep ∪ (8) eh ∈Eh Eh ), ye ∈ [0, 1], ∀ ep ∈ Ep , cycle inequalities, odd-wheel inequalities [18], ∀ eh ∈ Eh , higher-order inequalities (7). [sent-110, score-1.143]
55 Note that the proposed higher-order correlation clustering follows the concept of soft constraints: superpixels within a hyperedge are encouraged to merge if a hyperedge is highly homogeneous. [sent-111, score-1.114]
56 The pairwise feature vector ϕep (x) reflects the correspondence between neighboring superpixels, and the higher-order feature vector ϕeh (x) characterizes a more complex relations among superpixels in a broader region to measure homogeneity. [sent-115, score-0.733]
57 5 • Texture difference ϕt : The 64 texture distances (absolute differences, χ2 -distances, earth mover’s distances) between two adjacent superpixels using 15 Leung-Malik (LM) filter banks [19]. [sent-121, score-0.335]
58 • Joint visual word posterior ϕv : The 100-dimensional vector holding the joint visual word posteriors for a pair of neighboring superpixels using 10 visual words and the 400-dimensional vector holding the joint posteriors based on 20 visual words [21]. [sent-124, score-0.415]
59 eh e eh • Variance ϕva : The 14 color variances and 30 texture variances among superpixels in a hyperedge. [sent-127, score-0.86]
60 The 100-dimensional template matching feature vector is composed of the matching scores between a region defined by a hyperedge and templates using the Gaussian RBF kernel. [sent-130, score-0.289]
61 3 Structural learning The proposed discriminant function is defined over the superpixel graph, and therefore, the groundtruth segmentation needs to be transformed to the ground-truth edge labels in the superpixel graph. [sent-132, score-0.98]
62 For this, we first assign a single dominant segment label to each superpixel by majority voting over the superpixel’s constituent pixels and then obtain the ground-truth edge labels. [sent-133, score-0.324]
63 Given N training samples {xn , yn }N where yn is the ground-truth edge labels n=1 for the nth training image, the S-SVM [9] optimizes w by minimizing a quadratic objective function subject to a set of linear margin constraints: min w,ξ s. [sent-135, score-0.379]
64 The cutting plane algorithm [9, 18] with LP relaxation for loss-augmented inference is used to solve the optimization problem of S-SVM, since fast convergence and high robustness of the cutting plane algorithm in handling a large number of margin constraints are well-known [22, 23]. [sent-140, score-0.342]
65 A loss function is usually a non-negative function, and a loss function that is decomposable is preferred, since it enables the loss-augmented inference in the cutting plane algorithm to be performed efficiently. [sent-141, score-0.233]
66 The most popular loss function that is decomposable is the Hamming distance which is equivalent to the number of mismatches between yn and y at the edge level in this correlation clustering. [sent-142, score-0.457]
67 Unfortunately, in the proposed correlation clustering for image segmentation, the number of edges which are labeled as 1 is considerably higher than that of edges which are labeled as 0. [sent-143, score-0.693]
68 This unbalance makes other learning methods such as the perceptron algorithm inappropriate, since it leads to the clustering of the whole image as one segment. [sent-144, score-0.407]
69 3 gPb- oiem H 20 25 30 35 Average number of regions 40 Corr - luster - airwise C P 20 25 30 35 Average number of regions 40 Corr - luster - igher C H Figure 4: Obtained evaluation measures from segmentation results on the SBD. [sent-155, score-0.698]
70 Therefore, we use the following loss function: ) ) ∑( ∑( n n n n ∆(yn , y)= Rp yep +yep − (Rp + 1)yep yep +D Rh yeh +yeh − (Rh + 1)yeh yeh (10) ep ∈Ep eh ∈Eh where D is the relative weight of the loss at higher-order edge level to that of the loss at pairwise edge level. [sent-157, score-1.567]
71 In addition, Rp and Rh control the relative importance between the incorrect merging of the superpixels and the incorrect separation of the superpixels by imposing different weights to the false negative and the false positive. [sent-158, score-0.536]
72 4 Experiments To evaluate segmentations obtained by various algorithms against the ground-truth segmentation, we conducted image segmentations on three benchmark datasets: Stanford background dataset [24] (SBD), Berkeley segmentation dataset (BSDS) [25], MSRC dataset [26]. [sent-160, score-0.737]
73 For image segmentation based on correlation clustering, we initially obtain baseline superpixels (438 superpixels per image on average) by the gPb contour detector and the oriented watershed transform [19] and then construct a hypergraph. [sent-161, score-1.448]
74 Note that the relaxed solutions in loss-augmented inference are used during training, while in testing, our simple rounding method is used to produce valid segmentation results. [sent-163, score-0.397]
75 Rounding is only necessary in case we obtain fractional solutions from LP-relaxed correlation clustering. [sent-164, score-0.233]
76 We compared the proposed pairwise/higher-order correlation clustering to the following state-of-theart image segmentation algorithms: multiscale NCut [27], gPb-owt-ucm [19], and gPb-Hoiem [20] that grouped the same superpixels based on pairwise same-label likelihoods. [sent-165, score-1.504]
77 The pairwise samelabel likelihoods were independently learnt from the training data with the same 611-dimensional pairwise feature vector. [sent-166, score-0.506]
78 We consider four performance measures: probabilistic Rand index (PRI) [28], variation of information (VOI) [29], segmentation covering (SCO) [19], and boundary displacement error (BDE) [30]. [sent-167, score-0.392]
79 As the predicted segmentation is close to the ground-truth segmentation, the PRI and SCO are increased while the VOI and BDE are decreased. [sent-168, score-0.358]
80 Figure 4 shows the obtained four measures from segmentation results according to the average number of regions. [sent-172, score-0.358]
81 Irrespective of the measure, the proposed higher-order correlation clustering (Corr-Cluster-Higher) performed better than other algorithms including the pairwise correlation clustering (Corr-Cluster-Pairwise). [sent-179, score-1.163]
82 The proposed higher-order correlation clustering yielded the best segmentation results. [sent-181, score-0.848]
83 Regarding the runtime of our algorithm, we observed that for test-time inference it took on average around 15 seconds (graph construction and feature extraction: 14s, LP: 1s) per image on a 2. [sent-225, score-0.222]
84 Note that other segmentation algorithms such as the multiscale NCut and the gPb-owt-ucm took on average a few minutes per image. [sent-227, score-0.387]
85 2 Berkeley segmentation dataset and MSRC dataset The BSDS contains 300 natural images split into the 200 training images and 100 test images. [sent-229, score-0.398]
86 Since each image is segmented by multiple human subjects, we defined a single probabilistic (real-valued) ground-truth segmentation of each image for training in the proposed correlation clustering. [sent-230, score-0.952]
87 On average, all segmentation algorithms were set to produce 30 disjoint regions per image on the BSDS and 15 disjoint regions per image on the MSRC dataset. [sent-234, score-0.876]
88 As shown in Table 1, the proposed higher-order correlation clustering gave the best results on both datasets. [sent-235, score-0.49]
89 5 Conclusion This paper proposed the higher-order correlation clustering over a hypergraph to merge superpixels into homogeneous regions. [sent-237, score-1.111]
90 The LP relaxation was used to approximately solve the higher-order correlation clustering over a hypergraph where a rich feature vector was defined based on several visual cues involving higher-order relations among superpixels. [sent-238, score-0.953]
91 The S-SVM was used for supervised training of parameters in correlation clustering, and the cutting plane algorithm with LP-relaxed inference was applied to solve the optimization problem of S-SVM. [sent-239, score-0.452]
92 Experimental results showed that the proposed higher-order correlation clustering outperformed other image segmentation algorithms on various datasets. [sent-240, score-0.993]
93 Wu, “An efficient k-means clustering algorithm: Analysis and implementation,” PAMI, vol. [sent-250, score-0.226]
94 Malik, “Blobworld: image segmentation using expectationmaximization and its application to image querying,” PAMI, vol. [sent-262, score-0.648]
95 Joachims, “Supervised clustering with support vector machines,” in ICML, 2005. [sent-295, score-0.226]
96 Yilmaz, “Image segmentation as learning on hypergraphs,” in Proc. [sent-304, score-0.358]
97 Laget, “A multilevel spectral hypergraph partitioning approach for color image segmentation,” in Proc. [sent-314, score-0.524]
98 Shi, “Spectral segmentation with multiscale graph decomposition,” in CVPR, 2005. [sent-368, score-0.438]
99 Rand, “Objective criteria for the evaluation of clustering methods,” Journal of the American Statistical Association, vol. [sent-371, score-0.226]
100 Lee, “Learning full pairwise affinities for spectral segmentation,” in CVPR, 2010. [sent-389, score-0.258]
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