nips nips2013 nips2013-83 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Karen Simonyan, Andrea Vedaldi, Andrew Zisserman
Abstract: As massively parallel computations have become broadly available with modern GPUs, deep architectures trained on very large datasets have risen in popularity. Discriminatively trained convolutional neural networks, in particular, were recently shown to yield state-of-the-art performance in challenging image classification benchmarks such as ImageNet. However, elements of these architectures are similar to standard hand-crafted representations used in computer vision. In this paper, we explore the extent of this analogy, proposing a version of the stateof-the-art Fisher vector image encoding that can be stacked in multiple layers. This architecture significantly improves on standard Fisher vectors, and obtains competitive results with deep convolutional networks at a smaller computational learning cost. Our hybrid architecture allows us to assess how the performance of a conventional hand-crafted image classification pipeline changes with increased depth. We also show that convolutional networks and Fisher vector encodings are complementary in the sense that their combination further improves the accuracy. 1
Reference: text
sentIndex sentText sentNum sentScore
1 Discriminatively trained convolutional neural networks, in particular, were recently shown to yield state-of-the-art performance in challenging image classification benchmarks such as ImageNet. [sent-5, score-0.251]
2 In this paper, we explore the extent of this analogy, proposing a version of the stateof-the-art Fisher vector image encoding that can be stacked in multiple layers. [sent-7, score-0.302]
3 This architecture significantly improves on standard Fisher vectors, and obtains competitive results with deep convolutional networks at a smaller computational learning cost. [sent-8, score-0.22]
4 Our hybrid architecture allows us to assess how the performance of a conventional hand-crafted image classification pipeline changes with increased depth. [sent-9, score-0.303]
5 We also show that convolutional networks and Fisher vector encodings are complementary in the sense that their combination further improves the accuracy. [sent-10, score-0.22]
6 1 Introduction Discriminatively trained deep convolutional neural networks (CNN) [18] have recently achieved impressive state of the art results over a number of areas, including, in particular, the visual recognition of categories in the ImageNet Large-Scale Visual Recognition Challenge [4]. [sent-11, score-0.28]
7 Indeed, several standard features and pipelines in computer vision, such as SIFT [19] and a spatial pyramid on Bag of visual Words (BoW) [16] can be seen as corresponding to layers of a standard CNN. [sent-15, score-0.387]
8 We then show how this 1 representation can be modified to be used as a layer in a deeper architecture (Sect. [sent-24, score-0.266]
9 3) and how the latter can be discriminatively learnt to yield a deep Fisher network (Sect. [sent-25, score-0.209]
10 There exists a large variety of different encodings that can be used for this purpose, including the BoW [9, 29] encoding, sparse coding [33], and the FV encoding [20]. [sent-35, score-0.263]
11 Since FV was shown to outperform other encodings [6] and achieve very good performance on various image recognition benchmarks [21, 28], we use it as the basis of our framework. [sent-36, score-0.309]
12 Most encodings are designed to disregard the spatial location of features in order to be invariant to image transformations; in practice, however, retaining weak spatial information yields an improved classification performance. [sent-40, score-0.576]
13 This can be incorporated by dividing the image into regions, encoding each of them individually, and stacking the result in a composite higher-dimensional code, known as a spatial pyramid [16]. [sent-41, score-0.575]
14 The alternative, which does not increase the encoding dimensionality, is to augment the local features with their spatial coordinates [24]. [sent-42, score-0.317]
15 DNNs can be trained greedily, in a layer-by-layer manner, as in Restricted Boltzmann Machines [12] and (sparse) auto-encoders [3, 17], or by learning all layers simultaneously, which is relatively efficient if the layers are convolutional [18]. [sent-44, score-0.289]
16 For instance, dense feature encoding using the bag of visual words was considered as a single layer of a deep network in [1, 8, 32]. [sent-48, score-0.637]
17 2 Fisher vector encoding for image classification The Fisher vector encoding φ of a set of features {xp } (e. [sent-49, score-0.453]
18 The encoding describes how the distribution of features of a particular image differs from the distribution fitted to the features of all training images. [sent-59, score-0.452]
19 The FV dimensionality is 2Kd, where K is the codebook size (the number of Gaussians in the GMM), and d is the dimensionality of the encoded feature vector. [sent-61, score-0.254]
20 rest linear SVMs classifier layer SSR & L2 norm. [sent-68, score-0.232]
21 2-nd Fisher layer (global pooling) FV encoder L2 norm. [sent-69, score-0.299]
22 & PCA Spatial stacking FV encoder Dense feature extraction SSR & L2 norm. [sent-70, score-0.311]
23 FV encoder 1-st Fisher layer (with optional global pooling branched out) 0-th layer SIFT, raw patches, … Dense feature extraction SIFT, raw patches, … input image Figure 1: Left: Fisher network (Sect. [sent-72, score-1.003]
24 6, making the conventional pipeline slightly deeper by injecting a single Fisher layer substantially improves the classification accuracy. [sent-76, score-0.377]
25 As can be seen from (1), the (unnormalised) FV encoding is additive with respect to image features, i. [sent-77, score-0.262]
26 the encoding of an image is an average of the individual encodings of its features. [sent-79, score-0.391]
27 Finally, the high-dimensional FV is usually coupled with a one-vs-rest linear SVM classifier, and together they form a conventional image classification pipeline [21] (see Fig. [sent-81, score-0.269]
28 3 Fisher layer The conventional FV representation of an image (Sect. [sent-83, score-0.438]
29 SIFT) into a high-dimensional representation, and then aggregates these encodings into a single vector by global sum-pooling over the whole image (followed by normalisation). [sent-86, score-0.306]
30 This means that the representation describes the image in terms of the local patch features, and can not capture more complex image structures. [sent-87, score-0.329]
31 Deep neural networks are able to model the feature hierarchies by passing an output of one feature computation layer as the input to the next one. [sent-88, score-0.361]
32 We adopt a similar approach here, and devise a feed-forward feature encoding layer (which we term a Fisher layer), which is based on off-the-shelf Fisher vector encoding. [sent-89, score-0.391]
33 The layers can then be stacked into a deep network, which we call a Fisher network. [sent-90, score-0.231]
34 The architecture of the l-th Fisher layer is depicted in Fig. [sent-91, score-0.266]
35 On the input, it receives dl -dimensional features (dl ∼ 102 ), densely computed over multiple scales on a regular image grid. [sent-93, score-0.467]
36 The layer then performs feed-forward feature transformation in three sub-layers. [sent-95, score-0.283]
37 The first one computes semi-local FV encodings by pooling the input features not from the whole image, but from a dense set of semi-local regions. [sent-96, score-0.412]
38 The resulting FVs form a new set of densely sampled features that are more discriminative than the input ones and less local, as they integrate information from larger image areas. [sent-97, score-0.359]
39 2) uses a layer-specific GMM with Kl components, so the dimensionality of each FV is 2Kl dl , which, considering that FVs are computed densely, might be too large for practical applications. [sent-99, score-0.256]
40 Therefore, we decrease FV dimensionality by projection onto hl -dimensional subspace using a discriminatively trained linear projection Wl ∈ Rhl ×2Kl dl . [sent-100, score-0.516]
41 In the second sub-layer, the spatially adjacent features are stacked in a 2 × 2 window, which produces 4hl -dimensional dense feature representation. [sent-103, score-0.267]
42 3 dl Compressed semi-local Fisher local vector encoding mixture of Kl Gaussians hl projection Wl from 2Kldl to hl Spatial stacking (2×2) 4hl L2 norm. [sent-106, score-0.615]
43 Left: the arrows illustrate the data flow through the layer; the dimensionality of densely computed features is shown next to the arrows. [sent-108, score-0.246]
44 Right: spatial pooling (the blue squares) and stacking (the red square) in sub-layers 1 and 2 respectively. [sent-109, score-0.402]
45 2); compared to the regions used in global or spatial pyramid pooling [20], these are smaller and sampled much more densely. [sent-113, score-0.309]
46 As a result, instead of a single FV, describing the whole image, the image is represented by a large number of densely computed semi-local FVs, each of which describes a spatially adjacent set of local features, computed by the previous layer. [sent-114, score-0.303]
47 Thus, the new feature representation can capture more complex image statistics with larger spatial support. [sent-115, score-0.31]
48 The high dimensionality of Fisher vectors, however, brings up the computational complexity issue, as storing and processing thousands of dense FVs per image (each of which is 2Kl dl -dimensional) is prohibitive at large scale. [sent-117, score-0.454]
49 We tackle this problem by employing discriminative dimensionality reduction for high-dimensional FVs, which makes the layer learning procedure supervised. [sent-118, score-0.425]
50 The dimensionality reduction is carried out using a linear projection Wl onto an hl -dimensional subspace. [sent-119, score-0.287]
51 As noted in [8], such encodings require large codebooks to produce discriminative feature representations. [sent-123, score-0.284]
52 2, FV encoders do not require large codebooks, and by employing supervised dimensionality reduction, we can preserve the discriminative ability of FV even after the projection onto a low-dimensional space, similarly to [10]. [sent-126, score-0.229]
53 3), an image is represented as a spatially dense set of low-dimensional multi-scale discriminative features. [sent-129, score-0.311]
54 To capture the spatial structure within each feature’s neighbourhood, we incorporate the stacking sub-layer, which concatenates the spatially adjacent features in a 2 × 2 window (Fig. [sent-131, score-0.425]
55 In practice, the Fisher layer computation is repeated at multiple scales by changing the pooling window size ql (the PCA projection in sub-layer 3 is the same for all scales). [sent-139, score-0.534]
56 This allows a single layer to capture multi-scale statistics, which is different from typical DNN architectures, which use a single pooling window size per layer. [sent-140, score-0.414]
57 The resulting dense multi-scale features, computed by the layer, form the input of the next layer (similarly to the dense multi-scale SIFT features). [sent-141, score-0.383]
58 6 we show that a multi-scale Fisher layer indeed brings an improvement, compared to a fixed pooling window size. [sent-143, score-0.414]
59 4 4 Fisher network Our image classification pipeline, which we coin Fisher network (shown in Fig. [sent-144, score-0.256]
60 1) is constructed by stacking several (at least one) Fisher layers (Sect. [sent-145, score-0.258]
61 3) on top of dense features, such as SIFT or raw image patches. [sent-146, score-0.219]
62 We call this layer the global Fisher layer, and it effectively computes a full-dimensional normalised Fisher vector encoding (the dimensionality reduction stage is omitted since the computed FV is directly used for classification). [sent-148, score-0.542]
63 The final layer is an off-the-shelf ensemble of one-vs-rest binary linear SVMs. [sent-149, score-0.232]
64 Each subsequent Fisher layer is designed to capture more complex, higher-level image statistics, but the competitive performance of shallow FV-based frameworks [21] suggests that low-level SIFT features are already discriminative enough to distinguish between a number of image classes. [sent-152, score-0.768]
65 These image representations are then concatenated to produce a rich, multi-layer image descriptor. [sent-154, score-0.33]
66 1 Learning The Fisher network is trained in a supervised manner, since each Fisher layer (apart from the global layer) depends on discriminative dimensionality reduction. [sent-157, score-0.487]
67 Here we discuss how the (non-global) Fisher layer can be efficiently trained in the large-scale scenario, and introduce two options for the projection learning objective. [sent-159, score-0.346]
68 3, we need to learn a discriminative projection W to significantly reduce the dimensionality of the densely-computed semi-local FVs. [sent-162, score-0.229]
69 One approach to discriminative dimensionality reduction learning consists in finding the projection onto a subspace where the image classes are as linearly separable as possible [10, 31]. [sent-172, score-0.428]
70 This corresponds to the bilinear class scoring function: T vc W ψ, where W is the linear projection which we seek to optimise and vc is the linear model (e. [sent-173, score-0.213]
71 The max-margin optimisation problem for W and the ensemble {vc } takes the following form: max i vc − vc(i) T W ψi + 1, 0 + c =c(i) λ 2 vc c 2 2 + µ W 2 2 F, (2) where ci is the ground-truth class of an image i, λ and µ are the regularisation constants. [sent-176, score-0.286]
72 If a specific target dimensionality is required, PCA dimensionality reduction can be further applied to the classifier scores [10], but in our case we applied PCA after spatial stacking (Sect. [sent-184, score-0.517]
73 This suggests the fast computation procedure: each dl -dimensional input feature xp is first hard-assigned to a Gaussian k based on (3). [sent-194, score-0.31]
74 Then, the corresponding dl -D (1),(2) (k,1) (k,2) differences Φk (p) are computed and projected using small hl ×dl sub-matrices Wl , Wl , which is fast. [sent-195, score-0.218]
75 The algorithm avoids computing high-dimensional FVs, followed by the projection using a large matrix Wl ∈ Rhl ×2Kl dl , which is prohibitive since the number of dense FVs is high. [sent-196, score-0.297]
76 dim-ty reduction classifier scores classifier scores classifier scores bi-convex stacking L2 norm-n top-1 59. [sent-225, score-0.318]
77 11 Table 2: Evaluation of multi-scale pooling and multi-layer image description on the subset of ILSVRC-2010. [sent-233, score-0.289]
78 pooling window size q1 5 {5, 7, 9, 11} {5, 7, 9, 11} pooling stride δ1 1 2 2 multi-layer top-1 61. [sent-239, score-0.339]
79 In our experiments, we used SIFT as the first layer of the network, followed by two Fisher layers (the second one is global, as explained in Sect. [sent-247, score-0.328]
80 Here we quantitatively assess the three sub-layers of a Fisher layer (Sect. [sent-250, score-0.232]
81 We compare the two proposed dimensionality reduction learning schemes (bi-convex learning and classifier scores), and also demonstrate the importance of spatial stacking and L2 normalisation. [sent-252, score-0.396]
82 As can be seen, both spatial stacking and L2 normalisation improve the performance, and dimensionality reduction via projection onto the space of SVM classifier scores performs on par with the projection learnt using the bi-convex formulation (2). [sent-254, score-0.669]
83 From Table 2 it is clear that using multiple pooling window sizes is beneficial compared to a single window size. [sent-260, score-0.229]
84 Also, the multi-layer image descriptor obtained by stacking globally pooled and normalised FVs, computed by the two Fisher layers, outperforms each of these FVs taken separately. [sent-262, score-0.425]
85 We also note that in this experiment, unlike the previous one, both Fisher layers utilized spatial coordinate augmentation of the input features, which leads to a noticeable boost in the shallow baseline performance (from 78. [sent-263, score-0.366]
86 Apart from our Fisher network, multi-scale pooling can be readily employed in convolutional networks. [sent-266, score-0.22]
87 2 Evaluation on ILSVRC-2010 Now that we have evaluated various Fisher layer configurations on a subset of ILSVRC, we assess the performance of our framework on the full ILSVRC-2010 dataset. [sent-268, score-0.232]
88 We use off-the-shelf SIFT and colour features [20] in the feature extraction layer, and demonstrate that significant improvements can be achieved by injecting a single Fisher layer into the conventional FV-based pipeline [23]. [sent-269, score-0.585]
89 The first Fisher layer uses a large number of GMM components Kl , since it was found to be beneficial for shallow FV encodings [23], used here as a baseline. [sent-272, score-0.442]
90 First, we note that the globally pooled Fisher vector, branched out of the first Fisher layer (which effectively corresponds to the conventional FV encoding [23]), results in better accuracy than reported in [23], which validates our implementation. [sent-275, score-0.5]
91 Using the 2nd Fisher layer on top of the 1st one leads to a significant performance improvement. [sent-276, score-0.232]
92 We also specify the dimensionality of SIFT-based image representations. [sent-279, score-0.238]
93 pipeline setting 1st Fisher layer 2nd Fisher layer multi-layer (1st and 2nd Fisher layers) S´ nchez et al. [sent-280, score-0.574]
94 We conclude that injecting a single intermediate layer leads to a significant performance boost (+4. [sent-312, score-0.286]
95 7 Conclusion We have shown that Fisher vectors, a standard image encoding method, are amenable to be stacked in multiple layers, in analogy to the state-of-the-art deep neural network architectures. [sent-327, score-0.448]
96 Adding a single layer is in fact sufficient to significantly boost the performance of these shallow image encodings, bringing their performance closer to the state of the art in the large-scale classification scenario [14]. [sent-328, score-0.491]
97 The fact that off-the-shelf image representations can be simply and successfully stacked indicates that deep schemes may extend well beyond neural networks. [sent-329, score-0.311]
98 Modeling the spatial layout of images beyond spatial a ı pyramids. [sent-500, score-0.236]
99 Beyond spatial pyramids: A new feature extraction framework with dense spatial sampling for image classification. [sent-557, score-0.511]
100 Linear spatial pyramid matching using sparse coding for image classification. [sent-566, score-0.331]
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