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Dimensionality reduction with adaptive graph
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Graph-based dimensionality reduction (DR) methods have been applied successfully in many practical problems, such as face recognition, where graphs play a crucial role in modeling the data distribution or structure. However, the ideal graph is, in practice, difficult to discover. Usually, one needs to construct graph empirically according to various motivations, priors, or assumptions; this is independent of the subsequent DR mapping calculation. Different<br>from the previous works, in this p
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Dimensionality Reduction with Adaptive Graph
1
Lishan Qiao
1
Limei Zhang
1
Songcan Chen
2,*
2
1
Department of Mathematics Science, Liaocheng University, 252000, Liaocheng, P.R. China
3
2
Department of Computer Science and Engineering, Nanjing University of Aeronautics & Astronautics,
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210016, Nanjing, P.R. China
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Abstract:
Graph-based dimensionality reduction (DR) methods have
6
been applied successfully in many practical problems such as face
7
recognition, where graph plays a crucial role with the aim of modeling the
8
data distribution or structure. However, the ideal graph is difficult to be
9
known in practice. Usually, one needs to construct graph empirically
10
according to various motivations, priors or assumptions, which is
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independent of the subsequent DR mapping calculation. In this paper, we
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instead attempt to learn a graph closely linking to DR process, and
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propose an algorithm called Dimensionality Reduction with Adaptive
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Graph (DRAG), whose idea is to, during seeking projection matrix,
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simultaneously learn a graph in the neighborhood of a pre-specified one.
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Moreover, the pre-specified graph is treated as a noisy observation of the
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true one, and the square Frobenius divergence is used to measure their
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difference in the objective function. As a result, we achieve an elegant
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graph update formula which naturally fuses the original and transformed
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data information. In particular, the optimal graph is shown to be a
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weighted sum of the pre-defined graph in the original space and a new
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graph depending on transformed space. Empirical results on several face
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datasets demonstrate the effectiveness of the proposed algorithm.
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Keywords:
Dimensionality reduction; Graph construction; Face
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recognition
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1.
Introduction
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High-dimensional data exist pervasively, especially in pattern
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recognition, machine learning, and some related fields such as image
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*
Corresponding author: Tel: +86-25-84896481 Ext. 12221; Fax: +86-25-84892400; E-mail: s.chen@nuaa.edu.cn
(S. Chen)
2
retrieval, text classification, face verification, and so on. High dimensions
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not only bring about expensive computation cost, but also result in the
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so-called curse of dimensionality [1]. Dimensionality reduction (DR) is a
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direct and effective approach for handling these issues by mapping data
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into a low-dimensional space. To this day, researchers have developed
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abundant DR algorithms [2-7] including linear vs. nonlinear, supervised
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vs. unsupervised, global vs. local categories, etc. We recommend the
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readers refer to [4, 8-9] for detailed developments of DR methods. In this
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paper, we mainly focus on graph-based dimensionality reduction (GDR),
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a kind of recently developed DR methods which reduce data dimensions,
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while using graph as a tool to preserve some properties of the original
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data.
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At present, GDR has been becoming increasingly popular due to its
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flexibility, effectiveness and tractability. The popular GDR methods
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include ISOMAP [10], LLE [11], and Laplacian Eignmap [12], etc.,
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which are non-linear and deal with the data lying on or nearby a manifold.
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Empirical research [13] has shown that “such nonlinear techniques
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perform well on selected artificial tasks, but that this strong performance
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does not necessarily extend to real-world tasks”. More recently, there
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appear linear versions of GDR methods, such as Locality Preserving
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Projections (LPP) [2] and Neighborhood Preserving Embedding (NPE)
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[3], which are verified to be more effective than their non-linear
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counterparts in many practical applications [14]. In fact, current study [4]
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has shown that most DR algorithms even including classical PCA and
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LDA, can be unified within a common graph embedding framework
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.
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Moreover, different methods only correspond to different intrinsic and
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penalty graphs, and thus, a “good” graph is essentially important to their
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1
Of course, there exist many DR methods, such as independent component analysis, self-organizing feature
mapping, principal curve, which currently can not be nicely interpreted under such a framework. However, those
methods go beyond the main focus of this paper.
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success.
1
However, in practice such a graph is generally unknown, and one needs
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to specify graph empirically or learn it from data. For example, the
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often-used graphs have k nearest neighbor graph and
ball
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neighborhood graph, where some parameters, like k and
, are selected
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empirically [15] or via neighborhood optimization [16]. More recently,
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some researchers addressed graph construction by some novel strategies
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including minimal spanning tree [17], b-matching [18], sparse
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representation [19-21], and so on, but generally speaking, constructing a
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high-quality graph is still an open problem[22].
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In addition, a common point in the aforementioned strategies is that the
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graph constructing process is independent of the subsequent tasks such as
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projection learning for DR here. Instead, a recently-proposed DR
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algorithm called Graph-optimized Locality Preserving Projections
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(GoLPP) [23] attempts to learn a graph and a corresponding projection
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matrix simultaneously in one single objective function, resulting in an
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automatically-updated graph. Comparing with classical LPP which is
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based on a pre-specified neighborhood graph, GoLPP has been
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demonstrated to have superior performance on some public datasets [23].
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However, the optimal graph in GoLPP depends heavily on the
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transformed data, without making the best use of the original data
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information. This easily leads to the instability of performance, especially
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when encountering a “bad” projection/transform matrix. Figure 4 in
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section 4 gives an intuitive illustration to this point.
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To address this problem, in this paper, we propose a novel linear GDR
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algorithm called
D
imensionality
R
eduction with
A
daptive
G
raph
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(DRAG). The proposed method shares the graph optimization advantage
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of GoLPP, while the original data information is embedded into the graph
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optimization process. More concretely, during seeking a projection matrix,
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