jmlr jmlr2013 jmlr2013-86 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Binbin Lin, Xiaofei He, Chiyuan Zhang, Ming Ji
Abstract: We propose a novel local isometry based dimensionality reduction method from the perspective of vector fields, which is called parallel vector field embedding (PFE). We first give a discussion on local isometry and global isometry to show the intrinsic connection between parallel vector fields and isometry. The problem of finding an isometry turns out to be equivalent to finding orthonormal parallel vector fields on the data manifold. Therefore, we first find orthonormal parallel vector fields by solving a variational problem on the manifold. Then each embedding function can be obtained by requiring its gradient field to be as close to the corresponding parallel vector field as possible. Theoretical results show that our method can precisely recover the manifold if it is isometric to a connected open subset of Euclidean space. Both synthetic and real data examples demonstrate the effectiveness of our method even if there is heavy noise and high curvature. Keywords: manifold learning, isometry, vector field, covariant derivative, out-of-sample extension
Reference: text
sentIndex sentText sentNum sentScore
1 EDU Department of Computer Science University of Illinois at Urbana Champaign Urbana, IL 61801, USA Editor: Mikhail Belkin Abstract We propose a novel local isometry based dimensionality reduction method from the perspective of vector fields, which is called parallel vector field embedding (PFE). [sent-8, score-1.055]
2 We first give a discussion on local isometry and global isometry to show the intrinsic connection between parallel vector fields and isometry. [sent-9, score-1.131]
3 The problem of finding an isometry turns out to be equivalent to finding orthonormal parallel vector fields on the data manifold. [sent-10, score-0.768]
4 Therefore, we first find orthonormal parallel vector fields by solving a variational problem on the manifold. [sent-11, score-0.425]
5 Then each embedding function can be obtained by requiring its gradient field to be as close to the corresponding parallel vector field as possible. [sent-12, score-0.605]
6 Keywords: manifold learning, isometry, vector field, covariant derivative, out-of-sample extension 1. [sent-15, score-0.456]
7 However, these linear methods may fail to recover the intrinsic manifold structure when the data manifold is not a low dimensional subspace or an affine manifold. [sent-27, score-0.549]
8 , 2000), locally linear embedding (LLE, Roweis and Saul, 2000), Laplacian eigenmaps (LE, Belkin and Niyogi, 2001), Hessian eigenmaps (HLLE, Donoho and Grimes, 2003) and diffusion maps (Coifman and Lafon, 2006; Lafon and Lee, 2006; Nadler et al. [sent-30, score-0.392]
9 Isomap generalizes MDS to the nonlinear manifold case which tries to preserve pairwise geodesic distances on the data manifold. [sent-32, score-0.385]
10 Isomap is an instance of global isometry based dimensionality reduction techniques, which tries to preserve the distance function or the metric of the manifold globally. [sent-34, score-0.832]
11 HLLE is based on local isometry criterion, which successfully overcomes this problem. [sent-36, score-0.382]
12 MVU can be thought of as an instance of local isometry with additional consideration that the distances between two points that are not neighbors are maximized. [sent-47, score-0.429]
13 Tangent space based methods have also received considerable interest recently, such as local tangent space alignment (LTSA, Zhang and Zha, 2004), manifold charting (Brand, 2003), Riemannian Manifold Learning (RML, Lin and Zha, 2008) and locally smooth manifold learning (LSML, Doll´ r et al. [sent-48, score-0.781]
14 LSML tries to learn smooth tangent spaces of the manifold by proposing a smoothness regularization term of tangent spaces. [sent-54, score-0.742]
15 Vector diffusion maps (VDM, Singer and Wu, 2011) is a much recent work which considers the tangent spaces structure of the manifold to define and preserve the vector diffusion distance. [sent-55, score-0.725]
16 In this paper, we propose a novel dimensionality reduction method, called parallel vector field embedding (PFE), from the perspective of vector fields. [sent-56, score-0.673]
17 We first give a discussion on local isometry and global isometry to show the intrinsic connection between parallel vector fields and isometry. [sent-58, score-1.131]
18 The problem of finding an isometry turns out to be equivalent to finding orthonormal parallel vector fields on the data manifold. [sent-59, score-0.768]
19 Therefore, we first find orthonormal parallel vector fields by minimizing the covariant derivative of a vector field. [sent-60, score-0.659]
20 We then find an embedding function whose gradient field is as close to the parallel field as possible. [sent-61, score-0.56]
21 Naturally, the corresponding embedding consisted 2946 PARALLEL V ECTOR F IELD E MBEDDING of embedding functions preserves the metric of the manifold. [sent-63, score-0.526]
22 Our theoretical study shows that, if the manifold is isometric to a connected open subset of Euclidean space, our method can faithfully recover the metric structure of the manifold. [sent-67, score-0.388]
23 The organization of the paper is as follows: In the next section, we provide a description of the dimensionality reduction problem from the perspectives of isometry and vector fields. [sent-68, score-0.464]
24 A Riemannian metric is a Euclidean inner product g p on each of the tangent space Tp M , where p is a point on the manifold M . [sent-74, score-0.511]
25 In this paper, we only consider manifolds that are diffeomorphic to an open connected subset of Euclidean space like semi-sphere, swiss roll, swiss roll with hole, and so on. [sent-91, score-0.548]
26 1 Local Isometry and Global Isometry With the assumption that the manifold is diffeomorphic to an open subset of Rd , the goal of dimensionality reduction is to preserve the intrinsic geometry of the manifold as much as possible. [sent-93, score-0.711]
27 Here we consider two kinds of isometry, that are, 2947 L IN , H E , Z HANG AND J I local isometry and global isometry. [sent-96, score-0.436]
28 1 In the following we give the definitions and properties of local isometry and global isometry. [sent-97, score-0.436]
29 For a map between manifolds F : M → N , F is called local isometry if h(dFp (v), dFp (v)) = g(v, v) for all p ∈ M , v ∈ Tp M . [sent-99, score-0.47]
30 Intuitively, local isometry preserves the metric of the manifold locally. [sent-110, score-0.7]
31 If the local isometry F is also a diffeomorphism, then it becomes global isometry. [sent-111, score-0.436]
32 Definition 3 (Global Isometry, Lee, 2003) A map F : M → N is called global isometry between manifolds if it is a diffeomorphism and also a local isometry. [sent-112, score-0.596]
33 This is because that, although local isometry maps geodesics to geodesics, the shortest geodesic between two points on M may not be the shortest geodesic on N . [sent-121, score-0.578]
34 On the other hand, based on our assumption that the manifold M is diffeomorphic to an open subset of Rd , it suffices to find a local isometry which is also a diffeomorphism, according to Definition 3. [sent-130, score-0.684]
35 In many differential geometry textbooks, global isometry is often referred to as isometry or Riemannian isometry. [sent-132, score-0.78]
36 2948 PARALLEL V ECTOR F IELD E MBEDDING Figure 1: Local isometry but not global isometry. [sent-133, score-0.397]
37 2 Gradient Fields and Local Isometry Our analysis has shown that finding a global isometry is equivalent to finding a local isometry which is also a diffeomorphism. [sent-138, score-0.779]
38 , fd ) : M → Rd , there is a deep connection between local isometry and the differential dF = (d f1 , . [sent-142, score-0.484]
39 For the relationship between local isometry and differential, we have the following proposition: Proposition 2 Consider a map F : M ⊂ Rm → Rd . [sent-148, score-0.433]
40 Since we use the induced metric for the manifold M , the computation of inner product in tangent space is the same as the standard inner product in Euclidean space. [sent-168, score-0.511]
41 This proposition indicates that finding a local isometry F is equivalent to finding d orthonormal differentials d fi , i = 1, . [sent-175, score-0.596]
42 Parallel Field Embedding In this section, we introduce our parallel field embedding (PFE) algorithm for dimensionality reduction. [sent-183, score-0.549]
43 We first try to find orthonormal parallel vector fields on the manifold. [sent-191, score-0.425]
44 In Theorem 2, we show that if the manifold can be isometrically embedded into the Euclidean space, then there exist orthonormal parallel fields and each parallel field is exactly a gradient field. [sent-193, score-1.028]
45 We will also discuss the relationship among local isometry, global isometry and parallel fields. [sent-197, score-0.71]
46 Definition 4 (Parallel Field, Petersen, 1998) A vector field X on manifold M is a parallel vector field (or parallel field) if ∇X ≡ 0, where ∇ is the covariant derivative on the manifold M . [sent-198, score-1.343]
47 Given a point p on the manifold and a vector v p on the tangent space Tp M , then ∇v p X is a vector at point p which measures how the vector field X changes along the direction v p at point p. [sent-200, score-0.619]
48 For parallel fields, we also have the following proposition: Proposition 3 Let V and W be parallel fields on M associated with the metric g. [sent-203, score-0.575]
49 This corollary tells us if we want to check the orthogonality of the parallel fields at every point, it suffices to compute the integral of the inner product of the parallel fields. [sent-221, score-0.572]
50 Also we have the following corollary: Corollary 2 Let V be a parallel vector field on M , then ∀p ∈ M , Vp = constant where Vp represents the vector at p of the vector field V . [sent-223, score-0.409]
51 Since every tangent vector of a parallel field has a constant length, we can perform normalization 2951 L IN , H E , Z HANG AND J I of the parallel field simply as dividing every tangent vector of the parallel field by a same length. [sent-225, score-1.364]
52 According to these results, finding orthonormal parallel fields becomes much easier: we first find orthogonal parallel fields on the manifold one by one by requiring that the integral of the inner product of two parallel fields is zero. [sent-226, score-1.233]
53 Before presenting our main result, we still need to introduce some concepts and properties on the relationship between isometry and parallel fields. [sent-228, score-0.617]
54 Next we show that the differential of an isometry preserves covariant derivative. [sent-246, score-0.569]
55 More importantly, we show that an isometry preserves parallelism, that is, its differential carries a parallel vector field to another parallel vector field. [sent-248, score-1.054]
56 dF is an isometric isomorphism on the space of parallel fields. [sent-252, score-0.419]
57 Proof Let Y be a parallel field on M , we show that dF(Y ) is also a parallel field. [sent-253, score-0.548]
58 Combining these two facts, dF is an isometric isomorphism on the space of parallel fields. [sent-264, score-0.419]
59 Now we show that the gradient fields of a local isometry are also parallel fields. [sent-265, score-0.709]
60 Proof According to the property of local isometry, for very point p ∈ M , there is a neighborhood U ⊂ M of p such that F|U : U → F(U) is a global isometry of U onto an open subset F(U) of Rd (please see Lee, 2003, pg. [sent-273, score-0.436]
61 This proposition tells us that the gradient field of a local isometry is also a parallel field. [sent-289, score-0.737]
62 Since it is usually not easy to find a global isometry directly, in this paper, we try to find a set of orthonormal parallel fields first, and then find an embedding function whose gradient field is equal to the parallel field. [sent-290, score-1.337]
63 As we discussed in Section 2, the gradient fields of the isometry have to be orthonormal parallel fields. [sent-312, score-0.776]
64 Then the covariant derivative along the direction dγ(t) |t=0 dt can be computed by projecting dV |t=0 to the tangent space Tx M at x. [sent-318, score-0.415]
65 After finding the parallel vector fields Vi on M , the embedding function can be obtained by minimizing the following objective function: Φ( f ) = M ∇ f −V 2 dx. [sent-342, score-0.552]
66 If there exist a global isometry ϕ : M → D ⊂ Rd , where D is an open connected subset of Rd , then there is an orthonormal basis {Vi }d of the parallel fields on M , and embedding function fi : M → R whose i=1 gradient field satisfies ∇ fi = Vi , i = 1, . [sent-346, score-1.223]
67 According to Proposition 5, we know that a global isometry preserves parallelism, that is, its differential carries a parallel vector field to another parallel vector field. [sent-358, score-1.108]
68 Thus for a global isometry ϕ, dϕ maps parallel fields to parallel fields and dϕ is an isometric isomorphism, so is dϕ−1 . [sent-359, score-1.088]
69 Thus the space of parallel fields on M is isomorphic to the space of parallel fields on D. [sent-360, score-0.548]
70 Therefore there exists an orthonormal basis {Vi }d of the space of the parallel fields on M . [sent-361, score-0.38]
71 Next we show that such F is a global isometry on M . [sent-385, score-0.397]
72 Thus F is a local isometry according to Proposition 2. [sent-394, score-0.382]
73 Since F is a local isometry and a diffeomorphism, it is a global isometry. [sent-417, score-0.436]
74 The first variation is the choice of the orthonormal basis of parallel fields. [sent-424, score-0.38]
75 Thus the space of global isometry on M is actually O(d) × Rd . [sent-426, score-0.397]
76 When there is no isometry between the manifold M and Rd , our approach can still find a reasonably good embedding function. [sent-429, score-0.834]
77 It would be important to note that, in such cases, the isometric embedding or nearly isometric embedding does not exist. [sent-435, score-0.672]
78 The implementation includes two steps, first we estimate parallel vector fields on manifold from random points, and then we reconstruct embedding functions by requiring that the gradient fields are as close to the parallel fields as possible. [sent-439, score-1.137]
79 After finding d orthonormal vector fields and the corresponding embedding function fi , the final map F is given by F = ( f1 , . [sent-446, score-0.515]
80 According to the definition of vector fields, Vxi should be a tangent vector in the tangent space Txi M . [sent-472, score-0.542]
81 Thus it can be represented by local coordinates of the tangent space, (5) Vxi = Ti vi , T where vi ∈ Rd . [sent-473, score-0.42]
82 Then the covariant derivative of vector field V along ei j is given by (please see Figure 3) ∇ei j V dV |t=0 ) dt V (γ(t)) −V (γ(0)) = Pi lim t→0 t (Vx j −Vxi ) = Pi di j √ wi j (PiVx j −Vxi ), ≈ = Pi ( √ where di j ≈ 1/ wi j approximates the geodesic distance di j of xi and x j . [sent-486, score-0.461]
83 Recall Corollary 2 tells us for any point p ∈ M , the tangent vector Vp of a parallel field V has a constant length. [sent-529, score-0.545]
84 2 E MBEDDING Once the parallel vector fields Vi are obtained, the embedding functions fi : M → R can be constructed by requiring their gradient fields to be as close to Vi as possible. [sent-543, score-0.685]
85 Recall that, if the manifold is isometric to Euclidean space, then the vector field computed via Equation (1) is also a gradient field. [sent-544, score-0.459]
86 However, if the manifold is not isometric to Euclidean space, V may not be a gradient field. [sent-545, score-0.414]
87 The exponential map expx : Tx M → M maps the tangent space Tx M to the manifold M . [sent-550, score-0.617]
88 , vn+n′ , where Vo denotes the tangent vectors on the n n+1 o n 1 original training points and Vn denotes the tangent vectors on new points. [sent-638, score-0.476]
89 The idea of computing the covariant derivative is to find a way to compute the difference between vectors on different tangent spaces. [sent-693, score-0.415]
90 VDM proposed an intrinsic way to compute covariant derivative using the concept of parallel transport. [sent-694, score-0.496]
91 They first transported the vectors to the same tangent space using the parallel transport, and then compute the difference of vectors on the tangent space. [sent-695, score-0.726]
92 The way of finding the parallel transport between two points is to compute the orthogonal transformation between two corresponding tangent spaces. [sent-696, score-0.573]
93 The data set contains 2000 points sampled from a swiss roll with a hole, which is a 2D manifold embedded in R3 . [sent-711, score-0.608]
94 On the other hand, the swiss roll is a flat manifold with zero-curvature everywhere and thus can be isometrically embedded in R2 . [sent-714, score-0.61]
95 A local isometry preserving embedding is considered faithful and thus would get a low R-score. [sent-787, score-0.615]
96 MVU unfolds the manifold correctly, but does not preserve the isometry well. [sent-809, score-0.643]
97 6 Classification after Embedding In many scenarios, calculating the low-dimensional embedding of the data manifold is not the final step. [sent-985, score-0.491]
98 Conclusion We have introduced a novel local isometry based dimensionality reduction method from the perspective of vector field. [sent-1059, score-0.503]
99 If the manifold is isometric to Euclidean space, then the obtained vector field is parallel. [sent-1072, score-0.406]
100 Local procrustes for manifold embedding: a measure of embedding quality and embedding algorithms. [sent-1165, score-0.724]
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