nips nips2011 nips2011-64 knowledge-graph by maker-knowledge-mining

64 nips-2011-Convergent Bounds on the Euclidean Distance


Source: pdf

Author: Yoonho Hwang, Hee-kap Ahn

Abstract: Given a set V of n vectors in d-dimensional space, we provide an efficient method for computing quality upper and lower bounds of the Euclidean distances between a pair of vectors in V . For this purpose, we define a distance measure, called the MS-distance, by using the mean and the standard deviation values of vectors in V . Once we compute the mean and the standard deviation values of vectors in V in O(dn) time, the MS-distance provides upper and lower bounds of Euclidean distance between any pair of vectors in V in constant time. Furthermore, these bounds can be refined further in such a way to converge monotonically to the exact Euclidean distance within d refinement steps. An analysis on a random sequence of refinement steps shows that the MS-distance provides very tight bounds in only a few refinement steps. The MS-distance can be used to various applications where the Euclidean distance is used to measure the proximity or similarity between objects. We provide experimental results on the nearest and the farthest neighbor searches. 1

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 kr Abstract Given a set V of n vectors in d-dimensional space, we provide an efficient method for computing quality upper and lower bounds of the Euclidean distances between a pair of vectors in V . [sent-3, score-0.395]

2 For this purpose, we define a distance measure, called the MS-distance, by using the mean and the standard deviation values of vectors in V . [sent-4, score-0.208]

3 Once we compute the mean and the standard deviation values of vectors in V in O(dn) time, the MS-distance provides upper and lower bounds of Euclidean distance between any pair of vectors in V in constant time. [sent-5, score-0.501]

4 Furthermore, these bounds can be refined further in such a way to converge monotonically to the exact Euclidean distance within d refinement steps. [sent-6, score-0.249]

5 An analysis on a random sequence of refinement steps shows that the MS-distance provides very tight bounds in only a few refinement steps. [sent-7, score-0.105]

6 The MS-distance can be used to various applications where the Euclidean distance is used to measure the proximity or similarity between objects. [sent-8, score-0.236]

7 We provide experimental results on the nearest and the farthest neighbor searches. [sent-9, score-0.399]

8 1 Introduction The Euclidean distance between two vectors x and y in d-dimensional space is a typical distance measure that reflects their proximity in the space. [sent-10, score-0.402]

9 Measuring the Euclidean distance is a fundamental operation in computer science, including the areas of database, computational geometry, computer vision and computer graphics. [sent-11, score-0.099]

10 In machine learning, the Euclidean distance, denoted by dist(x, y), or it’s variations(for example, e||x−y|| ) are widely used to measure data similarity for clustering [1], classification [2] and so on. [sent-12, score-0.05]

11 Given two sets X and Y of vectors in d-dimensional space, our goal is to find a pair (x, y), for x ∈ X and y ∈ Y , such that dist(x, y) is the optimum (minimum or maximum) over all such pairs. [sent-14, score-0.113]

12 For the nearest or farthest neighbor searches, X is the set consisting of a single query point while Y consists of all candidate data points. [sent-15, score-0.451]

13 In d dimensional space, a single distance computation already takes O(d) time, thus the cost for finding the nearest or farthest neighbor becomes O(dnm) time, where n and m are the cardinalities of X and Y , respectively. [sent-18, score-0.515]

14 1 If we restrict ourselves to the nearest neighbor search, some methods using space partitioning trees such as KD-tree [4], R-tree [5], or their variations have been widely used. [sent-23, score-0.291]

15 Recently, cover tree [6] has been used for high dimensional nearest neighbor search, but its construction time increases drastically as the dimension increases [7]. [sent-25, score-0.383]

16 One of such methods is to compute a distance bound using the inner product approximation [8]. [sent-27, score-0.153]

17 Another method is to compute a distance bound using bitwise operations [9]. [sent-29, score-0.164]

18 In this paper, we define a distance measure, called the MS-distance, by using the mean and the standard deviation values of vectors in V . [sent-33, score-0.208]

19 Once we compute the mean and the standard deviation values of vectors in V in O(dn) time, the MS-distance provides tight upper and lower bounds of Euclidean distance between any pair of vectors in V in constant time. [sent-34, score-0.534]

20 Furthermore, these bounds can be refined further in such a way to converge monotonically to the exact Euclidean distance within d refinement steps. [sent-35, score-0.249]

21 We provide an analysis on a random sequence of k refinement steps for 0 ≤ k ≤ d, which shows a good expectation on the lower and upper bounds. [sent-37, score-0.091]

22 This can justify that the MS-distance provides very tight bounds in a few refinement steps of a typical sequence. [sent-38, score-0.146]

23 The MS-distance can be used to various applications where the Euclidean distance is a measure for proximity or similarity between objects. [sent-41, score-0.236]

24 Among them, we provide experimental results on the nearest and the farthest neighbor searches. [sent-42, score-0.399]

25 , xd ], we denote its mean by µx = d i=1 xi and its d 1 2 2 variance by σx = d i=1 (xi − µx ) . [sent-46, score-0.057]

26 For a pair of vectors x and y, we can reformulate the squared Euclidean distance between x and y as follows. [sent-47, score-0.273]

27 d dist(x, y)2 = (xi − yi )2 i=1 d ((µx + ai ) − (µy + bi ))2 = i=1 d (µ2 + 2ai µx + a2 + µ2 + 2bi µy + b2 − 2(µx µy + ai µy + bi µx + ai bi )) (1) x i y i = i=1 d (µ2 − 2µx µy + µ2 + a2 + b2 − 2ai bi ) x y i i = (2) i=1 d = d (µx − µy )2 + (σx + σy )2 − 2dσx σy − 2 ai bi (3) ai bi . [sent-55, score-1.962]

28 (4) i=1 d = d (µx − µy )2 + (σx − σy )2 + 2dσx σy − 2 i=1 2 d d d 1 2 By the definitions of ai and bi , we have i=1 ai = i=1 bi = 0, and d i=1 a2 = σx . [sent-56, score-0.704]

29 By the first i properties, equation (1) is simplified to (2), and by the second property, equations (2) becomes (3) and (4). [sent-57, score-0.078]

30 Note that equations (3) and (4) are composed of the mean and variance values (their products and squared values, multiplied by d) of x and y, except the last summations. [sent-58, score-0.074]

31 Thus, once we preprocess V of n vectors such that both µx and σx for all x ∈ V are computed in O(dn) time and stored in a table of size O(n), this sum can be computed in constant time for any pair of vectors, regardless of the dimension. [sent-59, score-0.17]

32 d The last summation, i ai bi , is the inner product a, b , and therefore by applying the CauchySchwarz inequality we get d d | a, b | = | ai bi | ≤ d a2 )( i ( i=1 i=1 b2 ) = dσx σy . [sent-60, score-0.755]

33 i (5) i=1 This gives us the following upper and lower bounds of the squared Euclidean distance from equations (3) and (4). [sent-61, score-0.313]

34 Lemma 1 For two d-dimensional vectors x, y, the followings hold. [sent-62, score-0.078]

35 However, in some applications these bounds may not be tight enough. [sent-64, score-0.105]

36 In this section, we introduce the MSdistance which not only provides lower and upper bounds of the Euclidean distance in constant time, but also could be refined further in such a way to converge to the exact Euclidean distance within d steps. [sent-65, score-0.43]

37 To do this, we reformulate the last term of equations (3) and (4), that is, the inner product a, b . [sent-66, score-0.111]

38 If d d d 2 2 the norms ||a|| = i=1 ai or ||b|| = i=1 bi are zero, then i=1 ai bi = 0, thus the upper and lower bounds become the same. [sent-67, score-0.867]

39 This implies that we can compute the exact Euclidean distance in constant time. [sent-68, score-0.144]

40 We can also get equation (10) by i i switching the roles of the term −dσx σy and the term dσx σy in the above equations. [sent-72, score-0.053]

41 Now we define the MS-distance between x and y in its lower bound form, denoted by MSL (x, y, k), by replacing the last term of equation (3) with equation (9), and in its upper bound form, denoted by MSU(x, y, k) by replacing the last term of equation (4) with equation (10). [sent-74, score-0.437]

42 The MS-distance makes use of the nonincreasing intermediate values for its upper bound and the nondecreasing intermediate values for its lower bound. [sent-75, score-0.21]

43 k MSL (x, y, k) = d (µx − µy )2 + (σx − σy )2 + σx σy i=0 k MSU (x, y, k) = d (µx − µy )2 + (σx + σy )2 − σx σy i=0 bi ai − σy σx 2 bi ai + σy σx 2 (11) (12) Properties. [sent-77, score-0.704]

44 Note that equation (11) is nondecreasing and equation (12) is nonincreasing while i ai bi ai bi increases from 0 to d, because d, σx , and σy are all nonnegative, and ( σy − σx )2 and ( σy + σx )2 are also nonnegative for all i. [sent-78, score-0.884]

45 This is very useful because, in equation (11), the first term, MSL(x, y, 0), is already a lower bound of dist(x, y)2 by inequality (6) , and the lower bound can be refined further nondecreasingly over the summation in the second term. [sent-79, score-0.237]

46 If we stop the summation at i = k, for k < d, the intermediate result is also a refined lower bounds of dist(x, y)2 . [sent-80, score-0.204]

47 Similarly, in equation (12), the first term, MSU(x, y, 0), is already an upper bound of dist(x, y)2 by inequality (7) , and the upper bound can be refined further nonincreasingly over the summation in the second term. [sent-81, score-0.259]

48 This means we can stop the summation as soon as we find a bound good enough for the application under consideration. [sent-82, score-0.097]

49 Lemma 2 (Monotone Convergence) Let MSL(x, y, k) and MSU(x, y, k) be the lower and upper bounds of MS-distance as defined above, respectively. [sent-85, score-0.163]

50 Let φ denote a threshold for filtering defined in some proximity search problem under consideration. [sent-94, score-0.147]

51 If φ < MSL(x, y, 0) in case of nearest search or φ > MSL(x, y, 0) in case of farthest search, we do not need to consider this pair (x, y) as a candidate, thus we can save time from computing their exact Euclidean distance. [sent-95, score-0.423]

52 , xd ] into a pair of points, ˆ ˆ ˆ ˆ x+ and x− , in the 2-dimensional plane such that x+ = [µx , σx ] and x− = [µx , −σx ]. [sent-99, score-0.094]

53 x ˆ (13) (14) To see why it is useful in fast filtering, consider the case of finding the nearest vector. [sent-101, score-0.167]

54 For ddimensional vectors in V of size n, we have n pairs of points in the plane as in Figure 1. [sent-102, score-0.13]

55 Let q be a query vector, and let q+ denote the point mapped in the plane as defined above. [sent-104, score-0.113]

56 Among these n points lying on or below µ-axis, let ˆi ˆ x− be the point that is nearest to q+ . [sent-105, score-0.136]

57 Note that the closest point from the query can be computed efficiently in 2-dimensional space, for example, after constructing some space partitioning structures such as kd-trees or R-trees, each query can be answered in poly-logarithmic search time. [sent-106, score-0.182]

58 4 ˆ Then we can ignore all d-dimensional vectors x whose mapped point x+ lies outside the circle ˆ centered at q+ and of radius dist(ˆ + , x− ) in the plane, because they are strictly farther than xi q ˆi from q. [sent-107, score-0.225]

59 All d-dimensional vectors x whose ˆ mapped point x+ lies outside the circle are strictly farther than xi from q. [sent-109, score-0.225]

60 Observe that MSL(x, y, k) is almost the same as MSL(x, y, k − 1) if bk /σy ≈ ak /σx . [sent-111, score-0.076]

61 Hence, in the worst case, MSL(x, y, 0) = MSL(x, y, d − 1) < MSL(x, y, d) = dist(x, y)2 when bk /σy = ak /σx for all k = 0, 1, . [sent-112, score-0.076]

62 Therefore, if we need a lower bound strictly better than MSL (x, y, 0), then we need to go through all d refinement steps, which takes O(d) time. [sent-116, score-0.085]

63 Consider a random order for the last term in equation MSL (x, y, k) and for the last term in equation MSU (x, y, k). [sent-119, score-0.152]

64 We measure the expected quality of the bounds by the difference between the bounds, that is, MSU(x, y, k) − MSL(x, y, k) as follows. [sent-122, score-0.092]

65 k MSU (x, y, k) − MSL (x, y, k) = 4dσx σy − 2σx σy i=0 = 4dσx σy − 2σx σy k d d i=0 aγ(i) σx 2 ai σx 2 + + bi σy 2 bγ(i) σy = 4dσx σy − 4kσx σy = 4σx σy (d − k) (15) 2 (16) (17) (18) Let us explain how we get Equation (16) from (15). [sent-123, score-0.352]

66 d d 2 2 Equations (17) and (18) are because i=1 a2 = dσx and i=1 b2 = dσy by definitions of ai and bi . [sent-128, score-0.352]

67 This shows a good theoretical expectation on the lower and upper bounds. [sent-133, score-0.091]

68 This can justify that the MS-distance provides very tight bounds in a few refinement steps of a typical sequence. [sent-134, score-0.146]

69 5 Applications : Proximity Searches The MS-distance can be used to application problems where the Euclidean distance is a measure for proximity or similarity of objects. [sent-135, score-0.236]

70 As a case study, we implemented the nearest neighbor search (NNS) and the farthest neighbor search (FNS) using the MS-distance. [sent-136, score-0.638]

71 Given a set X of d-dimensional vectors xi , for i = 1, . [sent-137, score-0.111]

72 , n, and a d-dimensional query vector q, we use the following simple randomized algorithm for NNS. [sent-140, score-0.052]

73 Consider the vectors in X one at a time according to this sequence. [sent-143, score-0.098]

74 , d : if MSL(q, xi , j) > φ : break; if j = d: φ = MSL(q, xi , d); NN = i; 2. [sent-148, score-0.066]

75 return NN as the nearest neighbor of q with the squared Euclidean distance φ. [sent-149, score-0.38]

76 Note that the first line of the pseudocodes filters out the vectors whose distance to q is larger than φ as in the fast filtering in Section 3. [sent-150, score-0.247]

77 In the for loop, we compute MSL(q, xi , j) from MSL(q, xi , j −1) in constant time. [sent-151, score-0.083]

78 From the last two lines of the pseudocodes, we update φ to the exact Euclidean distance between q and xi and store the index as the current nearest neighbor (NN). [sent-152, score-0.438]

79 The algorithm for the farthest neighbor search is similar to this one, except that it uses MSU(xi , y, j) and maintains the maximum distance. [sent-153, score-0.323]

80 For empirical comparison, we implemented a linear search algorithm that simply computes distances from q to every xi and chooses the one with the minimum distance. [sent-154, score-0.134]

81 We also used the implementation of the cover tree [6]. [sent-155, score-0.091]

82 A cover tree is a data structure that supports fast nearest neighbor queries given a fixed intrinsic dimensionality [7]. [sent-156, score-0.438]

83 We selected data sets D from various dimensions (from 10 to 100, 000), and randomly selected 30 queries points Q ⊂ D, and queried them on D \ Q. [sent-158, score-0.063]

84 Probably this is because the distances from queries to their nearest vectors tend to converge to the distances to their farthest vectors as described in [13]. [sent-166, score-0.586]

85 However, on such high dimensions, both the linear search and the cover tree algorithm also show poor performance. [sent-168, score-0.151]

86 Figure 3 shows the preprocessing time of the MS-distance and the cover tree for NNS. [sent-169, score-0.135]

87 Cover Figure 3: Preprocessing time for nearest neighbor search in log-scaled second. [sent-175, score-0.335]

88 This is because for the MS-distance it requires only O(dn) time to compute the mean and the standard deviation values. [sent-177, score-0.051]

89 Linear Figure 5: Relative running time for the farthest neighbor search queries, normalized by linear search time. [sent-189, score-0.403]

90 The graph shows the query time that is normalized by the linear search time. [sent-191, score-0.132]

91 It is clear that the filtering algorithm based on the MS-distance beats the linear search algorithm, even on high dimensional data in the results. [sent-192, score-0.077]

92 The cover tree, which is designed exclusively for NNS, shows slightly better query performance than ours. [sent-193, score-0.117]

93 However, the MS-distance is more general and flexible: it supports addition of a new vector to the data set (our data structure) in O(d) time for computing the mean and the standard deviation values of the vector. [sent-194, score-0.068]

94 This is outstanding compared to the linear search algorithm. [sent-198, score-0.06]

95 6 Conclusion We introduce a fast distance bounding technique, called the MS-distance, by using the mean and the standard deviation values. [sent-200, score-0.161]

96 The MS-distance between two vectors provides upper and lower bounds of Euclidean distance between them in constant time, and these bounds converge monotonically to the exact Euclidean distance over iteration. [sent-201, score-0.606]

97 The MS-distance can be used to application problems where the Euclidean distance is a measure for proximity or similarity of objects. [sent-202, score-0.236]

98 The experimental results show that our method is efficient enough even to replace the best known algorithms for proximity searches. [sent-203, score-0.087]

99 Dimensionality reduction and similarity computation by inner g g product approximations. [sent-250, score-0.058]

100 idistance: An adaptive b+-tree based indexing method for nearest neighbor search. [sent-270, score-0.255]


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