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68 nips-2004-Face Detection --- Efficient and Rank Deficient


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Author: Wolf Kienzle, Matthias O. Franz, Bernhard Schölkopf, Gökhan H. Bakir

Abstract: This paper proposes a method for computing fast approximations to support vector decision functions in the field of object detection. In the present approach we are building on an existing algorithm where the set of support vectors is replaced by a smaller, so-called reduced set of synthesized input space points. In contrast to the existing method that finds the reduced set via unconstrained optimization, we impose a structural constraint on the synthetic points such that the resulting approximations can be evaluated via separable filters. For applications that require scanning large images, this decreases the computational complexity by a significant amount. Experimental results show that in face detection, rank deficient approximations are 4 to 6 times faster than unconstrained reduced set systems. 1

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 de Abstract This paper proposes a method for computing fast approximations to support vector decision functions in the field of object detection. [sent-4, score-0.273]

2 In the present approach we are building on an existing algorithm where the set of support vectors is replaced by a smaller, so-called reduced set of synthesized input space points. [sent-5, score-0.18]

3 In contrast to the existing method that finds the reduced set via unconstrained optimization, we impose a structural constraint on the synthetic points such that the resulting approximations can be evaluated via separable filters. [sent-6, score-0.785]

4 For applications that require scanning large images, this decreases the computational complexity by a significant amount. [sent-7, score-0.171]

5 Experimental results show that in face detection, rank deficient approximations are 4 to 6 times faster than unconstrained reduced set systems. [sent-8, score-0.913]

6 1 Introduction It has been shown that support vector machines (SVMs) provide state-of-the-art accuracies in object detection. [sent-9, score-0.12]

7 When classifying image patches of size h × w using plain gray value features, the decision function requires an h · w dimensional dot product for each SV. [sent-16, score-0.459]

8 As an example, the evaluation of a single 20 × 20 patch on a 320 × 240 image at 25 frames per second already requires 660 million operations per second. [sent-18, score-0.419]

9 In the past, research towards speeding up kernel expansions has focused exclusively on the first issue, i. [sent-19, score-0.106]

10 In [2], Burges introduced a method that, for a given SVM, creates a set of so-called reduced set vectors (RSVs) that approximate the decision function. [sent-22, score-0.214]

11 This approach has been successfully applied in the image classification domain — speedups on the order of 10 to 30 have been reported [2, 3, 4] while the full accuracy was retained. [sent-23, score-0.224]

12 Additionally, for strongly unbalanced classification problems such as face detection, the average number of RSV evaluations can be further reduced using cascaded classifiers [5, 6, 7]. [sent-24, score-0.327]

13 While this could be remedied by switching to a sparser image representation (e. [sent-29, score-0.124]

14 a wavelet basis), one could argue that in connection with SVMs, not only are plain gray values straightforward to use, but they have shown to outperform Haar wavelets and gradients in the face detection domain [8]. [sent-31, score-0.357]

15 In this paper, we develop a method that combines the simplicity of gray value correlations with the speed advantage of more sophisticated image representations. [sent-33, score-0.279]

16 To this end, we borrow an idea from image processing: by constraining the RSVs to have a special structure, they can be evaluated via separable convolutions. [sent-34, score-0.4]

17 linear, polynomial, Gaussian and sigmoid) and decreases the average computational complexity of the RSV evaluations from O(h · w) to O(r · (h + w)), where r is a small number that allows the user to balance between speed and accuracy. [sent-37, score-0.181]

18 To evaluate our approach, we examine the performance of these approximations on the MIT+CMU face detection database (used in [10, 8, 5, 6]). [sent-38, score-0.407]

19 2 Burges’ method for reduced set approximations The present section briefly describes Burges’ reduced set method [2] on which our work is based. [sent-39, score-0.364]

20 For reasons that will become clear below, h × w image patches are written as h × w matrices (denoted by bold capital letters) whose entries are the respective pixel intensities. [sent-40, score-0.302]

21 The decision rule for a test pattern X reads m f (X) = sgn yi αi k(Xi , X) + b . [sent-50, score-0.139]

22 (1) i=1 In SVMs, the decision surface induced by f corresponds to a hyperplane in the reproducing kernel Hilbert space (RKHS) associated with k. [sent-51, score-0.143]

23 The corresponding normal m yi αi k(Xi , ·) Ψ= (2) i=1 can be approximated using a smaller, so-called reduced set (RS) {Z1 , . [sent-52, score-0.113]

24 3 From separable filters to rank deficient reduced sets We now describe the concept of separable filters in image processing and show how this idea extends to a broader class of linear filters and to a special class of nonlinear filters, namely those used by SVM decision functions. [sent-64, score-1.05]

25 Using the image-matrix notation, it will become clear that the separability property boils down to a matrix rank constraint. [sent-65, score-0.366]

26 1 Linear separable filters Applying a linear filter to an image amounts to a two-dimensional convolution of the image with the impulse response of the filter. [sent-67, score-0.632]

27 (6) If H has size h × w, the convolution requires O(h · w) operations for each output pixel. [sent-71, score-0.197]

28 However, in special cases where H can be decomposed into two column vectors a and b, such that H = ab (7) holds, we can rewrite (6) as J = [I ∗ a] ∗ b , (8) since the convolution is associative and in this case, ab = a ∗ b . [sent-72, score-0.11]

29 This splits the original problem (6) into two convolution operations with masks of size h×1 and 1×w, respectively. [sent-73, score-0.197]

30 As a result, if a linear filter is separable in the sense of equation (7), the computational complexity of the filtering operation can be reduced from O(h · w) to O(h + w) per pixel by computing (8) instead of (6). [sent-74, score-0.381]

31 2 Linear rank deficient filters In view of (7) being equivalent to rank(H) ≤ 1, we now generalize the above concept to linear filters with low rank impulse responses. [sent-76, score-0.743]

32 Consider the singular value decomposition (SVD) of the h × w matrix H, H = USV , (9) and recall that U and V are orthogonal matrices of size h × h and w × w, respectively, whereas S is diagonal (the diagonal entries are the singular values) and has size h × w. [sent-77, score-0.474]

33 Due to rank(S) = rank(H), we may write H as a sum of r rank one matrices r s i u i vi H= (10) i=1 where si denotes the ith singular value of H and ui , vi are the iths columns of U and V (i. [sent-79, score-0.596]

34 As a result, the corresponding linear filter can be evaluated (analogously to (8)) as the weighted sum of r separable convolutions r J= si [I ∗ ui ] ∗ vi (11) i=1 and the computational complexity drops from O(h × w) to O(r · (h + w)) per output pixel. [sent-82, score-0.398]

35 In SVMs this amounts to using a kernel of the form k(H, X) = g(c(H, X)). [sent-88, score-0.142]

36 (12) If H has rank r, we may split the kernel evaluation into r separable correlations plus a scalar nonlinearity. [sent-89, score-0.651]

37 As a result, if the RSVs in a kernel expansion such as (5) satisfy this constraint, the average computational complexity decreases from O(m · h · w) to O(m · r · (h + w)) per output pixel. [sent-90, score-0.221]

38 While linear, polynomial and sigmoid kernels are defined as functions of input space dot products and therefore immediately satisfy equation (12), the idea applies to kernels based on the Euclidian distance as well. [sent-92, score-0.135]

39 For instance, the Gaussian kernel reads k(H, X) = exp(γ(c(X, X) − 2c(H, X) + c(H, H))). [sent-93, score-0.104]

40 (13) Here, the middle term is the correlation which we are going to evaluate via separable filters. [sent-94, score-0.224]

41 The last term is merely a constant scalar independent of the image data. [sent-96, score-0.155]

42 4 (14) 2 F F , (15) is the Frobenius norm for Finding rank deficient reduced sets In our approach we consider a special class of the approximations given by (3), namely those where the RSVs can be evaluated efficiently via separable correlations. [sent-99, score-0.858]

43 In particular, we restrict the RSV search space to the manifold spanned by all image patches that — viewed as matrices — have a fixed, small rank r (which is to be chosen a priori by the user). [sent-101, score-0.633]

44 (16) The rank constraint can then be imposed by allowing only the first r diagonal elements of Si to be non-zero. [sent-103, score-0.365]

45 The minimization problem is solved via 2 In this paper we call a non-square matrix orthogonal if its columns are pairwise orthogonal and have unit length. [sent-106, score-0.2]

46 the components of the decomposed RSV image patches, i. [sent-111, score-0.166]

47 Intuitively, this amounts to rotating (rather than translating) the columns of Ui,r and Vi,r , which ensures that the resulting matrices are still orthogonal, i. [sent-117, score-0.175]

48 The first part illustrates the behavior of rank deficient approximations for a face detection SVM in terms of the convergence rate and classification accuracy for different values of r. [sent-127, score-0.827]

49 In the second part, we show how an actual face detection system, similar to that presented in [5], can be sped up using rank deficient RSVs. [sent-128, score-0.6]

50 It consisted of 19 × 19 gray level image patches containing 16081 manually collected faces (3194 of them kindly provided by Sami Romdhani) and 42972 non-faces automatically collected from a set of 206 background scenes. [sent-130, score-0.342]

51 The set was split into a training set (13331 faces and 35827 non-faces) and a validation set (2687 faces and 7145 non-faces). [sent-132, score-0.166]

52 The resulting decision function (1) achieved a hit rate of 97. [sent-135, score-0.108]

53 0% false positives on the validation set using m = 6910 SVs. [sent-137, score-0.233]

54 1 Rank deficient faces In order to see how m and r affect the accuracy of our approximations, we compute rank deficient reduced sets for m = 1 . [sent-140, score-0.549]

55 3 (the left array in Figure 1 illustrates the actual appearance of rank deficient RSVs for the m = 6 case). [sent-146, score-0.411]

56 Accuracy of the resulting decision functions is measured in ROC score (the area under the ROC curve) on the validation set. [sent-147, score-0.196]

57 The results for our approximations are depicted in Figure 2. [sent-150, score-0.138]

58 As expected, we need a larger number of rank deficient RSVs than unconstrained RSVs to obtain similar classification accuracies, especially for small r. [sent-151, score-0.521]

59 First, a rank as m' = 1 r=ful + + r=3 = + = + . [sent-153, score-0.331]

60 The left array shows the RSVs (Zi ) of the unconstrained (top row) and constrained (r decreases from 3 to 1 down the remaining rows) approximations for m = 6. [sent-157, score-0.413]

61 This supports the fact that the classification accuracy for r = 3 is similar to that of the unconstrained approximations (cf. [sent-159, score-0.378]

62 The right array shows the m = 1 RSVs (r = full, 3, 2, 1, top to bottom row) and their decomposition into rank one matrices according to (10). [sent-161, score-0.444]

63 For the unconstrained RSV (first row) it shows an approximate (truncated) expansion based on the three leading singular vectors. [sent-162, score-0.305]

64 This illustrates that the approach is clearly different from simply finding unconstrained RSVs and then imposing the rank constraint via SVD (in fact, the norm (4) is smaller for the r = 1 RSV than for the leading singular vector of the r = full RSV). [sent-164, score-0.729]

65 low as three seems already sufficient for our face detection SVM in the sense that for equal sizes m there is no significant loss in accuracy compared to the unconstrained approximation (at least for m > 2). [sent-165, score-0.565]

66 The associated speed benefit over unconstrained RSVs is shown in the right plot of Figure 2: the rank three approximations achieve accuracies similar to the unconstrained functions, while the number of operations reduces to less than a third. [sent-166, score-1.101]

67 2 A cascade-based face detection system In this experiment we built a cascade-based face detection system similar to [5, 6], i. [sent-170, score-0.612]

68 a cascade of RSV approximations of increasing size m . [sent-172, score-0.219]

69 As the benefit of a cascaded classifier heavily depends on the speed of the first classifier which has to be evaluated on the whole image [5, 6], our system uses a rank deficient approximation as the first stage. [sent-173, score-0.681]

70 9 using 114 multiply-adds, whereas the simplest possible unconstrained approximation m = 1, r = full needs 361 multiply-adds to achieve a ROC score of only 0. [sent-176, score-0.364]

71 In particular, if the threshold of the first stage is set to yield a hit rate of 95% on the validation set, scanning the MIT+CMU set (130 images, 507 faces) with m = 3, r = 1 discards 91. [sent-179, score-0.178]

72 5% of the false positives, whereas the m = 1, r = full can only reject 70. [sent-180, score-0.155]

73 At the same time, when scanning a 320 × 240 image 3 , the three separable convolutions plus nonlinearity require 55ms, whereas the single, full kernel evaluation takes 208ms on a Pentium 4 with 2. [sent-182, score-0.62]

74 Moreover, for the unconstrained 3 For multi-scale processing the detectors are evaluated on an image pyramid with 12 different scales using a scale decay of 0. [sent-184, score-0.366]

75 This amounts to scanning 140158 patches for a 320 × 240 image. [sent-186, score-0.267]

76 6 2 10 3 10 4 10 #operations (m'⋅r⋅(h+w)) Figure 2: Effect of the rank parameter r on classification accuracies. [sent-195, score-0.331]

77 The left plots shows the ROC score of the rank deficient RSV approximations (cf. [sent-196, score-0.535]

78 Additionally, the solid line shows the accuracy of the RSVs without rank constraint (cf. [sent-204, score-0.381]

79 The right plot shows the same four curves, but plotted against the number of operations needed for the evaluation of the corresponding decision function when scanning large images (i. [sent-206, score-0.318]

80 Figure 3: A sample output from our demonstration system (running at 14 frames per second). [sent-209, score-0.112]

81 In this implementation, we reduced the number of false positives by adjusting the threshold of the final classifier. [sent-210, score-0.29]

82 cascade to catch up in terms of accuracy, the (at least) m = 2, r = full classifier (also with an ROC score of roughly 0. [sent-213, score-0.164]

83 The subsequent stages of our system consist of unconstrained RSV approximations of size m = 4, 8, 16, 32, respectively. [sent-216, score-0.398]

84 These sizes were chosen such that the number of false positives roughly halves after each stage, while the number of correct detections remains close to 95% on the validation set (with the decision thresholds adjusted accordingly). [sent-217, score-0.419]

85 To eliminate redundant detections, we combine overlapping detections via averaging of position and size if they are closer than 0. [sent-218, score-0.151]

86 The application now classifies a 320x240 image within 54ms (vs. [sent-225, score-0.124]

87 Note that this will not significantly affect the speed of our system (currently 14 frames per second) since 0. [sent-229, score-0.179]

88 034% false positives amounts to merely 47 patches to be processed by subsequent classifiers. [sent-230, score-0.387]

89 6 Discussion We have presented a new reduced set method for SVMs in image processing, which creates sparse kernel expansions that can be evaluated via separable filters. [sent-231, score-0.646]

90 To this end, the user-defined rank (the number of separable filters into which the RSVs are decomposed) provides a mechanism to control the tradeoff between accuracy and speed of the resulting approximation. [sent-232, score-0.638]

91 Our experiments show that for face detection, the use of rank deficient RSVs leads to a significant speedup without losing accuracy. [sent-233, score-0.503]

92 Especially when rough approximations are required, our method gives superior results compared to the existing reduced set methods since it allows for a finer granularity which is vital in cascade-based detection systems. [sent-234, score-0.413]

93 At run-time, rank deficient RSVs can be used together with unconstrained RSVs or SVs using the same canonical image representation. [sent-236, score-0.645]

94 In addition, our approach allows the use of off-the-shelf image processing libraries for separable convolutions. [sent-238, score-0.314]

95 Since such operations are essential in image processing, there exist many (often highly optimized) implementations. [sent-239, score-0.22]

96 to go directly from the training data to a sparse, separable function as opposed to taking the SVM ’detour’. [sent-242, score-0.19]

97 Improving the accuracy and speed of support vector machines. [sent-267, score-0.15]

98 Training support vector machines: an application to face detection. [sent-279, score-0.174]

99 Rapid object detection using a boosted cascade of simple features. [sent-291, score-0.204]

100 [12] RDRSLIB – a matlab library for rank deficient reduced sets in object detection, http://www. [sent-324, score-0.472]


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