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311 nips-2012-Shifting Weights: Adapting Object Detectors from Image to Video


Source: pdf

Author: Kevin Tang, Vignesh Ramanathan, Li Fei-fei, Daphne Koller

Abstract: Typical object detectors trained on images perform poorly on video, as there is a clear distinction in domain between the two types of data. In this paper, we tackle the problem of adapting object detectors learned from images to work well on videos. We treat the problem as one of unsupervised domain adaptation, in which we are given labeled data from the source domain (image), but only unlabeled data from the target domain (video). Our approach, self-paced domain adaptation, seeks to iteratively adapt the detector by re-training the detector with automatically discovered target domain examples, starting with the easiest first. At each iteration, the algorithm adapts by considering an increased number of target domain examples, and a decreased number of source domain examples. To discover target domain examples from the vast amount of video data, we introduce a simple, robust approach that scores trajectory tracks instead of bounding boxes. We also show how rich and expressive features specific to the target domain can be incorporated under the same framework. We show promising results on the 2011 TRECVID Multimedia Event Detection [1] and LabelMe Video [2] datasets that illustrate the benefit of our approach to adapt object detectors to video. 1

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 edu Abstract Typical object detectors trained on images perform poorly on video, as there is a clear distinction in domain between the two types of data. [sent-3, score-0.501]

2 In this paper, we tackle the problem of adapting object detectors learned from images to work well on videos. [sent-4, score-0.413]

3 We treat the problem as one of unsupervised domain adaptation, in which we are given labeled data from the source domain (image), but only unlabeled data from the target domain (video). [sent-5, score-1.064]

4 Our approach, self-paced domain adaptation, seeks to iteratively adapt the detector by re-training the detector with automatically discovered target domain examples, starting with the easiest first. [sent-6, score-1.271]

5 At each iteration, the algorithm adapts by considering an increased number of target domain examples, and a decreased number of source domain examples. [sent-7, score-0.734]

6 To discover target domain examples from the vast amount of video data, we introduce a simple, robust approach that scores trajectory tracks instead of bounding boxes. [sent-8, score-1.378]

7 We also show how rich and expressive features specific to the target domain can be incorporated under the same framework. [sent-9, score-0.513]

8 1 Introduction Following recent advances in learning algorithms and robust feature representations, tasks in video understanding have shifted from classifying simple motions and actions [3, 4] to detecting complex events and activities in Internet videos [1,5,6]. [sent-11, score-0.632]

9 Because many events are characterized by key objects and their interactions, it is imperative to have robust object detectors that can provide accurate detections. [sent-13, score-0.397]

10 However, as seen in Figure 1, the domain of images and videos is quite different, as it is often the case that images of objects are taken in controlled settings that differ greatly from where they appear in real-world situations, as seen in video. [sent-17, score-0.511]

11 To adapt object detectors from image to video, we take an incremental, self-paced approach to learn from the large amounts of unlabeled video data available. [sent-19, score-0.911]

12 We make the assumption that within our unlabeled video data, there exist instances of our target object. [sent-20, score-0.773]

13 However, we do not assume that every video has an instance of the object, due to the noise present in Internet videos. [sent-21, score-0.411]

14 We start by introducing a simple, robust method for discovering examples in the video data using KanadeLucas-Tomasi (KLT) feature tracks [8,9]. [sent-22, score-0.688]

15 This is done by iteratively including examples from the video data into the training set, while removing examples from the image data based on the difficulty of the examples. [sent-25, score-0.763]

16 In addition, it is common to have discriminative features that are only available in the target domain, which we term target features. [sent-29, score-0.583]

17 For example, in the video domain, there are contextual features in the spatial and temporal vicinity of our detected object that we can take advantage of when performing detection. [sent-30, score-0.706]

18 2 Related Work Most relevant are works that also deal with adapting detectors to video [10–13], but these works typically deal with a constrained set of videos and limited object classes. [sent-32, score-0.968]

19 The work of [14] deals with a similar problem, but they adapt detectors from video to image. [sent-33, score-0.59]

20 More similar to our method are approaches based on optimizing Support Vector Machine (SVM) related objectives [19–24] or joint cost functions [25], that treat the features as fixed and seek to adapt parameters of the classifier from source to target domain. [sent-37, score-0.493]

21 However, with the exception of [18, 25], previous works deal with supervised or semi-supervised domain adaptation, which require labeled data in the target domain to generate associations between the source and target domains. [sent-38, score-1.023]

22 In our setting, unsupervised domain adaptation, the target domain examples are unlabeled, and we must simultaneously discover and label examples in addition to learning parameters. [sent-39, score-0.926]

23 The objective we optimize to learn our detector draws inspiration from [26–28], in which we include and exclude the loss of certain examples using binary-valued indicator variables. [sent-40, score-0.395]

24 However, our method is different from [26] in that we have three sets of weights that govern the source examples, target examples, and target features. [sent-42, score-0.674]

25 The weights are annealed in different directions, giving us the flexibility to iteratively include examples from the target domain, exclude examples from the source domain, and include parameters for the target features. [sent-43, score-1.024]

26 We assume that we are given a large amount of unlabeled video data with positive instances of our object class within some of these videos. [sent-51, score-0.671]

27 We start by initializing our detector using image positives and negatives (Step 1). [sent-53, score-0.736]

28 We then proceed to enter a loop in which we discover the top K video positives and negatives (Step 2), re-train our detector using these (Step 3), and then update the annealed parameters of the algorithm (Step 4). [sent-54, score-1.269]

29 We initialize our detector (Step 1 of Figure 2) by training a classifier on the labeled image positives and negatives, which we denote by our dataset (hx1 , y1 i, . [sent-55, score-0.59]

30 Our goal then is to discover the top K positive and negative examples from the unlabeled videos, and to use these examples to help re-train our detector. [sent-61, score-0.5]

31 We do not attempt to discover all instances, but simply a sufficient quantity to help adapt our detector to the video domain. [sent-62, score-0.793]

32 To discover the top K video positives and negatives (Step 2 of Figure 2), we utilize the strong prior of temporal continuity and score trajectory tracks instead of bounding boxes, which we describe in Section 3. [sent-63, score-1.328]

33 Given the discovered examples, we optimize a novel objective inspired by self-paced learning [26] that simultaneously selects easy examples and trains a new detector (Step 3 of Figure 2). [sent-65, score-0.418]

34 1 Discovering Examples in Video In this step of the algorithm, we are given weights w of an object detector that can be used to score bounding boxes in video frames. [sent-68, score-1.107]

35 A naive approach would run our detector on frames of video, taking the highest scoring and lowest scoring bounding boxes as the top K video positives and negatives. [sent-69, score-1.311]

36 An object that appears in one frame of a video is certain to appear close in neighboring frames as well. [sent-71, score-0.704]

37 We obtain tracks by running a KLT tracker on our videos, which tracks a sparse set of features over large periods of time. [sent-74, score-0.397]

38 Because of the large number of unlabeled videos we have, we elect to extract KLT tracks rather than computing dense tracks using optical flow. [sent-75, score-0.603]

39 Note that the number of bounding boxes in B is only dependent on the dimensions of the detector and the scales we search over. [sent-81, score-0.445]

40 The score bs is computed by pooling i scores of the bounding box along multiple points of the track in time. [sent-82, score-0.388]

41 max After scoring each track in our unlabeled videos, we select the top and bottom few scoring tracks, and extract bounding boxes from each using the associated box coordinates (bx , by ) to get our max max top K video positives and negatives. [sent-85, score-1.346]

42 For each box, we average the scores at each point along the track, and take the i i box with the maximum score as the score and associated bounding box coordinates for this track. [sent-89, score-0.47]

43 2 Self-Paced Domain Adaptation In this step of the algorithm, we are given the discovered top K video positives and negatives, which we denote by the dataset (hz1 , h1 i, . [sent-95, score-0.77]

44 Ideally, we would like to re-train with a set of easier examples whose labels we are confident of first, and then re-discover video examples with this new detector. [sent-105, score-0.649]

45 By repeating this process, we can avoid bad examples and iteratively refine our set of top K video positives and negatives before having to train with all of them. [sent-107, score-1.099]

46 Formulating this intuition, our algorithm selects easier examples to learn from in the discovered video examples, and simultaneously selects harder examples in the image examples to stop learning from. [sent-108, score-0.888]

47 The number of examples selected from the video examples and image examples are governed by weights that will be annealed over iterations (Step 4 of Figure 2). [sent-110, score-0.976]

48 To prevent the algorithm from assigning all examples to be difficult, we introduce parameters K source and K target that control the number of examples considered from the source and target domain, respectively. [sent-120, score-0.982]

49 (wt+1 , v t+1 , ut+1 ) = arg min r(w) + C w,v,u n ⇣X vi Loss(xi , yi ; w) + i=1 1 K source n X vi i=1 1 K target k X j=1 uj ! [sent-121, score-0.404]

50 k X j=1 uj Loss(zj , hj ; w) ⌘ (2) If K target is large, the algorithm prefers to consider only easy target examples with a small Loss(·), and the same is true for K source . [sent-122, score-0.774]

51 In the annealing of the weights for the algorithm (Step 4 of Figure 2), we decrease K target and increase K source to iteratively include more examples from the target domain and decrease examples from the source domain. [sent-123, score-1.267]

52 Leveraging target features Often, the target domain we are adapting to has additional features we can take advantage of. [sent-127, score-0.938]

53 However, as we iteratively adapt to the target domain and build more confidence in our detector, we can start utilizing these target features to help with detection. [sent-129, score-0.884]

54 We assume there are a set of features that are shared between the source and target domains as = [ shared shared , and a set of target domain-only features as target : target ]. [sent-131, score-1.287]

55 Since the source data doesn’t have target features, we initialize those features to be 0 so that wtarget doesn’t affect the loss on the source data. [sent-133, score-0.711]

56 The new objective function is formulated as: (wt+1 , v t+1 , ut+1 ) = arg min r(w) + C w,v,u n ⇣X vi Loss(xi , yi ; w) + i=1 + 1 K f eat ||wtarget ||1 k X uj Loss(zj , hj ; w) j=1 1 K source n X i=1 vi 1 K target k X j=1 uj ! [sent-134, score-0.475]

57 To anneal the weights for target features, we increase K f eat to iteratively reduce the L1 norm on the target features so that wtarget can become non-zero. [sent-136, score-0.827]

58 Intuitively, we are forcing the weights w to only use shared features first, and to consider more target features when we have a better model of the target domain. [sent-137, score-0.715]

59 4 Experiments We present experimental results for adapting object detectors on the 2011 TRECVID Multimedia Event Detection (MED) dataset [1] and LabelMe Video [2] dataset. [sent-141, score-0.381]

60 The detection scores are computed on annotated video frames from the respective video datasets that are disjoint from the unlabeled videos used in the adapting stage. [sent-145, score-1.431]

61 The spatial features are taken to be HOG features bordering the object with dimensions half the size of the object bounding box. [sent-150, score-0.58]

62 To isolate the effects of adaptation and better analyze our method, we restrict our experiments to the setting in which we fix the video negatives, and focus our problem on adapting from the labeled image positives to the unlabeled video positives. [sent-153, score-1.523]

63 This scenario is realistic and commonly seen, as we can easily obtain video negatives by sampling from a set of unlabeled or weakly-labeled videos. [sent-154, score-0.706]

64 For the K target and K source weights, we set values for the first and final iterations, and linearly interpolate values for the remaining iterations in between. [sent-238, score-0.409]

65 For the K target weight, we estimate the weights so that we start by considering only the video examples that have no loss, and end with all video examples considered. [sent-239, score-1.362]

66 For the target features, we set the algorithm to allow target features at the midpoint of total iterations. [sent-241, score-0.583]

67 Model selection The free model parameters that can be varied are the number of top K examples to discover, the ending K source weight, and whether or not to use target features. [sent-243, score-0.54]

68 In our results, we perform model selection by comparing the distribution of scores on the discovered video positives. [sent-244, score-0.538]

69 The distributions are compared between the initial models from iteration 1 for different model parameters to select K and K source , and between the final iteration 5 models for different model parameters to determine the use of target features. [sent-245, score-0.372]

70 This allows us to evaluate the strength of the initial model trained on the image positives and video negatives, as well as our final adapted model. [sent-246, score-0.723]

71 2 Baseline Comparisons InitialBL This baseline is the intial detector trained only on image positives and video negatives. [sent-249, score-1.037]

72 VideoPosBL This baseline uses the intial detector to discover the top K video positives from the unlabeled video, then trains with all these examples without iterating. [sent-250, score-1.33]

73 Thus, it incorporates our idea of discovering video positives by scoring tracks and re-training, but does not use self-paced domain adaptation for learning weights. [sent-251, score-1.193]

74 This is a state-of-the-art method for unsupervised domain adaptation [18] that models the domain shift in feature space. [sent-255, score-0.509]

75 Since we are not given labels in the target domain, most previous methods for domain adaptation cannot be applied to our setting. [sent-256, score-0.579]

76 3 TRECVID MED The 2011 TRECVID MED dataset [1] consists of a collection of Internet videos collected by the Linguistic Data Consortium from various Internet video hosting sites. [sent-262, score-0.587]

77 There are a total of 15 complex events, and videos are labeled with either an event class or no label, where an absence of label indicates the video belongs to no event class. [sent-263, score-0.721]

78 We select 6 object classes to learn object detectors for because they are commonly present in selected events: “Skateboard”, “Animal”, “Tire”, “Vehicle”, “Sandwich”, and “Sewing machine”. [sent-264, score-0.464]

79 After sets of iterations, we show samples of newly discovered video positives (red boxes) that were not in the set of top K of previous iterations (left, middle columns). [sent-266, score-0.807]

80 As our model adapts, it is able to iteratively refine its set of top K video positives. [sent-268, score-0.513]

81 Green boxes detections from our method, red boxes detections from “InitialBL”, blue boxes detections from “VideoPosBL”, and magenta boxes detections from Gopalan et al. [sent-271, score-0.904]

82 The video negatives were randomly sampled from the videos that were labeled with no event class. [sent-275, score-0.857]

83 To test our algorithm, we manually annotated approximately 200 frames with bounding boxes of positive examples for each object, resulting in 1234 annotated frames total from over 500 videos, giving us a diverse set of situations the objects can appear in. [sent-276, score-0.63]

84 For each object, we use 20 videos from the associated event as unlabeled video training data. [sent-277, score-0.746]

85 The video negatives were randomly sampled from the videos that were not annotated with any of these objects. [sent-283, score-0.814]

86 For each object class, we use the remaining videos that contain the object as the unlabeled video training data, resulting in around 9 videos per object. [sent-285, score-1.172]

87 This shows that if we discover the top K video positives and re-train our detector with all of them, we do not obtain consistent gains in performance. [sent-289, score-1.026]

88 As illustrated in Figure 4, our method is able to add new video positives from iteration to iteration that are good examples, and remove bad examples at the same time. [sent-291, score-0.813]

89 93% for classes that choose models with target features versus no target features. [sent-305, score-0.61]

90 However, we hypothesize that the inclusion of more complex target features such as temporal movement could help our method achieve even better results. [sent-307, score-0.393]

91 Although this is not a common occurrence, it can happen when our method of self-paced domain adaptation replaces good video positives taken in the first iteration with bad examples in future iterations. [sent-309, score-1.141]

92 This situation arises when there are incorrect examples present in the easiest of the top K video positives, causing our detector to re-train and iteratively become worse. [sent-310, score-0.898]

93 To discover examples in the unlabeled video data, we classify tracks instead of bounding boxes, allowing us to leverage temporal continuity to avoid spurious detections, and to discover examples we would’ve otherwise missed. [sent-313, score-1.191]

94 Furthermore, we introduce a novel self-paced domain adaptation algorithm that allows our detector to iteratively adapt from source to target domain, while also considering target features unique to the target domain. [sent-314, score-1.616]

95 We’ve shown convincing results that illustrate the benefit of our approach to adapting object detectors to video. [sent-316, score-0.381]

96 A measure that would allow us to estimate our performance on the target domain with theoretical guarantees would be an interesting direction. [sent-318, score-0.432]

97 Another possible direction would be to relax the assumption of having no labeled target domain examples, and to formulate similar methods for this scenario. [sent-319, score-0.47]

98 Labelme video: Building a video database with human annotations. [sent-348, score-0.411]

99 Detection by detections: Non-parametric detector adaptation for a video. [sent-410, score-0.387]

100 Exploiting weakly-labeled web images to improve object classification: a domain adaptation approach. [sent-462, score-0.509]


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