1 Related Work
There is little existing work on micro-video analysis, as the medium itself is new. Redi et al.  explore the problem of finding creative micro-videos, inspired by similar studies of image quality assessment. Sano et al.  analyze the problem of detecting loop micro-videos (that are designed to be played in a continuous six-second loop). Here we focus on analyzing general properties of micro-videos, with the explicit goal of constructing a new, large-scale benchmark for temporally-evolving tag prediction.
Viewpoint modeling: A unique contribution of our dataset is the diversity of camera viewpoints. Existing video benchmarks for action recognition have focused on third-person viewpoints (e.g., HMDB , UCF101 , Hollywood-2 , UT-Interaction  and Olympic Sports ). Wearable cameras such as Google Glass and GoPro have spurred interest in analyzing egocentric views [18, 19, 20, 21]. Compared to existing action and egocentric datasets, our user-generated micro-videos contain a wider variety of categories (tags) and viewpoints (e.g., self-facing) with richer narrative content, even in a single clip. To understand and highlight these differences we train mixture-of-viewpoint models that specifically target viewpoint variations in dynamic micro-videos (Sec. 3) and carry out an extensive comparison with HMDB  (Sec. 4).
Closed vs open vocabularies: Traditionally, video datasets in computer vision have been labeled with a fixed ontology of activities or events [9, 10, 22]. An alternative perspective (popular in the multimedia community) is to formulate the problem as a multi-label tag or concept prediction task [23, 24, 25, 26, 27]. Our dataset falls into this later camp. In terms of size and diversity, the most relevant prior work appears to be Sports-1M  which contains 1M videos in 487 categories, and EventNet , which contains 95K videos labeled with 5K concepts. Our dataset already includes 2X more videos and 10X more concept tags. Unlike other video datasets, our data also includes timestamps which allow us to study temporally-varying semantics, a relatively unexplored concept in vision, with the notable exception of . Importantly, tag frequency distributions are highly imbalanced, following a natural long-tail distribution (Fig. 3). While highly imbalanced class distributions are somewhat uncommon in current vision datasets, they appear to be a fundamental aspect of life-long learning in the open-world . With the advent of deep architectures that appear capable of transferring knowledge across imbalanced classes , we think the time is right to (re)consider learning in the open-world!
In this section, we describe our (continually-running) data-collection process and analyze the statistics of micro-video tags, shots and views that make our dataset distinct from existing video benchmarks.
Streaming dataset collection: We collect a stream of Vine videos by daily querying of Vine’s API . To ensure a diverse stream, we query the 300 most popular and 300 most recent videos across multiple community-curated channels (Comedy, Sports, Musics). We typically obtain 6000 videos daily. Each video is associated with a collection of hashtags that are added to our open vocabulary. On average, a video contains 1.56 hashtags, but this statistic is skewed by the fact that nearly half the videos do not contain any tags (56%).This indirectly motivates one practical application of our dataset - automatic prediction of hashtags. We attempted to automatically merge similar tags through linguistic normalization, but found this merging did little to change our label space (perhaps because users have an incentive to used normalized hashtags that are already searchable). We visualize our overall distribution of tags in Fig. 1. We use MV-58k to refer to a snapshot of this datastream collected during the period Dec-2015 to Feb-2016. We provide additional statistics of this data collection and projections in the supplemental materials.
Curation: To examine the amount of “noise” in the tag stream and perform diagnostic comparisons to existing activity datasets, we manually curated a subset of 40 tags and their associated 4000 videos. We selected 40 representative tags that span both common actions as well rare tags “in-the-tail”. We “clean” this dataset by merging synonymous tags (e.g., #horseback and #horsebackriding), removing mistagged (spam) videos, and removing videos in which there was only circumstantial visual evidence for the tagged activity (see Fig. 3 for an example). We added additional annotations to each video including viewpoint and a dominant (tag) category. The latter allows us to recast tag prediction as a K-way classification problem, simplifying our diagnostic analysis. We refer to this curated dataset as MV-40, and contrast this with (the fixed snapshot of) our uncurated, open-world dataset MV-58K. We organize MV-40 into broad categories of atomic actions, recreational activities, competitive sports, objects, and vine-specific. We visualize our two-level taxonomy and provide visual examples in Fig. 1.
Our collection process reveals a salient property of open-world microvideos; they follow long-tail distributions of tags. This significantly complicates learning because there will be some tags for which we have little training data. While traditionally a notorious challenge for machine learning, our analysis suggests that hierarchical feature learning (with CNNs) can learn to share, or transfer knowledge from the data-rich tags to the data-sparse tags (i.e.,one-shot learning). For example, even if we have few examples of #dunking, mid-level features learned for a data-rich tag such as #basketball may still be useful for the former class.
Temporal dynamics: Our open-world dataset collection has another notable property - both the frequency of tag usage and the visual appearance semantics associated with a given tag evolve over time. In probabilistic terms, we can interpret such dynamics and changes in the prior of labels (Fig. 2) and the likelihood of image features conditioned on the tag label (Fig. 10). This suggests developing approaches for continually retraining models as new data becomes available, an idea we explore in Sec.4. An extreme case arises when a new tag first appears in the data stream there are no training examples available (e.g., #trump2016 did not exist until recently). In our current experiments, we simply fail to predict such tags at test time. However, we point out that the temporal appearance of new tags provides a compelling natural example of zero-shot learning where side information about the semantic relation between tags could readily be exploited.
Shot statistics: Micro-videos have a unique shot structure due to the capturing mechanics and limited time constraint imposed by the app. A Vine or Instagram user creates videos by holding a button to capture and releasing the button to pause at any time. He/she can later resume capturing again until the content time limit is exceeded (6 seconds for Vine). This interface allows and even encourages splicing together of many shots within a single time-limited video. We plot a distribution shot length and frequency for various tags in Fig. 5. Some tags, such as #cooking tend to consistently involve a large number of shots. Indeed, we find 3.75% of videos have more than 6 shots, implying many shots are less than a second in length.
Viewpoint: To analyze the effect of viewpoint on recognition accuracy, we manually annotate MV-40 with the viewpoint of each video as egocentric, third-person, or self-facing. In some cases, the viewpoint changed between shots in the video in which case we record all the viewpoints present, as well as the dominant one. With this annotation we can treat viewpoint prediction as either a multi-label attribute prediction problem (where multiple viewpoints can be present in video) or a multi-class problem (where the goal is to predict the dominant viewpoint). We report the per-tag viewpoint statistics in Fig. 4. Interestingly, even at the shot-level, we sometimes find ambiguities in viewpoint. Some social activities (such as #bicycling) involve both the photographer and subjects in view, suggesting a simultaneous egocentric and third-person viewpoint.
In this section, we describe several baseline models for tag prediction and viewpoint classification. As the focus of our work is not on video feature extraction, we make use of standard feature sets. Recent results from the THUMOS evaluation benchmark suggest that CNN spatial features (VGG ) combined with temporal motion features (IDT ) make for a reasonable video descriptor. We briefly outline our feature descriptor and associated classification engines here.
Recent work has shown that Convolutional Neural Networks (CNN) produce quite effective visual descriptors for recognition tasks[35, 33] including video analysis [36, 37, 22, 38]. We experimented with many open-source implementations of CNN architectures for video-feature extractors, and found that while many were effective, some placed significant demands on processing time and descriptor storage. We refer the reader to our supplementary material for a detailed description of our diagnostic experiments. We found a good tradeoff in speed, storage, and accuracy with the following simple pipeline: given a video, (1) run off-the-shelf VGG-16 models  on 15 equally-spaced frames from a video, (2) for each frame, extract (6144-dimensional) multi-scale features across multiple layers 
, and (4) max-pool the resulting features across the 15 frames. When compared to standard single-frame CNN feature extraction (that extract 4096-dimensional features ), our final video pipeline was only 15x slower and 2x larger in storage costs.
Motion features: Recent state-of-the-art results on video datasets have made use of trajectory-based motion features. We include such features in our analysis, focusing on Improved Dense Track (IDT) . This method is based on a “interest-track” framework (rather than a space-time interest-point approach) in which short-term tracks are found from tracking interest points across frames. One then extracts various features aligned to these temporal tracks, including oriented gradient histograms and optical flow. We quantize these descriptors based on established guidelines for constructing a codebook, using
codebook entries found with K-means.
We train tag classifiers that aggregate features by combining multiple kernels. We experimented extensively with various feature encodings and kernel combinations before settling on the following strategy. We compute the similarity between two video clipsand by averaging their appearance and motion-feature similarities:
We define the motion similarity with a -RBF kernel:
where is the average distance between all videos in the training data, L is the number of motion channels (Traj, HOG, HOF, MBHx, MBHy), and is the distance between and with respect to the -th channel. We measure the appearance similarity between two clips using a linear kernel: summed over the static feature channels extracted by the CNN. Given a training vocabulary of tags, we train binary (one-vs-all) kernelized SVMs using the LIBSVM package . Finally, we calibrate each predictor using Platt scaling .
Viewpoint mixtures: Different viewpoints of the same tag can have significant differences in video content, as shown in Fig. 1. For example, a third-person and egocentric #bicycling video contain very different motions and appearances. To analyze such variations in the MV-40 diagnostic dataset, we train viewpoint-specific models for each tag. The final confidence associated with a tag prediction is the maximum score across the three viewpoint-specific models.
Temporally-adapted models: To explore the temporal evolution of tag semantics and videos, we evaluate models trained with videos sampled over different temporal windows. Consider the task of predicting the label for a video collected at time using with models trained on a stream of timestamped training videos indexed by time. We consider models trained on three different subsets of training data:
where is a specified window size. The above approach can be simplified by making some assumptions about the nature of temporal variation. For example, if we assume that the popularity of tags changes over time but their appearance models do not, one can model efficiently model temporal dynamics with a statistical prior shift . Intuitively, dynamics can be captured with a fixed set of posterior class predictions that are reweighted by dynamically-varying tag priors. Unfortunately, this requires access to tag priors on test data from the future, which violates causality. Instead, we assume that tag priors vary smoothly over time, and simply use a weighted estimate of recent tags’ popularity. We found that the simple approach of applying temporally-weighted Platt rescaling (using a weighted dataset where recent videos are given more importance) outperformed an explicit prior model.
In this section, we present an extensive set of experiments on our dataset and refer the reader to the supplementary materials for additional tables and figures. We focus on three sets of experiments: a diagnostic evaluation of features and viewpoints on our curated dataset (MV-40), its relation to popular benchmarks such as HMDB , and analysis of the open-world dynamics of MV-58K.
First, we analyze various aspects of our dataset and recognition pipeline, focusing on the curated and annotated MV-40 subset.
Feature comparisons: We begin by comparing the performance of various combinations of our features in Fig. 6
-(a). We observe CNN features outperform IDT in most category groups except ‘Atomic’. Trajectory-based motion and appearance-based deep features are particularly effective when combined, indicating that they capture complementary cues. Supplementary materials include the class confusion matrix over all 40 tags for the IDT+CNN feature combination.
View-specific mixtures: We next evaluate the performance of view-specific tag classifiers and compare to results with a single classifier per tag. To ensure sufficient training data for each mixture component, we only train a mixture for a specific view if there are more than 20 videos in that viewpoint (11 classes out of 40 satisfy this criteria in MV-40). When training, one can treat clips from the same tag but different viewpoints as positive training examples (Pos), negative training examples (Neg), or such clips can be treated as neutral and ignored (Neu). Fig 6-(b) summarizes of the performance. When averaged over all tags, the performance increase from view-specific mixtures is rather negligible ( 0.6% for our combined features). We also evaluate only those 11 classes for which additional views were trained. We see a small but definite improvement of 2.1%. Ignoring clips from other viewpoints (Neu) slightly increases performance.
Viewpoint prediction: We also investigate the task of viewpoint prediction: what is the viewpoint of a test video? Fig 6-(c) summarizes viewpoint confusions. Egocentric views are often confused with third-person and the accuracies for egocentric drops significantly. This is consistent with Fig. 4, which suggests many recreational activities involve both the photographer and subjects involved in the action. If we score viewpoint prediction as a multilabel problem (where each video could be labeled with more than one viewpoint), accuracy for ego, third, and self-facing jump to 92%, 90%, and 93%. This suggests the presence of any given viewpoint can be accurately predicted.
4.2 Comparison to existing benchmarks
We perform an extensive comparison of our data with a popular action recognition benchmark, HMDB . We use a subset of 15 tags (vine-15) that overlap with HMDB categories and evaluate the IDT+CNN based predictor. Overall, the average accuracy on vine-15 is lower than HMDB (65.99% vs. 71.27%), suggesting that our data is more challenging. Torralba and Efros  suggest that the ‘performance drop’ provides a way to quantify how biased or general a dataset may be. The drop for models trained on Vine data is 13%, while the drop for models trained with HMDB is 26.75%.
Viewpoint: One might hypothesize that since HMDB contains mostly third-person views, it won’t generalize to the other viewpoint in our data. To test this, we extract a smaller subset vine-3rd containing only third-person viewpoints. Fig 7-(g) shows that vine-3rd is more similar to HMDB, as models trained on HMDB data perform better on vine-3 than vine-15. However, performance drop for models trained on vine-3rd is still significantly smaller than those trained on HMDB (12.88% vs. 24.4%). This suggests that even accounting for viewpoint, our videos still generalize better than HMDB.
Temporally iconic videos: The concept of “iconic views” in object recognition refers to “easy” images with a clear and distinctive depiction of an object, often close cropped or in an uncluttered setting without occlusion. We apply this notion to video, defining a temporally iconic depiction of a tag as one where temporal clutter has been removed by trimming down the video clip to focus on the core action. Temporal cropping is relevant even in micro-videos, which often contain additional frames and shots surrounding those described by the tag. We manually segment each video in vine-15 to derive an iconic version vine-icon and use vine-3rd-icon to denote the third-person subset. Fig. 7 reveals that vine-icon and vine-3rd-icon are slightly easier than vine, and more similar to HMDB (following our previous analysis). However, the cross-dataset performance drop for models trained on vine-icon and vine-3rd-icon (14.06% and 12.86% respectively) are still significantly smaller than the drop by models trained HMDB data (25.35% and 21.89%). To summarize, even though third-person and temporal iconic-ness accounts for much of the difference between HMDB and our dataset, our micro-videos can still generalize better.
Qualitative differences: To better understand the differences, we visualize example videos in Fig. 7. #swordfighting in vine-15 tends to involve people playfully jousting in social everyday scenes, while HMDB clips tend to involve formally sparring or staged/scripted fights. This difference helps explain the why HMDB’s #swordfights don’t generalize to Vine and is consistent with the notion that models trained from purely iconic images of objects often generalize poorly . In the same figure, we also plot sample frames from videos with a particular tag. Perhaps surprisingly, many distinct clips in the dataset actually come from the same longer video. This reduces the amount of diversity in the dataset, and reinforces one of our motivations: it is
surprisingly hard to collect diverse video clips. This phenomena is not limited to HMDB, and also appears in other benchmarks such as ImageNet Video Challenge. Our micro-video dataset, however, is inherently diverse by construction due to its dynamic and pre-trimmed nature.
4.3 Open-World Dynamics
In this section, we analyze properties of our open-world MV-58K dataset. One immediate issue is that open-world tags are more naturally treated as multi-label tasks (since videos can be naturally labeled with many tags). We adopt two scoring criteria from the literature on multi-label classification . treats each tag as a individual binary prediction problem, and computes the average precision (AP) of each prediction task, returning the mean AP over all tags. This approach equally weights popular and infrequent tags, and is analogous with standard mAP measures used in object detection. Alternative, first computes an image-specific AP by ranking all tag predictions specific to that image, and then returns the mean AP over all images. This latter scheme places more importance on frequent tags, and also requires that confidences across different tag predictors be calibrated. By default, we use unless otherwise specified.
Curated vs raw data: When comparing models trained on curated versus open-world data (Fig. 8), we see that additional data always helps, but given a fixed amount of training data, curated data performs better. Curated data is designed to mimic existing recognition datasets which use concise notions of visual categories, while the raw dataset captures problem of tag-prediction “in the wild”. For example, videos of people talking about #archery are removed in MV-40, but exist in MV-58k (Fig. 3). When comparing accuracy on tag-prediction, we find that a clean dataset replicates the accuracy of a 2X larger raw dataset.
Closed vs open vocabularies: We now move beyond our 40 selected tags to evaluation of an open-world vocabulary. Because the amount of training data variables considerably per tag, we plot test accuracy as a function of training-set size in Fig. 9. We tend to see different performance regimes. “Easy” tags perform well even with little training data, likely due to a characteristic appearance that is easy to learn from little training data (#cavs, #warriors). “Challenging” tags appear to contain appearance variation, but are learnable with additional data (#dogs, #soccer). “Unlearnable” tags remain near-zero AP even given lots of training data (#revine, #lol). We posit that these can be treated as stopwords that fail to capture much semantic meaning of the video.
Temporal dynamics: We now examine the time-varying properties of micro-video content and tags (Fig. 10). We refer the reader to the caption for a detailed analysis, but we find that causal prediction (where one only has access to data from the past) is much harder than the non-causal counterpart, suggesting that micro-videos are not “iid” over time. Benchmarks that use iid resampling may over-estimate real-world performance on streaming data. We find that much of this temporal variation can be explained by fluctuations of tag popularity, whose degree of variation can vary dramatically across classes.
Conclusion: We introduce a open-world dataset of micro-videos, which lie in a regime between single images and typical videos, allowing for easy capture, storage, and processing. They contain micro-narratives captured from viewpoints typically not studied in computer vision. Because they are naturally diverse, pre-trimmed, and user-annotated, they can be used a live testbed for open-world evaluation of video understanding systems. We conclude with an intriguing thought: rather than distributing a fixed benchmark dataset (which historically leads to eventual overfitting ), our analysis suggests that we can instead distribute a benchmark script that evaluates models on live open-world micro-videos. We think the time is right to consider video recognition out in-the-open!
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