In this paper, we propose a discriminative video representation for event detection over a large scale video dataset when only limited hardware resources are available. The focus of this paper is to effectively leverage deep Convolutional Neural Networks (CNNs) to advance event detection, where only frame level static descriptors can be extracted by the existing CNN toolkit. This paper makes two contributions to the inference of CNN video representation. First, while average pooling and max pooling have long been the standard approaches to aggregating frame level static features, we show that performance can be significantly improved by taking advantage of an appropriate encoding method. Second, we propose using a set of latent concept descriptors as the frame descriptor, which enriches visual information while keeping it computationally affordable. The integration of the two contributions results in a new state-of-the-art performance in event detection over the largest video datasets. Compared to improved Dense Trajectories, which has been recognized as the best video representation for event detection, our new representation improves the Mean Average Precision (mAP) from 27.6 to 36.8 for the TRECVID MEDTest 14 dataset and from 34.0 to 44.6 for the TRECVID MEDTest 13 dataset. This work is the core part of the winning solution of our CMU-Informedia team in TRECVID MED 2014 competition.
Recent research efforts have shown that combining multiple features, including static appearance features @cite_15 @cite_57 @cite_13 , motion features @cite_32 @cite_42 @cite_5 @cite_33 @cite_35 and acoustic features @cite_36 , yields good performance in event detection, as evidenced by the reports of the top ranked teams in the TRECVID Multimedia Event Detection (MED) competition @cite_17 @cite_29 @cite_25 @cite_7 and research papers @cite_28 @cite_43 @cite_46 that have tackled this problem. By utilizing additional data to assist complex event detection, researchers propose the use of video attributes'' derived from other sources to facilitate event detection @cite_49 , or to utilize related exemplars when the training exemplars are very few @cite_2 . As we focus on improving video representation in this paper, this new method can be readily fed into those frameworks to further improve their performance.
@cite_35 @cite_33 @cite_7 @cite_36 @cite_28 @cite_29 @cite_42 @cite_46 @cite_32 @cite_57 @cite_43 @cite_49 @cite_2 @cite_5 @cite_15 @cite_13 @cite_25 @cite_17
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In this paper, we present recent experiments on using Artificial Neural Networks (ANNs), a new “delayed” approach to speech vs. non-speech segmentation, and extraction of large-scale pooling feature (LSPF) for detecting “events” within consumer videos, using the audio channel only. A “event” is defined to be a sequence of observations in a video, that can be directly observed or inferred. Ground truth is given by a semantic description of the event, and by a number of example videos. We describe and compare several algorithmic approaches, and report results on the 2013 TRECVID Multimedia Event Detection (MED) task, using arguably the largest such research set currently available. The presented system achieved the best results in most audio-only conditions. While the overall finding is that MFCC features perform best, we find that ANN as well as LSP features provide complementary information at various levels of temporal resolution. This paper provides analysis of both low-level and high-level features, investigating their relative contributions to overall system performance. Combining multiple low-level visual features is a proven and effective strategy for a range of computer vision tasks. However, limited attention has been paid to combining such features with information from other modalities, such as audio and videotext, for large scale analysis of web videos. In our work, we rigorously analyze and combine a large set of low-level features that capture appearance, color, motion, audio and audio-visual co-occurrence patterns in videos. We also evaluate the utility of high-level (i.e., semantic) visual information obtained from detecting scene, object, and action concepts. Further, we exploit multimodal information by analyzing available spoken and videotext content using state-of-the-art automatic speech recognition (ASR) and videotext recognition systems. We combine these diverse features using a two-step strategy employing multiple kernel learning (MKL) and late score level fusion methods. Based on the TRECVID MED 2011 evaluations for detecting 10 events in a large benchmark set of ∼45000 videos, our system showed the best performance among the 19 international teams. Complex events essentially include human, scenes, objects and actions that can be summarized by visual attributes, so leveraging relevant attributes properly could be helpful for event detection. Many works have exploited attributes at image level for various applications. However, attributes at image level are possibly insufficient for complex event detection in videos due to their limited capability in characterizing the dynamic properties of video data. Hence, we propose to leverage attributes at video level (named as video attributes in this work), i.e., the semantic labels of external videos are used as attributes. Compared to complex event videos, these external videos contain simple contents such as objects, scenes and actions which are the basic elements of complex events. Specifically, building upon a correlation vector which correlates the attributes and the complex event, we incorporate video attributes latently as extra informative cues into the event detector learnt from complex event videos. Extensive experiments on a real-world large-scale dataset validate the efficacy of the proposed approach. Compared to visual concepts such as actions, scenes and objects, complex event is a higher level abstraction of longer video sequences. For example, a "marriage proposal" event is described by multiple objects (e.g., ring, faces), scenes (e.g., in a restaurant, outdoor) and actions (e.g., kneeling down). The positive exemplars which exactly convey the precise semantic of an event are hard to obtain. It would be beneficial to utilize the related exemplars for complex event detection. However, the semantic correlations between related exemplars and the target event vary substantially as relatedness assessment is subjective. Two related exemplars can be about completely different events, e.g., in the TRECVID MED dataset, both bicycle riding and equestrianism are labeled as related to "attempting a bike trick" event. To tackle the subjectiveness of human assessment, our algorithm automatically evaluates how positive the related exemplars are for the detection of an event and uses them on an exemplar-specific basis. Experiments demonstrate that our algorithm is able to utilize related exemplars adaptively, and the algorithm gains good performance for complex event detection. We study the question of feature sets for robust visual object recognition; adopting linear SVM based human detection as a test case. After reviewing existing edge and gradient based descriptors, we show experimentally that grids of histograms of oriented gradient (HOG) descriptors significantly outperform existing feature sets for human detection. We study the influence of each stage of the computation on performance, concluding that fine-scale gradients, fine orientation binning, relatively coarse spatial binning, and high-quality local contrast normalization in overlapping descriptor blocks are all important for good results. The new approach gives near-perfect separation on the original MIT pedestrian database, so we introduce a more challenging dataset containing over 1800 annotated human images with a large range of pose variations and backgrounds.