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Paper in Artificial Intelligence (2009): “A novel sequence representation for unsupervised analysis of human activities”

September 20th, 2009 Irfan Essa Posted in Aaron Bobick, Activity Recognition, Charles Isbell, Papers, Raffay Hamid, Siddhartha Maddi No Comments »

A novel sequence representation for unsupervised analysis of human activities

  • R. Hamid, S. Maddi, A. Johnson, A. Bobick, I. Essa, and C. Isbell (2009), “A Novel Sequence Representation for Unsupervised Analysis of Human Activities,” Artificial Intelligence Journal, 2009. [BIBTEX]
    @article{2009-Hamid-NSRUAHA,
      Author = {R. Hamid and S. Maddi and A. Johnson and A. Bobick and I. Essa and C. Isbell},
      Date-Modified = {2011-12-08 21:27:48 +0000},
      Journal = {Artificial Intelligence Journal},
      Month = {May},
      Title = {A Novel Sequence Representation for Unsupervised Analysis of Human Activities},
      Year = {2009}}

Abstract

Formalizing computational models for everyday human activities remains an open challenge. Many previous approaches towards this end assume prior knowledge about the structure of activities, using which explicitly defined models are learned in a completely supervised manner. For a majority of everyday environments however, the structure of the in situ activities is generally not known a priori. In this paper we investigate knowledge representations and manipulation techniques that facilitate learning of human activities in a minimally supervised manner. The key contribution of this work is the idea that global structural information of human activities can be encoded using a subset of their local event subsequences, and that this encoding is sufficient for activity-class discovery and classification.

In particular, we investigate modeling activity sequences in terms of their constituent subsequences that we call event n-grams. Exploiting this representation, we propose a computational framework to automatically discover the various activity-classes taking place in an environment. We model these activity-classes as maximally similar activity-cliques in a completely connected graph of activities, and describe how to discover them efficiently. Moreover, we propose methods for finding characterizations of these discovered classes from a holistic as well as a by-parts perspective. Using such characterizations, we present a method to classify a new activity to one of the discovered activity-classes, and to automatically detect whether it is anomalous with respect to the general characteristics of its membership class. Our results show the efficacy of our approach in a variety of everyday environments.

Keywords: Temporal reasoning; Scene analysis; Computer vision

Hamid et al AIJ Paper

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Paper: IEEE Data Mining Conference 2007 “Detecting Subdimensional Motifs: An Efficient Algorithm for Generalized Multivariate Pattern Discovery”

October 28th, 2007 Irfan Essa Posted in Activity Recognition, Charles Isbell, David Minnen, Papers, Research, Thad Starner No Comments »

Detecting Subdimensional Motifs: An Efficient Algorithm for Generalized Multivariate Pattern Discovery

  • Minnen, Essa, Isbell, and Starner (2007), “Detecting Subdimensional Motifs: An Efficient Algorithm for Generalized Multivariate Pattern Discovery,” in Proceedings of IEEE International Conference on Data Mining (ICDM), 2007. [PDF] [DOI]
    [BIBTEX]

    @inproceedings{2007-Minnen-DSMEAGMPD,
      Author = {D. Minnen and I. Essa and C. Isbell and T. Starner},
      Booktitle = {Proceedings of IEEE International Conference on Data Mining (ICDM)},
      Doi = {10.1109/ICDM.2007.52},
      Month = {October},
      Pdf = {http://www.cc.gatech.edu/~irfan/p/2007-Minnen-DSMEAGMPD.pdf},
      Title = {Detecting Subdimensional Motifs: An Efficient Algorithm for Generalized Multivariate Pattern Discovery},
      Year = {2007}}

Abstract

ICDMPaper Discovering recurring patterns in time series data is a fundamental problem for temporal data mining. This paper addresses the problem of locating subdimensional motifs in real-valued, multivariate time series, which requires the simultaneous discovery of sets of recurring patterns along with the corresponding relevant dimensions. While many approaches to motif discovery have been developed, most are restricted to categorical data, univariate time series, or multivariate data in which the temporal patterns span all of the dimensions. In this paper, we present an expected linear-time algorithm that addresses a generalization of multivariate pattern discovery in which each motif may span only a subset of the dimensions. To validate our algorithm, we discuss its theoretical properties and empirically evaluate it using several data sets including synthetic data and motion capture data collected by an on-body inertial sensor.

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Funding: NSF/SGER (2007) “Persistent, Adaptive, Collaborative Synthespians”

September 15th, 2007 Irfan Essa Posted in Charles Isbell, Numerical Machine Learning No Comments »

Award#0749181 – SGER Collaborative Research: Persistent, Adaptive, Collaborative Synthespians
ABSTRACT

This project explores the development of methodologies for populating worlds with persistent, adaptive, collaborative, believable synthetic actors, referred to as Synthespians. These methods are extensions of adaptive models of learning and planning to accommodate the complex, dynamic environments in massive multi-player online games. The intellectual merit includes the development and evaluation of: 1. A behavior development language, with discovery, machine learning, and adaptation of behaviors directly integrated into the language, allowing for the rapid development and deployment of Synthespians. 2. A framework for the actors to recognize and discover plans by observing and modeling the activities of the other agents. An expected outcome of this research is the ability to author complex virtual worlds with many participants that support intelligent and effective interaction between people and machines. Broader Impact: A scientific understanding of how we interact with each other and collaborate will benefit from our ability to simulate complex environments with dynamic and evolving individual and group behaviors. In this project, building and modeling such environments and behaviors is done within a gaming context. This work will in the long run effect and change the fields of education and entertainment. In addition, being able to model large collaborative and interactive scenarios will also help us understand and model large social dynamics phenomenon of interest to sociologists and economists.

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Paper: AAAI 2007: “Discovering Multivariate Motifs using Subsequence Density Estimation and Greedy Mixture Learning”

July 24th, 2007 Irfan Essa Posted in Activity Recognition, Charles Isbell, David Minnen, Papers, Research, Thad Starner No Comments »

Discovering Multivariate Motifs using Subsequence Density Estimation and Greedy Mixture Learning

Abstract
The problem of locating motifs in real-valued, multivariate time series data involves the discovery of sets of recurring patterns embedded in the time series. Each set is composed of several non-overlapping subsequences and constitutes a motif because all of the included subsequences are similar. The ability to automatically discover such motifs allows intelligent systems to form endogenously meaningful representations of their environment through unsupervised sensor analysis. In this paper, we formulate a unifying view of motif discovery as a problem of locating regions of high density in the space of all time series subsequences. Our approach is efficient (sub-quadratic in the length of the data), requires fewer user-specified parameters than previous methods, and naturally allows variable length motif occurrences and nonlinear temporal warping. We evaluate the performance of our approach using four data sets from different domains including on-body inertial sensors and speech.

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Paper: IEEE ISWC (2006) “Discovering Characteristic Actions from On-Body Sensor Data”

October 14th, 2006 Irfan Essa Posted in Activity Recognition, Charles Isbell, David Minnen, Papers, Research, Thad Starner No Comments »

Discovering Characteristic Actions from On-Body Sensor Data (IEEEXplore)

Minnen, D. Starner, T. Essa, I. Isbell, C.
College of Computing, Georgia Institute of Technology, Atlanta, GA 30332 USA.
This paper appears in: Wearable Computers, 2006 10th IEEE International Symposium on
Publication Date: Oct. 2006
On page(s): 11 – 18
Number of Pages: 11 – 18
Location: Montreux, Switzerland
ISSN: 1550-4816
ISBN: 1-4244-0598-x
Digital Object Identifier: 10.1109/ISWC.2006.286337
Posted online: 2007-01-22 09:58:15.0

Abstract

We present an approach to activity discovery, the unsupervised identification and modeling of human actions embedded in a larger sensor stream. Activity discovery can be seen as the inverse of the activity recognition problem. Rather than learn models from hand-labeled sequences, we attempt to discover motifs, sets of similar subsequences within the raw sensor stream, without the benefit of labels or manual segmentation. These motifs are statistically unlikely and thus typically correspond to important or characteristic actions within the activity. The problem of activity discovery differs from typicalmotif discovery, such as locating protein binding sites, because of the nature of time series data representing human activity. For example, in activity data, motifs will tend to be sparsely distributed, vary in length, and may only exhibit intra-motif similarity after appropriate time warping. In this paper, we motivate the activity discovery problem and present our approach for efficient discovery of meaningful actions from sensor data representing human activity. We empirically evaluate the approach on an exercise data set captured by a wrist-mounted, three-axis inertial sensor. Our algorithm successfully discovers motifs that correspond to the real exercises with a recall rate of 96.3% and overall accuracy of 86.7% over six exercises and 864 occurrences.

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