TechTalks from event: ICML 2011

Time Series

  • Time Series Clustering: Complex is Simpler! Authors: Lei Li; B. Aditya Prakash
    Given a motion capture sequence, how to identify the category of the motion? Classifying human motions is a critical task in motion editing and synthesizing, for which manual labeling is clearly inefficient for large databases. Here we study the general problem of time series clustering. We propose a novel method of clustering time series that can (a) learn joint temporal dynamics in the data; (b) handle time lags; and (c) produce interpretable features. We achieve this by developing complex-valued linear dynamical systems (CLDS), which include real-valued Kalman filters as a special case; our advantage is that the transition matrix is simpler (just diagonal), and the transmission one easier to interpret. We then present Complex-Fit, a novel EM algorithm to learn the parameters for the general model and its special case for clustering. Our approach produces significant improvement in clustering quality, 1.5 to 5 times better than well-known competitors on real motion capture sequences.
  • Learning Discriminative Fisher Kernels Authors: Laurens Van der Maaten
    Fisher kernels provide a commonly used vectorial representation of structured objects. The paper presents a technique that exploits label information to improve the object representation of Fisher kernels by employing ideas from metric learning. In particular, the new technique trains a generative model in such a way that the distance between the log-likelihood gradients induced by two objects with the same label is as small as possible, and the distance between the gradients induced by two objects with different labels is as large as possible. We illustrate the strong performance of classifiers trained on the resulting object representations on problems in handwriting recognition, speech recognition, facial expression analysis, and bio-informatics.