José A. Rodriguez, Florent Perronnin
IEEE Transactions on Pattern Analysis and Machine Intelligence, 34 (11), pp. 2108-2120, Nov. 2012
This article proposes a novel similarity measure between vector sequences. We work in the framework of modelbased
approaches, where each sequence is first mapped to a Hidden Markov Model (HMM) and then a measure of similarity is
computed between the HMMs. We propose to model sequences with semi-continuous HMMs (SC-HMMs). This is a particular
type of HMM whose emission probabilities in each state are mixtures of shared Gaussians. This crucial constraint provides two
major benefits. First, the a priori information contained in the common set of Gaussians leads to a more accurate estimate of the
HMM parameters. Second, the computation of a similarity between two SC-HMMs can be simplified to a Dynamic Time Warping
(DTW) between their mixture weight vectors, which reduces significantly the computational cost. Experiments are carried out on a
handwritten word retrieval task in three different datasets - an in-house dataset of real handwritten letters, the GeorgeWashington
dataset and the IFN/ENIT dataset of Arabic handwritten words. These experiments show that the proposed similarity outperforms
the traditional DTWbetween the original sequences, and the model-based approach which uses ordinary continuous HMMs. We also show that this increase in accuracy can be traded against a significant reduction of the computational cost.
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