Fast HMM-Filler approach for Key Word Spotting in Handwritten Documents

The so-called filler or garbage Hidden Markov Models (HMM) are among the most widely used models for lexicon-free, query by string key word spotting in the fields of speech recognition and (lately) handwritten text recognition. An important drawback of this approach is the large computational cost of the keyword-specific HMM Viterbi decoding process needed to obtain the confidence scores of each word to be spotted. This paper presents a novel way to compute such confidence scores, directly from character lattices produced during a single Viterbi decoding process using only the "filler" model (i.e. no explicit keyword-specific decoding is needed). Experiments show that, as compared with the classical HMM-filler approach, the proposed method obtains essentially the same spotting results, while requiring between one and two orders of magnitude less query computing time.