Text recognition using two-dimensional stochastic models
First Claim
1. A method of using a computer to spot a keyword in a document, which comprises the steps of:
- storing first signals representing a first pseudo two-dimensional hidden Markov model in said computer, said first pseudo two-dimensional hidden Markov model representing said keyword and including a one-dimensional hidden Markov model having at least one superstate associated with a first dimension of said keyword and, for each superstate, a one-dimensional hidden Markov model having at least one state associated with a second dimension of said keyword,storing second signals representing a second pseudo two-dimensional hidden Markov model in said computer, said second pseudo two-dimensional hidden Markov model representing a plurality of extraneous words, other than said keyword, that may appear in said text and including a one-dimensional hidden Markov model having at least one superstate associated with a first dimension of said plurality of extraneous words and, for each superstate, a one dimensional hidden Markov model having at least one state associated with a second dimension of said plurality of extraneous words,scanning said document to generate third signals representing a pixel map for each text word in said document, said pixel map having rows and columns of pixels,for each text word;
responsive to said third signals, comparing the pixel map for said text word with said first pseudo two-dimensional hidden Markov model, by applying the Viterbi algorithm, to generate a first comparison signal indicating a first probability that said first pseudo two-dimensional hidden Markov model represents said text word,also responsive to said third signals, comparing the pixel map for said text word with said second pseudo two-dimensional hidden Markov model, by applying the Viterbi algorithm, to generate a second comparison signal indicating a second probability that said second pseudo two-dimensional hidden Markov model represents said text word, andresponsive to said first and second comparison signals, generating an output signal identifying said text word as said keyword if said first probability is greater than said second probability.
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Abstract
Pseudo two-dimensional hidden Markov models (HMMs) are used to represent text elements, such as characters or words. Observation vectors for each text element are based on pixel maps obtained by optical scanning. A character is represented by a pseudo two-dimensional HMM having a number of superstates, with each superstate having at least one state. Text elements are compared with such models by using the Viterbi algorithm, first in connection with the states in each superstate, then the superstates themselves, to calculate the probability that a particular model represents the text element. Parameters for the models are generated by training routines. Probabilities can be adjusted to compensate for changes in scale, translations, slant, and rotation.
An embodiment is also disclosed for identifying keywords in a body of text. A first pseudo two-dimensional HMM is created for the words that may appear in the text. Each word in the text is compared with both models, again using the Viterbi algorithm, to calculate probabilities that the model represents the subject word. If the probability for the keyword is greater than that for the extraneous words, the subject word is identified as being the keyword. Preprocessing steps for reducing the number of words to be compared can be added.
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Citations
17 Claims
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1. A method of using a computer to spot a keyword in a document, which comprises the steps of:
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storing first signals representing a first pseudo two-dimensional hidden Markov model in said computer, said first pseudo two-dimensional hidden Markov model representing said keyword and including a one-dimensional hidden Markov model having at least one superstate associated with a first dimension of said keyword and, for each superstate, a one-dimensional hidden Markov model having at least one state associated with a second dimension of said keyword, storing second signals representing a second pseudo two-dimensional hidden Markov model in said computer, said second pseudo two-dimensional hidden Markov model representing a plurality of extraneous words, other than said keyword, that may appear in said text and including a one-dimensional hidden Markov model having at least one superstate associated with a first dimension of said plurality of extraneous words and, for each superstate, a one dimensional hidden Markov model having at least one state associated with a second dimension of said plurality of extraneous words, scanning said document to generate third signals representing a pixel map for each text word in said document, said pixel map having rows and columns of pixels, for each text word; responsive to said third signals, comparing the pixel map for said text word with said first pseudo two-dimensional hidden Markov model, by applying the Viterbi algorithm, to generate a first comparison signal indicating a first probability that said first pseudo two-dimensional hidden Markov model represents said text word, also responsive to said third signals, comparing the pixel map for said text word with said second pseudo two-dimensional hidden Markov model, by applying the Viterbi algorithm, to generate a second comparison signal indicating a second probability that said second pseudo two-dimensional hidden Markov model represents said text word, and responsive to said first and second comparison signals, generating an output signal identifying said text word as said keyword if said first probability is greater than said second probability. - View Dependent Claims (2, 3, 4)
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5. A method of using a computer to identify unknown text elements in a document, which comprises the steps of:
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storing first signals representing a plurality of pseudo two-dimensional hidden Markov models in said computer, each pseudo two-dimensional hidden Markov model having at least one superstate associated with a first dimension of said known text element and, for each superstate, a one-dimensional hidden Markov model having at least one state associated with a second dimension of said known text element, scanning said document to generates second signals representing a pixel map for each unknown text element, said pixel map having rows and columns of pixels, and for each unknown text element; responsive to said second signals, comparing said pixel map for said unknown text element with each pseudo two-dimensional hidden Markov model, by applying the Viterbi algorithm, to generate a comparison signal indicating the probability that said pseudo two-dimensional hidden Markov model represents said unknown text element, and responsive to said comparision signals, generating an output signal identifying said unknown text element as the known text element represented by the pseudo two-dimensional hidden Markov model for which said probability is highest. - View Dependent Claims (6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
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Specification