Image classification by a two dimensional hidden Markov model

Traditional block-based image classification algorithms, such as CART and VQ based classification, ignore the statistical dependency among image blocks. Consequently, these algorithms often suffer from over-localization. In order to benefit from the inter-block dependency, an image classification algorithm based on a hidden Markov model (HMM) is developed. An HMM for image classification, a two dimensional extension from the one dimensional HMM used for speech recognition, has transition probabilities conditioned on the states of neighboring blocks from both directions. Thus, the dependency in two dimensions can be reflected simultaneously. The HMM parameters are estimated by the EM algorithm. A two dimensional version of the Viterbi algorithm is also developed to classify optimally an image based on the trained HMM. An application of the HMM algorithm to document image and aerial image segmentation shows that the algorithm performs better than CART.