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We present a probabilistic framework—namely, multiscale generative models known as Dynamic Trees (DT)—for unsupervised image segmentation and subsequent matching of segmented regions in a given set of images. Beyond these novel applications of DTs, we propose important additions for this modeling paradigm. First, we introduce a novel DT architecture, where multilayered observable data are incorporated at all scales of the model. Second, we derive a novel probabilistic inference algorithm for DTs—Structured Variational Approximation (SVA)—which explicitly accounts for the statistical dependence of node positions and model structure in the approximate posterior distribution, thereby relaxing poorly justified independence assumptions in previous work. Finally, we propose a similarity measure for matching dynamic-tree models, representing segmented image regions, across images. Our results for several data sets show that DTs are capable of capturing important component-subcomponent relationships among objects and their parts, and that DTs perform well in segmenting images into plausible pixel clusters. We demonstrate the significantly improved properties of the SVA algorithm—both in terms of substantially faster convergence rates and larger approximate posteriors for the inferred models—when compared with competing inference algorithms. Furthermore, results on unsupervised object recognition demonstrate the viability of the proposed similarity measure for matching dynamic-structure statistical models.
Index Terms- Generative models, Bayesian networks, dynamic trees, variational inference, image segmentation, image matching, object recognition.

M. C. Nechyba and S. Todorovic, "Dynamic Trees for Unsupervised Segmentation and Matching of Image Regions," in IEEE Transactions on Pattern Analysis & Machine Intelligence, vol. 27, no. , pp. 1762-1777, 2005.
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