Recognizing Visual Categories by Commonality and Diversity
Davis, Larry Steven
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Visual categories refer to categories of objects or scenes in the computer vision literature. Building a well-performing classifier for visual categories is challenging as it requires a high level of generalization as the categories have large within class variability. We present several methods to build generalizable classifiers for visual categories by exploiting commonality and diversity of labeled samples and the cat- egory definitions to improve category classification accuracy. First, we describe a method to discover and add unlabeled samples from auxil- iary sources to categories of interest for building better classifiers. In the literature, given a pool of unlabeled samples, the samples to be added are usually discovered based on low level visual signatures such as edge statistics or shape or color by an unsupervised or semi-supervised learning framework. This method is inexpensive as it does not require human intervention, but generally does not provide useful information for accuracy improvement as the selected samples are visually similar to the existing set of samples. The samples added by active learning, on the other hand, provide different visual aspects to categories and contribute to learning a better classifier, but are expensive as they need human labeling. To obtain high quality samples with less annotation cost, we present a method to discover and add samples from unlabeled image pools that are visually diverse but coherent to cat- egory definition by using higher level visual aspects, captured by a set of learned attributes. The method significantly improves the classification accuracy over the baselines without human intervention. Second, we describe now to learn an ensemble of classifiers that captures both commonly shared information and diversity among the training samples. To learn such ensemble classifiers, we first discover discriminative sub-categories of the la- beled samples for diversity. We then learn an ensemble of discriminative classifiers with a constraint that minimizes the rank of the stacked matrix of classifiers. The resulting set of classifiers both share the category-wide commonality and preserve diversity of subcategories. The proposed ensemble classifier improves recognition accuracy significantly over the baselines and state-of-the-art subcategory based en- semble classifiers, especially for the challenging categories. Third, we explore the commonality and diversity of semantic relationships of category definitions to improve classification accuracy in an efficient manner. Specif- ically, our classification model identifies the most helpful relational semantic queries to discriminatively refine the model by a small amount of semantic feedback in inter- active iterations. We improve the classification accuracy on challenging categories that have very small numbers of training samples via transferred knowledge from other related categories that have a lager number of training samples by solving a semantically constrained transfer learning optimization problem. Finally, we summarize ideas presented and discuss possible future work.