Automatic Image Annotation Based on WordNet and Hierarchical Ensembles

摘要

Automatic image annotation concerns a process of automatically labeling image contents with a pre-defined set of keywords, which are regarded as descriptors of image high-level semantics, so as to enable semantic image retrieval via keywords. A serious problem in this task is the unsatisfactory annotation performance due to the semantic gap between the visual content and keywords. Targeting at this problem, we present a new approach that tries to incorporate lexical semantics into the image annotation process. In the phase of training, given a training set of images labeled with keywords, a basic visual vocabulary consisting of visual terms, extracted from the image to represent its content, and the associated keywords is generated at first, using K-means clustering combined with semantic constraints obtained from WordNet, then the statistical correlation between visual terms and keywords is modeled by a two-level hierarchical ensemble model composed of probabilistic SVM classifiers and a co-occurrence language model. In the phase of annotation, given an unla-beled image, the most likely associated keywords are predicted by the posterior probability of each keyword given each visual term at the first-level classifier ensemble, then the second-level language model is used to refine the annotation quality by word co-occurrence statistics derived from the annotated keywords in the training set of images. We carried out experiments on a medium-sized image collection from Corel Stock Photo CDs. The experimental results demonstrated that the annotation performance of this method outperforms some traditional annotation methods by about 7% in average precision, showing the feasibility and effectiveness of the proposed approach.