Abstract

Automatic image annotation aims at predicting a set of tex- tual labels for an image that describe its semantics. These are usually taken from an annotation vocabulary of few hundred labels. Because of the large vocabulary, there is a high variance in the number of im- ages corresponding to different labels (“class-imbalance”). Additionally, due to the limitations of manual annotation, a significant number of available images are not annotated with all the relevant labels (“weak- labelling”). These two issues badly affect the performance of most of the existing image annotation models. In this work, we propose 2PKNN, a two-step variant of the classical K-nearest neighbour algorithm, that addresses these two issues in the image annotation task. The first step of 2PKNN uses “image-to-label” similarities, while the second step uses “image-to-image” similarities; thus combining the benefits of both. Since the performance of nearest-neighbour based methods greatly depends on how features are compared, we also propose a metric learning frame- work over 2PKNN that learns weights for multiple features as well as distances together. This is done in a large margin set-up by generaliz- ing a well-known (single-label) classification metric learning algorithm for multi-label prediction. For scalability, we implement it by alternating between stochastic sub-gradient descent and pro jection steps. Extensive experiments demonstrate that, though conceptually sim- ple, 2PKNN alone performs comparable to the current state-of-the-art on three challenging image annotation datasets, and shows significant improvements after metric learning.