Insufficiency of labeled training data is a major obstacle for automatically annotating large-scale video databases with semantic concepts. Existing semi-supervised learning algorithms based on parametric models try to tackle this issue by incorporating the information in a large amount of unlabeled data. However, they are based on a "model assumption" that the assumed generative model is correct, which usually cannot be satisfied in automatic video annotation due to the large variations of video semantic concepts. In this paper, we propose a novel semi-supervised learning algorithm, named Semi Supervised Learning by Kernel Density Estimation (SSLKDE), which is based on a non-parametric method, and therefore the "model assumption" is avoided. While only labeled data are utilized in the classical Kernel Density Estimation (KDE) approach, in SSLKDE both labeled and unlabeled data are leveraged to estimate class conditional probability densities based on an extended form of KDE. We also investigate the connection between SSLKDE and existing graph-based semi-supervised learning algorithms. Experiments prove that SSLKDE significantly outperforms existing supervised methods for video annotation.
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