I. Introduction
Digital images are subject to a broad spectrum of distortions during the process of acquisition, compression, transmission and reproduction. Therefore, it is essential to guarantee the quality of image content for end-users. Subjective viewing test is a natural way to evaluate visual image quality. Despite of its high accuracy and reliability, subjective evaluation is cumbersome, expensive, time consuming, and non-reproducible, which makes it difficult to be embedded into practical applications such as real-time quality monitoring and prediction [1], [2].