I. Introduction
Objective image quality assessment (IQA), i.e., to automatically estimate the perceptual quality of a distorted image, has been a long-standing research topic. Objective IQA methods are divided into three categories based on the availability of pristine reference images: full-reference IQA (FR-IQA), reduced-reference IQA (RR-IQA), and no-reference IQA (NR-IQA). To develop and evaluate these methods, a number of benchmark databases have been proposed, some of which are compared in Table I.