Image Quality Metrics

Abstract

Images produced on monitors, printers, copiers, and films contain a variety of elements, including text, graphics, and pictorial segments. As viewers evaluate the quality of images, they may notice that various image elements are not well formed or do not communicate the desired intent. Examples of some of the most significant deficiencies seen in images that have pictorial segments include lack of clarity in details, noise in areas expected to be smooth, loss of information in highlights or darker areas, and inaccurate representation of color. These characteristics form the basis for defining sharpness, graininess, tone scale, and color rendition, respectively. These four attributes are significant factors that viewers consider when judging image quality. In images containing text and graphics, perception of quality is critically affected by roughness of edges or lines that should be smooth, blurred edges, or nonuniform density. These defects in image rendering. In addition to the imperfections described, artifacts are often introduced into the images because of mechanical or material problems during imaging. In this article, physical and perceptual aspects of image quality are discussed. Objective methods, which relate the design parameters to the physical parameters of the image are reviewed. Psychophysical methods that are used to characterize the percept in terms of the underlying perceptual or physical scale are then reviewed. Finally, the problem of formulating and validating computational models of image fidelity is discussed.