Image Quality Metrics

INTRODUCTION 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. Figure 1 illustrates these defects in image rendering. In addition to the imperfections described before, artifacts are often introduced into the images because of mechanical or material problems during imaging. For example, mechanical noise in the imaging chain of a printer may produce uneven development of density, called banding. Other failures of the equipment may produce a variety of artifacts such as streaks or white or black spots. Such image defects are usually produced by equipment failures and are labeled artifacts, which are distinguished from the characteristics of the image that result from normal operation of the imaging device. Figure 2 illustrates three such artifacts — banding, repetitive patterns, and streaking. Throughout this article, we have used a set of high-quality original images to demonstrate the effects of various image quality factors. Many of these images are from the widely available Kodak PhotoCD sampler. All artifacts and defects have been simulated using image processing algorithms that have been designed to mimic closely, in a carefully controlled way, the actual effect that is being illustrated. The perceived quality of an image for which the original is not available to the observer is governed by factors such as how ''pleasing'' the overall rendered image is and how well it conforms to the observer's expectation of what it should look like. These aspects of quality are related to a number of visual attributes of the image. For example, perceived quality generally increases for increasing degree of sharpness and decreasing levels of graininess. The presence of artifacts at any visible level tends to …