<title>Important distortion factors in the encoding of very high quality images</title>

Abstract

The determination of an objective quality scale for image coding that corresponds to the subjective evaluation of quality is a long standing problem that has eluded researchers for many years because of its complexity. As a consequence, SNR, which is recognized as an inappropriate method for evaluating image coding, has been widely used for convenience. Recently, several of the authors have developed a new approach to the quantitative determination of image quality, defining a picture quality scale (PQS) that correlates well with subjective ratings, for mean opinion scores (MOS) in the range of 2 to 4. In this work, we extend and specialize these results for the case where the quality requirements become very high and where the distortion should be barely perceptible. We consider the detailed spatial distortion maps, which are the local contributions for distortion factors that make up PQS, as spatial indicators of perceptible distortions. We determine the applicability of these distortion maps to the type of distortions that remain perceptible at high quality. We propose some modifications to the PQS metric that would apply at high quality. The experimental results are established for images processed using the JPEG coding standard.