Subjective quality assessment of compressed images

With limited additional effort, one can obtain valuable information on the subjective performance of image compression schemes. The choice of an appropriate experimental technique, however, is not always trivial, since both the subject's task and the experimental method itself should depend on the properties of the stimulus set (quality range, kind of artifacts). The goal of this paper is to demonstrate how to use a number of numerical scaling techniques in assessing perceived image quality. We show how to analyze data obtained in numerical category scaling experiments and how to set up such experiments. The results of several subjective experiments illustrate that numerical category scaling techniques provide an efficient means not only for obtaining compression ratio versus quality curves that characterize coder performance over a broad range of compression ratios, but also for assessing perceived image quality in a much smaller range (e.g., close to threshold level). However, the nature of the artifacts introduced by different coders can cause problems when evaluations are carried out using direct numerical category scaling. The latter is demonstrated by comparing the results of a direct scaling method and a scaling technique in which subjects have to determine quality differences between all possible combinations of coded images