The problem of evaluating the quality of a compressed image in specific applications may be solved using a suitable rate-distortion measure. In practice, however, this procedure may be difficult to apply for two reasons: (1) There is a lack of knowledge of which distortion measure is more suitable for images (Refs. , , , , and ), and (2) the characteristics of the human visual system are not well understood (Refs.  and ), so the goal of finding a general-enough measure of perceptual quality has proven to be elusive.
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