The role of perceptual contrast non-linearities in image transform quantization

The conventional quantizer design based on average error minimization over a training set does not guarantee a good subjective behaviour on individual images even if perceptual metrics are used. In this work a novel criterion for transform coder design is analyzed in depth. Its aim is to bound the perceptual distortion in each individual quantization according to a non-linear model of early human vision. A common comparison framework is presented to describe the qualitative behaviour of the optimal quantizers under the proposed criterion and the conventional rate-distortion based criterion. Several underlying metrics, with and without perceptual non-linearities, are used with both criteria. Analytical results show that the proposed design criterion gives rise to a JPEG-like quantization if a simple linear metric is used. Experimental results show that signiicant improvements over the perceptually weighted rate-distortion approach are obtained if a more meaningful non-linear metric is used.