The JPEG XR image compression standard, originally developed under the name
HD-Photo by Microsoft, offers the feature of spatial variably quantization;
its codestream syntax allows to select one out of a limited set of possible
quantizers per macro block and per frequency band. In this paper, an
algorithm is presented that finds the rate-distortion optimal set of quantizers,
and the optimal quantizer choice for each macro block. Even though it
seems plausible that this feature may provide a huge improvement for images
whose statistics is non-stationary, e.g. compound images, it is demonstrated that
the PSNR improvement is not larger than 0.3dB for a two-step heuristics
of feasible complexity, but improvements of up to 0.8dB for
compound images are possible by a much more complex optimization strategy.
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