Visual sensitivity considerations for subband coding

Sensitivity to uniform quantization noise in wavelet subbands within an image was evaluated psychovisually with an uncontrolled survey and a controlled test. Quantizer step sizes for a single band were varied while the other bands were not quantized, allowing spatial masking to occur. The results of the survey are incorporated into a quantization strategy using standard Lagrangian bit allocation to provide images at a range of compression ratios that exhibit similar visual characteristics and are suitable for general-population use on the Web. The results of the second test suggest that quantizer step sizes causing just-noticeable-distortion are proportional to the square root of the standard deviations of the subbands rather than to the standard deviations themselves, as commonly used.