Contrast based quantization and rate control for wavelet coded images

A visually-optimal quantization and rate-control strategy based on results of recent contrast sensitivity and suprathreshold summation experiments is proposed. At suprathreshold contrasts, masked detection thresholds for wavelet subband quantization distortions were approximately equal for scale-3, 4, and 5 distortions; approximately 52% greater for scale-2 distortions; and approximately 84% greater for scale-1 distortions. Base contrasts for individual subbands are selected to match these contrast ratios, and are adjusted to account both for changes in relative sensitivity at suprathreshold contrasts and for suprathreshold error-pooling effects such that the combined distortions exhibit a target contrast. Quantizer step sizes are then computed from the adjusted base contrasts. Rate control is performed by scaling target contrast linearly until a specific bit rate is met.