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. Based on a suprathreshold error-pooling model, contrasts for individual subbands are selected to match these contrast ratios, and are adjusted to account for changes in relative sensitivity at suprathreshold contrasts. Quantization step sizes are then computed from the adjusted base contrasts. A target contrast is estimated from the desired rate, and rate control is performed by adjusting this contrast until the rate is met. Images compressed with the proposed algorithm show improved visual quality at low bit rates.