JPEG is an international standard for still image compression [l]. The P E G baseline algorithm allows users to supply the custom quantization table and Huffman table to control the compression ratio and the quality of the encoded image. Methods for determining the quantization matrix are usually based on i) ratedistortion theory [2,8] and ii) spatial masking effects of the human visual system [9]. In [2], Wu and Gersho proposed a recursive algorithm for generating pictureadaptive quantization tables based on rate-distortion approach but the complexity of the encoding algorithm is rather high, In this paper, we propose improvements to the Wu-Gersho's algorithm and a new bit allocation algorithm. Simulation results show that our new algorithm is superior to the Wu-Gersho's algorithm in terms of speed and peak signal to noise ratio (PSNR). Moreover, by incorporating the Human Visual System (HVS), our proposed coder can encode images with better visual quality.

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In this paper, a method to determine the visibility thrcsholds for a given subband systcm is introduced. Our approach identifies the important worsc case combination of the quantization error in cach basis functiori and determine the corresponding visibility thresholds using results previously measured in the DCT domain. Using th:: proposed method, it is relatively simple to determine the visibility thresholds for any subband system in the YCrCb domain. Simulation results on the Lapped Orthogonal Transform (LOT) show that the visibility thresholds are useful in visually lossless coding and perceptual weighted quantimtion.