Abstract: In this paper, we have investigated the compression behavior of each processing step of the JPEG baseline image coder. The two main objectives of this research are to provide a better understanding of the JPEG system and to provide a guideline for improving the performance of any JPEG-like image coders. For performance evaluation, we have chosen the estimated entropy as a means for performance measure. The key results of this paper are: (1) Generally, the psychovisually-weighted plays a dominant role of the overall system performance. (2) The compression gain provided by the entropy coding procedure is also significant. Since there is a gap between the estimated entropies and the actual coding rates, a more efficient entropy coding procedure which reflects the signal statistics should improve the system performance. (3) The common concept of the optimal transform is variance-based, which requires a zonal selection of the transform coefficients. Since the JPEG adopts the thresholding quantization, the ordinary discussion of an optimal transform is not appropriate. A truly optimal transform should take the transform and its subsequent operations into account. In consequence, to improve the overall system performance it would be effective to focus on the quantization and entropy coding procedures.!13
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