Mahmoud R. El-Sakka, Khaled Hassanein, and Mohamed S. Kamel, "An Optimized JPEG Quantizer for Financial Document Images", SPIE International Conference on Applications of Digital Image Processing XXI, Vol. 3460, pp. 530-538, July 1998, San Diego, California, USA.
In this paper, two schemes for optimizing the JPEG quantizer are investigated. The first scheme starts from a given quantization-table and executes a sub-optimal search for quantization-table parameters. Those parameters, when changed by a pre-specified incremental step-size, result in the most optimal move towards decreasing (increasing) the compressed file-size, with a minimal increase (maximum decrease) of error. This procedure is repeated, until a pre-specified file-size is reached. The second scheme is based on performing a mapping from the JPEG default quantization-table to an optimized one. This mapping adapts the default quantization-table to the statistics of the DCT coefficients of the image at hand.
The superiority of these two optimized JPEG quantizers is established in terms of the visual quality of their reconstructed images. This was done by running comparative visual image quality experiments involving 20 human subjects. These optimized quantizers were also demonstrated to result in a higher level of machine image quality, by improving the accuracy of cheque amount reading applications. Experimental results indicate that the second optimization scheme yields a higher level of visual image quality, while requiring a fraction of the processing time used by the first scheme.