A CIELab driven adaptive quantization scheme for DCT-based compression of CMYK images

In todays digital prepress workflow images are most often stored in the CMYK color representation. In the lossy compression of CMYK color images, most techniques don't take the tonal correlation between the color channels into account or they are not able to perform a proper color decorrelation in four dimensions. In a first stage a compression method has been developed that takes this type of redundancy into account. The basic idea is to divide the image into blocks. The color information in those blocks is then transformed from the original CMYK color space into a decorrelated color space. In this new color space not all components are of the same magnitude so here the gain for compression purposes becomes clear. After the color transformation step any regular compression scheme meant to reduce the spatial redundancy can be applied. In this paper a more advanced approach for the quantization procedure in the compression algorithm is presented. The proposed scheme tries to control the quantization parameters differently for all blocks and color components. Therefore the influence on the CIELab ΔE measure is investigated when making a small shift in the four main directions of the decorrelated color space.