Subband Compression of High Quality Color Still Images

This paper presents a complete coding system for compression of natural color still images represented with 24 bits per pixel. The original 8-bit red, green, and blue (RGB) image components are first transformed to a luminance (Y) and two chrominance components (Cb, Cr) according to the ITU-R Recommendation 601 [1]. Each of the Y, Cb, and Cr tristimulus values are then decomposed separately in a subband filter bank with different filters for the luminance and the chrominance components. The Y-component requires a significantly higher bit rate than the Cb- and Cr-components, which can be downsampled by a factor of 10 to 20. For high quality image compression the downsampling factor must not be too large to avoid color artifacts. The luminance and chrominance components are quantized in uniform threshold quantizers with step size adapted to the signal statistics and the human visual system. A bank of entropy coders adapted to the components' subband signal statistics is used for bit ef..