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 rst 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 lter bank with diierent lters for the lumi-nance and the chrominance components. The Y-component requires a signiicantly 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 down-sampling 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 eecient representation of the quantized samples. The coding results shows that a transparent image quality can be obtained at about 1.2 bits per pixel for the Mandrill image.