Optimized universal color palette design for error diffusion

Currently, many low cost computers can only simultaneously display a palette of 256 colors. However, this palette is usually selectable from a very large gamut of available colors. For many applications, this limited palette size imposes a significant constraint on the achievable image quality. In this paper, we propose a method for designing an optimized universal color palette for use with halftoning methods such as error diffusion. The advantage of a universal color palette is that it is fixed and therefore allows multiple images to be displayed simultaneously. In order to design the palette, we employ a new vector quantization method known as sequential scalar quantization (SSQ) to allocate the colors in a visually uniform color space. The SSQ method achieves near optimal allocation, but may be efficiently implemented using a series of look up tables. When used with error diffusion, SSQ adds little computational overhead, and may be used to minimize the visual error in an opponent color coordinate system. We compare the performance of the optimized algorithm to standard error diffusion by evaluating a visually weighted mean squared error measure. Our metric is based on the color difference in CIELab, but also accounts for the lowpass characteristic of human contrast sensitivity. This work was supported by an NEC Faculty Fellowship. Journal of Electronic Imaging, vol. 4, no. 2, pp. 131-143, April 1995. 1