METHOD AND APPARATUS FOR COMBINING PALETTES OF COLOR QUANTIZED IMAGES

A color-mapped display sub,stem efficiently combines palettes of multiple images into a single shared palette. As each image already received a degree of distortion during conventional palette selection, it is desirable to minimize further distortion during the palette combination method of this invention. A pairwise nearest neighbor (PNN) technique is used for combining colors from respective palettes to minimize further distortion. For a final 256-color shared palette, up to 256 (n-1) individual vector merges are performed (where n is the number of image palettes being combined). In one embodiment, two vectors are chosen at each step that yield the lowest increase in distortion when merged. A mean squared error distortion measure of gamma-corrected values defined in YIQ space is used to compare distortion. Searching time at each step is reduced from O(N.sup.2) to O(N), while also eliminating the need for extensive recalculation of color pair distortions between steps. Efficiency is enhanced because the matrix effectively caches distortion calculations between steps. One advantage of the invention is the ability to service run-time demands for the simultaneous display of multiple images on a personal computer or workstation platform having an 8-bit color-mapped display subsystem. Another advantage is the maintenance of image quality across similar multiple images using a shared palette.