Assignment of Indices Suitable for the Predictive Coding of a Color Quantized Image

Abstract

A color quantized image is represented by providing each pel with an index corresponding to a representative color of the pel. There must be a one-to-one correspondence between representative colors and their respective indices, and this correspondence can be designated freely. To encode efficiently a color quantized image using conventional predictive coding schemes such as JPEG lossless coding, this paper proposes a new method which assigns an almost optimal index to each representative color on the basis of evaluation parameters that indicate approximately the predictive efficiency or coding rates. Although the method renews repeatedly assignment of indices, the introduction of a two-dimensional histogram of the indices reduces remarkably the amount of computation required for the repetition. Simulation results show that the application of the method to JPEG lossless coding decreases the coding rates of 8-bit color quantized images by 0.39-0.80 bits/pel and that these coding rates are 0.78-1.81 bits/pel lower than those of GIF.