Lossy compression of palettized images

Abstract

Many digital display systems economize by rendering color images with the use of a limited palette. Palettized images differ from continuous-tone images in two important ways: they are less continuous due their use of lookup table indices instead of physical intensity values, and pixel values may be dithered for better color rendition. These image characteristics reduce the spatial continuity of the image, leading to high bit rates and low image quality when compressing these images using a conventional lossy coder. We present an algorithm that uses a debinarization technique to approximate the original continuous-tone image, before palettization. The color components of the reconstructed image are then compressed using standard lossy compression techniques. The decoded images must be color quantized to obtain a palettized image. We compare our results with a second algorithm that applies a combination of lossy and lossless compression directly to the color quantized image in order to avoid color quantization after decoding.