Lossy LZW algorithm with adaptive threshold for palettized image compression

Color palettized image file formats especially the GIF format are widely used in the World Wide Web of the internet. In these file formats, only lossless compression techniques are used for the encoding of palettized image, thus the compression ratio is only 1.2 to 2.5. To achieve higher compression ratio, lossy compression techniques have to be use. However, significant visual artifact due to lossy encoding of the palettized image may occur in the reconstructed images, and the compressed file format is not compatible to the conventional formats. To avoid these two problems, a Lossy LZW algorithm is proposed for palettized images compression. In the proposed algorithm, adaptive thresholds according to the human visual characteristics are used to constrain the color pixel's error of the reconstructed image. In addition, the algorithm can be embedded in the GIF encoder so that the original GIF decoder can be used to decode the image compressed by the new algorithm.