LZW compression as used by Gif, Tif and Zip is completely lossless. There are different flavours of LZW depending on implementation, but they all work by looking for repeating patterns of data and replacing them with a single copy and a count of repeats. ("Run length encoding") e.g. 0,2,4,5,0,2,4,5,0,2,4,5 could be replaced by 3,4,0,2,4,5 or 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 by 15,1,0 To uncompress the data the process is reversed. - Mark

Mark