Patents

One note of caution regarding the use of the LZW algorithm. Terry Welch filed for, and was awarded, a U.S. patent covering at least some portions of his algorithm. This patent is presently assigned to Unisys, which has made public its intention to protect its intellectual property rights, and has in recent years begun to do so. LZW compression is defined as part of the CCITT V.42bis specification, and Unisys has defined specific terms under which it will license the algorithm to modem manufacturers. Unisys is now in the process of seeking license payments from software vendors who use the LZW algorithm in programs, most notably in the GIF graphics file format.

Clearly LZW is a derivative work of the LZ78 algorithm, but defining the boundaries of what is covered by the patent and what is not probably requires the assistance of a skilled patent attorney. Over the past ten years, quite a few software copyright battles have been fought in the courts, enough so that software developers can sensibly use some general rules. The same cannot be said for software patents. The U.S. patent office has only begun issuing these patents in a major way since the mid 1980s, and very little significant litigation has made its way through the courts. Programmers and manufacturers would be wise to seek competent counsel before stepping into these waters.