 Some algorithms are patented in the United States and other countries and their legal usage requires licensing by the patent holder.Because of patents on certain kinds of LZW compression, and in particular licensing practices by patent holder Unisys that many developers considered abusive, some open source proponents encouraged people to avoid using the Graphics Interchange Format (GIF) for compressing still image files in favor of Portable Network Graphics (PNG), which combines the LZ77-based deflate algorithm with a selection of domain-specific prediction filters.However, the patents on LZW expired on June 20, 2003.