Unisys's US patent on the LZW algorithm expired on June 20, 2003. It was vigorously enforced. Many programs used huffman to avoid paying royalties. PNG was cre

Unisys's US patent on the LZW algorithm expired on June 20, 2003. It was vigorously enforced. Many programs used huffman to avoid paying royalties. PNG was created to avoid problems with LZW patents. A lot of compression technology choices were based on avoiding proprietary algorithms.