How a Patent Letter Made PNG in Two Months

On December 28, 1994, Unisys announced that anyone shipping software that wrote GIF files would owe royalties on the LZW compression patent. CompuServe had picked LZW for GIF in 1987 without realizing it was patented; Unisys had filed in 1983 and quietly held the rights ever since. The web ran on GIF. Suddenly the web ran on a patent.

Seven days later, on January 4, 1995, Thomas Boutell posted a draft for a replacement to comp.graphics. He called it PBF, Portable Bitmap Format, though contributors also joked about Chunky GIF and Peanut Butter Format. Within a week, the newsgroup thread had locked in most of the format defining choices: deflate compression (patent-free, courtesy of LZ77 and Huffman coding, championed by Tom Lane and Mark Adler), delta filtering for smoother gradients, full 24-bit color, and internal CRC checks for corruption.

On January 6, Oliver Fromme suggested PING, a recursive acronym for PING is not GIF. It got abbreviated to PNG in Draft 5, on January 23. By March 7, Draft 9 was frozen.

That is roughly nine weeks from a patent threat on Usenet to a stable spec. PNG was better than GIF in almost every dimension, deeper color, lossless compression, alpha channels, and it still spread slowly because browsers took years to support it cleanly (Internet Explorer did not handle PNG transparency correctly until version 7 in 2006). The LZW patent itself expired in 2003. PNG outlasted it. RFC 2083 went out in January 1997 and the format has barely changed since.