The story of the PNG format

In 1977 and 1978, two Israeli researchers, Jacob Ziv and Abraham Lempel worked on a set of compression algorithms, which were later named LZ77 and LZ78. Later, in 1983, Terry Welch from Sperry (which later merged with Burroughs to form Unisys) developed a very fast variant of LZ78 called LZW. Welch, together with two IBM researchers, Victor Miller and Mark Wegman, filled for a patent for LZW.

At the same time, Compuserve, specifically Bob Berry, was designing a new algorithm for a compressed image format, where they used the LZW compression, without knowing that LZW algorithms were patented. After the GIF format was already released and popular, CompuServe found out from Unisys that the LZW compression in patented. There was already no way back, so they continued to use LZW, motivated by Unisys’s promises that it would not ask royalties from “pure software developers”. They were instead charging royalties from modem factories, which used LZW.

But at the end of 1993, Unisys started to demand that software developers would pay royalties for them. Of course, the first targeted company was CompuServe. After that, a great panic emerged online, especially on the Usenet newsgroups. Among the panic, a team led by Thomas Boutell was working on creating a new format that would substitute PNG.

Its first name was PBF (Portable Bitmap Format). The first specification contained three-byte signature, chunk numbers rather than chunk names, maximum pixel depth of 8 bits and no specified compression method, but it would afterwards be improved. During a week, the format was discussed, and in the end it came with delta-filtering for improved compression  deflate compression 24-bit support the PNG name itself, internal CRCs, gamma chunk , and 48- and 64-bit support.

After 9 subsequent drafts, the format standards were frozen, with some minor changes along time.  Nowadays, Glenn Randers-Pehrson still keeps some PNGs created at the 9^th Draft. They are still readable by any PNG decoder today