Graphic artists will use PNG as an intermediate format because of its lossless 24-bit (and up) compression and as a final format because of its ability to store gamma and chromaticity information for platform-independence.
— Greg Roelofs (January 1997} "History of the Portable Network Graphics (PNG) Format" Linux Gazette


Back in 1977 and 1978, two Israeli researchers, Jacob Ziv and Abraham Lempel, first published a pair of papers on a new class of lossless data-compression algorithms, now collectively referred to as "LZ77" and "LZ78". Some years later, in 1983, Terry Welch of Sperry Corporation (which later merged with Burroughs Corporation to form Unisys) developed a very fast variant of LZ78 called LZW. Welch also filed for a patent on LZW, as did two IBM researchers, Victor Miller and Mark Wegman.

Meanwhile, CompuServe -- specifically, Bob Berry -- was busily designed a new, portable, compressed image format in 1987. Its name was GIF, for "Graphics Interchange Fromat", and Berry blithely settled on LZW as the compression method. Tim Oren, Vice President of Future Technology at CompuServe wrote : "The LZW algorithm was incorporated from an open publication, and without knowledge that Unisys was pursuing a patent. The patent was brought to our attention, much to our displeasure, after the GIF spec had been published and passed into wide use."

Whether due to ongoing financial difficulties or as part of the industry-wide' World Wide Web disruptions, Unisys in 1993 began aggressively pursuing commercial vendors of software-only LZW implementations. CompuServe seems to have been its primary target at first, culminating in an agreement -- quietly announced on 28 December 1994, right in the middle of the Christmas holidays -- to begin collecting royalties from authors of GIF-supporting software.

The spit hit the fan on the Internet the following week, what whas then the newsgroup went nuts. Much ire was directed at CompuServe for making the announcement, and then at Unisys once the details became a little clearer, but mixed in with the noise was the genesis of an informal Internet working group led by Thomas Boutell. Its purpose was not only to design a replacement for the GIF format, but a successor to it : better, smaller, more extensible, and free.

The very first PNG draft -- then called "PBF", for Portable Bitmap Format -- was posted by Tom to, comp.compression and comp.infosystems.www.providers on 4 January 1995. Within one week, most of the major features of PNG had been proposed : delta-filtering for improved compresion (Scott Elliott and Mark Adler), deflate compression (Tom Lane, the Info-ZIP gang and many others), 24-bit support (many folks), the PNG name itself (Oliver Frommer), internal CRCs (Greg Roelofs), gamma chunk (Paul Haeberli) and 48 & 64-bit support (Jonathan Shekter).

On of the real strengths of the PNG group was its ability to weigh the pros and cons of various issues in a rational manner (most of the time), reach some sort of consensus, and then move on to the next issue without prolonging discussion on "dead" topics indefinitely. In part this was probably due to the fact that the group was relatively small, yet possessed of a sufficiently broad range of graphics and compression expertise that no one felt unduly "shut out" when a decision went against him. But equally important was Tom Boutell, who, as the initiating force behind the PNG project, held the role of benevolent dictator. When consensus was impossible, Tom would make a decision, and that would settle the matter. On one or two rare occasions, he might later have been persuaded to reverse the decision, but this generally only happened if new information came to light.

By the beginning of February 1995, seven drafts had been produced and the PNG format was settling down. CompuServe was impressed enough by the design that on the 7th of February they announced support for PNG as the designated successor to GIF, supplanting what they had initially referred to as the GIF24 development project. By the beginning of March, PNG Draft 9 was released and the specification was officially frozen. Although further drafts followed, they merely added clarifications, some recommended behaviors for encoders and decoders, and a tutorial or two.

Once Draft 9 was released, many people set about writing PNG encoders and decoders. The true glory is really reserved for three people : Info-ZIP's Jean-loup Gailly and Mark Adler (both also of gzip fame), who originally wrote Zip's deflate() and UnZip's inflate() routines, and then -- for PNG -- rewrote them as a portable library called zlib. And Guy Eric Schalnat of Group 42, who almost single-handedly wrote the libpng reference implementation from scratch. The first truly usable versions of the libraries were released on the first of May, 1995.

Later, the pace of subsequent development slowed at that point. This was partly due to the fact that, after four months of intense development and dozens of e-mail messages every day, everyone was burned out. Partly because Guy controlled libpng's development and became busy with other things at work. Partly because of the perception that PNG was basically "done", emphasized by a CompuServe press release mid-June.
—Greg Roelofs (January 1997} "History of the Portable Network Graphics (PNG) Format" Linux Gazette


In December, at the Fourth International World Wide Web Conference, the World Wide Web Consortium (W3C) released the PNG Specification version 0.92 as an official standards-track Working Draft. In July 1996, the W3C promoted the spec to Proposed Recommendation status and then to full Recommendation status on October. Finally, in mid-October, the Internet Assigned Numbers Authority (IANA) formally approved "image/png" as an official Internet Media Type, joining image/gif and image/jpeg as non-experimental image formats for the Web.

Microsoft recognized the benefits of PNG and apparently embraced it wholeheartedly. They have not only made it the native image format of the Office97 application suite, but have also repeatedly promised to put it into Internet Explorer. 5 May 1997, As promised, the first 4.0 beta of Microsoft Internet Explorer does indeed have native PNG support.

11 November 1997, Netscape Communicator 4.04 was released with native PNG support.

8 April 1998, The PNG-writing code in Netscape's second release of the Mozilla sources was removed for "legal reasons". According to the included ns/LEGAL file, Stac claims that a pair of their patents cover the deflate algorithm. Since no one but Netscape seems to have been contacted by Stac, and since both deflate and inflate received a clean bill of health in the Free Software Foundation's patent search, both the PNG Group and Info-ZIP are considerable puzzled by this.

29 November 1998, Note that while it is possible to write an infringing deflate encoder, the one in zlib was very carefully written to avoid all patents, and the deflate specification (RFC 1951) notes this explicitly : "... it is strongly recommended that the implementor of a compressor follow the general algorithm presented here, which is known not to be patented per se."
—Greg Roelofs (January 1997} "History of the Portable Network Graphics (PNG) Format" Linux Gazette