The PNG Guide is an eBook based on Greg Roelofs' book, originally published by O'Reilly.

History of the Portable Network Graphics Format

Internet GIF tax,
January '95.
PNG to the rescue!

--Glenn Randers-Pehrson[47]

[47] Alternatively, ``Unisys bombshell, / Christmas 1994. / PNG to the rescue!''

The Portable Network Graphics image format, or PNG for short, is the first general-purpose image format to achieve wide, cross-platform acceptance[48] since JPEG/JFIF arrived in the early 1990s. Almost every major feature in PNG exists in other general-purpose formats--specifically, GIF, JPEG, and TIFF--yet in January 1995, a group of strangers felt compelled to band together and design another image format from scratch. To understand why, it is necessary to delve even further into history.

[48] The choice of adjectives is intentional: there are other widely accepted formats, such as Windows BMPs, but they're not cross-platform, and there are cross-platform formats such as PostScript or the astronomical FITS format, but they're not general-purpose.

In 1977 and 1978, Israeli researchers Jacob Ziv and Abraham Lempel published a pair of papers on a new class of lossless data-compression algorithms in the journal IEEE Transactions on Information Theory. These algorithms, now collectively referred to as ``LZ77'' and ``LZ78,'' formed the basis for an entire industry of software, hardware, and subsequent research papers. One of the follow-up papers was by Terry Welch and was published in the June 1984 issue of IEEE Computer. Entitled ``A Technique for High-Performance Data Compression,'' it described his research at Sperry into a fast, efficient implementation of LZ78 called LZW.

By 1987, when CompuServe's Bob Berry was busy designing the GIF image format, LZW was well established in the Unix world in the form of the compress command, and in the PC world in the form of SEA's ARC. As a fast algorithm with good compression and relatively low memory requirements, LZW was ideally suited to the PCs of the day, and it became Berry's choice for a GIF compression method, too. In turn, GIF became the image format of choice on the Internet, particularly on the worldwide discussion forum known as Usenet.

And so things remained largely unchanged until 1994. The introduction (from a practical standpoint) of JPEG around 1992 or 1993 may have slowed GIF's rising star slightly, but computational requirements and the limitations of then-current graphics cards limited JPEG's acceptance for several years. With the advent of graphical browsers for the World Wide Web in 1992 and 1993, GIF's popularity only increased: simple graphics with few colors were the norm, and those were ideally suited to GIF's palette-based format. With the release of Netscape Navigator 1.0 in 1994, progressive rendering of images as they downloaded suddenly became widespread, and GIF's interlacing scheme worked in its favor once more.[49]

[49] Progressive capability had for quite some time been part of the JPEG specification, too, but since the Independent JPEG Group's free library didn't support the progressive mode until August 1995, neither did any applications--including web browsers.

Then, three days after Christmas 1994, CompuServe quietly dropped a small bombshell on an unsuspecting world: henceforth, all GIF-supporting software would require royalties. In fact, the announcement was apparently the culmination of more than a year of legal wrangling with Unisys, which had inherited the Welch LZW patent in the 1986 merger of Sperry and Burroughs, and which had by 1993 become considerably more aggressive about enforcing its patent in software-only applications.

In any case, shortly after the holidays ended, word of the announcement reached the Internet--specifically, the ever-volatile Usenet community. As one might expect, the results were spectacular: within days, a full-fledged conflagration of bluster, whining, flaming, vitriol, and general-purpose noise had engulfed several of the Usenet newsgroups, among them comp.compression and But mixed in with the noise was the genesis of an informal Internet working group led by Thomas Boutell. Its purpose was to design not only a replacement for the GIF format, but also a successor to it: better, smaller, more extensible, and free.

Last Update: 2010-Nov-26