History of PNG
The development of PNG is an interesting one that has its roots early in the development of a graphical Internet.
In 1987, the second distribution of Berkeley PDP-11 Software for UNIX Release 2.10 came out; Leonard Nemoy scored a big hit with Three Men and a Baby, which grossed $167 million; Athens Georgia rockers REM released Document; and computer users were itching for a way to send dirty pictures to one another over the newly discovered Internet.
CompuServe, the largest online subscription service at the time, obliged users by creating the now very familiar .gif (Graphic Interchange Format) image file format. Users were free to scan, upload, download, and trade any pictures that their hearts desired. And at a color depth of 8 bits, or 255 colors, it was exactly as colorful as most of the nice monitors at the time. To help save space on your hard driveand to save time when sending or receiving picturesthe GIF format made use of data compression. The compression algorithm was lossless, meaning that no original data is lost when the image is saved. This meant that a file could be opened, modified, and changed infinitely with no degradation to the image. The .gif format took off and rapidly became the de facto standard for image file types. FTP servers got crammed with photos of girls in short skirts, images of planets and galaxies, scanned album covers, and anything pertaining to Star Trek.
It takes an awful lot of data to properly represent an image file, so .gif images are compressed. When developing this file type, CompuServe chose a method of compression called LZW (named for its creators, Abraham Lempel, Jacob Ziv, and Terry Welch). LZW was described in the article "A Universal Algorithm for Sequential Data Compression," published by Lempel and Ziv in IEEE Transactions on Information Theory in May 1977. The compression technique later was refined by Terry Welch in his article "A Technique for High-Performance Data Compression," in Computer, June 1984. The LZW compression method was effective and easy to implement.
Seven years later, in 1994, Pulp Fiction and Forest Gump were tearing it up in the theaters, the O.J. Simpson trial was on everybody's television, Oklahoma City and "Government Shutdown" were phrases on the lips of Americans, and, that same year, CompuServe dropped a bomb on the Internet. Unbeknownst to CompuServe, LZW was a proprietary algorithm owned by Unisys. Unisys itself had been blissfully unaware that anybody was using the technologyand when Unisys found out, the company took action to protect the patented technologies.
The publication of Unisys's initial licensing agreement caused a furor in the Libertarian-minded Internet community, which had grown up on the exchange of free data. Immediately rumors began to spread that Unisys was going to start hunting down individual users and charging them for every digital image of a bikini-clad supermodel found on their hard drives. In reality, Unisys wanted a percentage of the royalties for products that utilized their compression algorithm (.45% of the selling price of GIF/LZW products (max $10) and .65% on GIF/TIFF/LZW products (max $25); it didn't want anything from the end user. But the fires had been lit, and already people began plotting for a royalty-free solution.
This was the impetus for Portable Network Graphic.