Why PNG Was Created: The GIF Patent Controversy

The creation of the Portable Network Graphics (PNG) format was directly triggered in late December 1994 when Unisys announced it would aggressively enforce its patent on the LZW compression algorithm used in GIF files. Faced with sudden royalty demands and the threat of litigation, developers across the early internet organized to produce a patent-free, technologically superior alternative. Within months, this collective effort yielded the PNG format, which not only solved the legal crisis but also modernized lossless web imagery.

In 1987, CompuServe introduced the Graphics Interchange Format (GIF) to allow users to view color images across different computer platforms. To keep file sizes small over slow dial-up modems, CompuServe utilized the Lempel-Ziv-Welch (LZW) data compression algorithm. However, CompuServe was unaware that Unisys held a 1985 patent covering the LZW technique. For years, the format spread across the burgeoning World Wide Web, becoming the universal standard for digital images without licensing friction.

The landscape shifted abruptly on December 28, 1994, when CompuServe and Unisys jointly announced that developers of commercial software utilizing GIF support would be required to pay licensing royalties for LZW. The announcement ignited widespread panic and outrage across the software and web development communities. Developers faced potential liability not just for future software, but retroactively for existing tools. Non-commercial and open-source developers were particularly vulnerable, as they lacked the funds to pay licensing fees for freely distributed software.

In direct response, developer Thomas Boutell initiated a discussion on the Usenet newsgroup comp.graphics on January 4, 1995. The goal was simple: design an open-source, patent-free replacement for GIF before the Unisys licensing policy took full effect. A group of internet architects and programmers quickly coalesced around the project, drafting specifications for what was initially called the Portable Bitmap Format, later renamed Portable Network Graphics (PNG).

Rather than merely cloning GIF, the working group addressed the technical limitations that had constrained GIF since the 1980s:

• Patent-Free Compression: PNG replaced LZW with the Deflate algorithm (the same open, unpatented algorithm used in PKZIP and gzip), which was verified to be free from intellectual property encumbrances while offering better compression ratios.
• Full Color Depth: While GIF was strictly limited to an 8-bit palette of 256 colors, PNG introduced support for 24-bit truecolor and 48-bit deep color, making it suitable for continuous-tone photographic images as well as graphics.
• Alpha Transparency: GIF only offered binary transparency (a pixel was either fully opaque or completely invisible). PNG introduced an 8-bit alpha channel, providing 256 levels of opacity for smooth edges and seamless blending over varied backgrounds.
• Integrity and Portability: PNG implemented 32-bit CRC checks for data corruption detection and platform-independent color management via gamma correction.

The group intentionally chose not to include multi-frame animation in the PNG specification, keeping the format lightweight and focused purely on single-image rendering.

By March 1995, the core PNG specification was essentially complete, and by October 1996, it gained formal endorsement as a W3C Recommendation. When Unisys subsequently tightened its licensing terms in 1999 to target non-commercial software and even website operators hosting GIFs, adoption of PNG accelerated rapidly. By the time the Unisys LZW patents finally expired worldwide between 2003 and 2004, PNG was firmly established as the web's premier lossless image standard.