PNG, the Portable Network Graphics format, is a lossless raster image format created in the mid-1990s as a deliberate, unencumbered replacement for GIF. The original specification was published by the IETF as RFC 2083 in March 1997, authored by Thomas Boutell. RFC 2083 describes PNG as “an extensible file format for the lossless, portable, well-compressed storage of raster images” and states plainly that it “provides a patent-free replacement for GIF.”
The motivation was direct. GIF used LZW compression, which was covered by a Unisys patent (US 4,558,302), and in late 1994 Unisys and CompuServe announced licensing terms that alarmed the free-software and web communities. A group of developers responded by designing a new format from scratch that would be free of patent encumbrances. PNG’s compression is built on the DEFLATE algorithm, the same zlib-based combination of LZ77 matching and Huffman coding used in gzip, which was not patent-restricted.
Technically, PNG stores images as a stream of typed chunks, each with a length, a four-letter type code, data, and a CRC checksum. This chunk structure makes the format extensible: critical chunks define the image dimensions, bit depth, and color type, while ancillary chunks carry optional information such as gamma, color profiles, text comments, and transparency. PNG supports grayscale, truecolor, and palette-indexed images, and unlike GIF it offers a full alpha channel, allowing smooth variable transparency rather than a single on-or-off transparent color. Before compression it applies per-row filtering (predictors that subtract neighboring pixel values) to make the data more compressible.
PNG is lossless, meaning the decoded image is bit-for-bit identical to the original, which makes it well suited to screenshots, diagrams, line art, and images with sharp edges and flat color regions where the lossy artifacts of JPEG would be objectionable. The tradeoff is that for photographic content PNG files are typically much larger than JPEG files. PNG does not natively support animation in its original form; the later APNG extension and the separate MNG format addressed moving images.
The format was subsequently adopted by the World Wide Web Consortium as a W3C Recommendation and standardized internationally. The W3C Recommendation at w3.org/TR/PNG carries the format forward, with the Third Edition published in 2025 adding animation support and aligning the specification for international standardization. PNG became one of the three dominant web image formats alongside JPEG and GIF, and its origin remains a clear case study in how a patent dispute over a compression algorithm can reshape an entire category of file formats.