Portable Network Graphics (PNG)

An Open, Extensible Image Format with Lossless Compression

(Not Related to Papua New Guinea, the Pawnee National Grassland,
the Professional Numismatists Guild or the "Pack 'N' Go" format)

Welcome to the PNG Home Site, maintained by Greg Roelofs. Our hero likes to speak of himself in the third person, but don't let that put you off; this is intended to be a mostly serious set of reference pages for locating information, applications and programming code related to the twelve-year-old PNG image format.

What It Be (An Informal History)

So what is PNG, and why is it worthy of its own home site? PNG (pronounced "ping") is the Portable Network Graphics format, a format for storing bitmapped (raster) images on computers. Unofficially its acronym stands for [ ] "PNG's Not GIF." PNG was designed to be the successor to the once-popular GIF format, which became decidedly less popular right around New Year's Day 1995 when Unisys and CompuServe suddenly announced that programs implementing GIF would require royalties, because of Unisys' patent on the LZW compression method used in GIF. Since GIF had been showing its age in a number of ways even prior to that, the announcement only catalyzed the development of a new and much-improved replacement format. PNG is the result.

(By the way, despite the implications in some of CompuServe's old press releases and in occasional trade-press articles, PNG's development was not instigated by either CompuServe or the World Wide Web Consortium, nor was it led by them. Individuals from both organizations contributed to the effort, but the PNG development group exists as a separate, Internet-based entity.)

That's only half the story, however; PNG would deserve a home page even if all that had not taken place, just because it's so darned nifty. Yes, it's not every day you come across an image format and say, "Outraaageous!" In fact, you may never say that in your entire lifetime (truly a pity), but PNG is still cool. Some of its spiffier features include:

* unambiguous pronunciation (ooo, baby!)
* multiple CRCs so that file integrity can be checked without viewing
* ultra-clever magic signature that can detect the most common types of file corruption
* better compression than GIF, typically 5% to 25% (but often 40% or 50% better on tiny images)
* non-patented (you betcha!), completely lossless compression
* majorly gnarly two-dimensional interlacing scheme
* 1-, 2-, 4- and 8-bit palette support (like GIF)
* 1-, 2-, 4-, 8- and 16-bit grayscale support
* 8- and 16-bit-per-sample (that is, 24- and 48-bit) truecolor support
* full alpha transparency in 8- and 16-bit modes, not just simple on-off transparency like GIF
* "palette-alpha" mode, effectively transforming normal RGB palette into RGBA
* gamma correction for cross-platform "brightness" control
* color correction for cross-platform, precision color
* both compressed and uncompressed text chunks for copyright and other info
* full Year 2000 (Y2K) support, and then some (good for at least 63 millenia! yowza!)
* free and complete reference implementation with full source code

Not exactly spiffy, but worth mentioning anyway:

* officially registered Internet media ("MIME") type: image/png

PNG also supports things like suggested quantization, "smart" extensibility, a standard color space and lots of other excellent stuff, but let us leave all that aside for now. Those who want a quick explanation of the main features can check out Greg's Basic Introduction to PNG Features. Those who want all of the gory details can either find a library with the July 1995 issue of Dr. Dobb's Journal and read Lee Crocker's PNG article; read a copy of Greg's O'Reilly book, PNG: The Definitive Guide; or else go read the full Portable Network Graphics Specification, a reasonably concise W3C Recommendation (the very first one!) that is amazingly well written and understandable. (Greg had no part in the actual writing of it, so he can say things like that.) See the PNG documentation page for links to plain ASCII, PostScript (US letter-size) and PDF versions, and see the W3C's PNG page and official press release for links to related documentation on gamma and color correction.

Note that the PNG specification was updated to version 1.1 on New Year's Eve 1998 (that is, 31 December 1998). It included new chunks for cross-platform color correction (sRGB and iCCP), a revised and much more sensible description of gamma correction, and a number of other minor improvements and clarifications (all fully backward compatible, of course!). A second, more minor update (version 1.2) was released in August 1999; its only change was the addition of the iTXt chunk (international text).

In addition, PNG began the long process of international standardization* in 1999 (see the 10 May 1999 news item for details), thanks largely to its inclusion in VRML97. It finally completed that process and became the joint ISO/IEC standard 15948:2004 nearly five years later (see the 3 March 2004 news item), a few months after it was also rereleased by the W3C (with identical content) as their "PNG Second Edition" Recommendation.

*PNG was already part of the UK profile for MHEG-5 on digital terrestrial television; MHEG-5 is the international standard for a next-generation teletext system that shares a number of features with HTML. PNG is also used in MHP, the Java-based Multimedia Home Platform for digital video systems, and was included in HAVi, the somewhat defunct Home Audio-Video Interoperability standard for 1394-based home networking.

And the design of the multi-image extension to PNG known as MNG is officially complete (version 1.0 of the MNG specification was released on 31 January 2001), with quite a number of applications available and a free reference library, too.

By now you're undoubtedly drooling over such an incredibly well-designed image format and wondering where you can find applications or programming tools that support it. Well, wonder no further! Greg aims to please.