Some of PNG 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
* 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.