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Chapter 21. PNG Format

The Portable Network Graphic format (PNG for short -- pronounced "ping") is a versatile and full-featured graphics file format that has been lurking in the shadows for several years. Despite some attractive features and the fact that it was created with web use specifically in mind, the PNG has been largely avoided by the web design community. This is primarily due to abysmal browser support and a lack of tools that can compress PNGs well enough to make them compete with GIFs.

The good news is that both browser and tool support has been gradually improving over the last several years. It remains to be seen, however, whether PNG will ever be permitted to live up to its potential.

21.1. The PNG Story

PNG was developed in January and February 1995 as an effort to find a non-proprietary alternative to GIF when Unisys threatened to enforce its patent on LZW compression and collect licensing fees from developers of GIF-supporting programs. This caused a flurry of outrage and activity on the Internet.

Days after the announcement, Thomas Boutell posted the first draft of the PNG specification to the comp.graphics newsgroup. A community of programmers then quickly cooperated in specifying and implementing an impressive list of features:

  • 8-bit palette support (like GIF), support of 16-bit grayscale, and up to 48-bit truecolor (RGB) support

  • A lossless compression scheme and better compression than GIF for indexed color (palette) images

  • Two-dimensional progressive display that is more sophisticated than GIF's one-dimensional interlacing

  • An alpha channel that can contain 8-bit or 16-bit transparency information, which means pixels can have up to 65,000 shades of transparency (not just "on" or "off" like GIF); 8-bit (256 shades of transparency) is far more common

  • Gamma correction information to make the PNG display with its intended brightness regardless of platform

  • Several methods for checking file integrity and corruption

  • Text storage capabilities for keyword information, such as copyright

  • Nonpatented compression free from licensing restrictions

The PNG format became an official W3C Recommendation in October of 1996 (see http://www.w3.org/Graphics/PNG/). Since then, browser and software developers have given the format more attention, but there is still a long way to go.

MNG for Motion

Here's another acronym for your graphic format arsenal -- the MNG (Multiple-image Network Graphic). As the name implies, MNG was designed based on the PNG format to handle animated (multi-image) graphics. It shares a number of PNG's best features. In addition, it offers a number of interesting animation features, including:

  • Object- or sprite-based animation

  • Nested loops for complex animations

  • Much better compression than animated GIFs

  • Support for frame differencing (for maximizing compression)

The MNG format is still in development, but it is already being supported by a number of programs, including Netscape 6. For complete information, see the official MNG home page at http://www.libpng.org/pub/mng/.

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