Chapter 21. PNG Format

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.

• 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

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/.