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
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Several methods for checking file integrity and corruption
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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:
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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