A Complete Guide to Web Design

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PNG

Web Design in a Nutshell, eMatter Edition

Chapter 16PNG

CHAPTER 16

PNG Format

ThePortable Network Format(PNG for short—pronounced “ping”) is a versatile
and full-featured graphics file format poised to make a big wave on the Web...
it’s just taking its time doing so.

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 thecomp.graphicsnewsgroup. A community of programmers then
quickly cooperated in specifying and implementing an impressive list of features:

• 8-bit palette support (like GIF) and 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 1-
  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

Despite this list of attractive capabilities, and despite an official W3C recommenda-
tion, PNGs have been the source of a lot of talk and relatively little action on the