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Web Design in a Nutshell by Jennifer Niederst

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Chapter 16. PNG Format

The Portable 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 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) 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 ...

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