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Responding to a need for faster loading, enhanced quality cross-platform Web graphics, The World Wide Web Consortium (W3C) recently issued the Portable Network Graphics (PNG) specification as a W3C Recommendation.
Designed to set a new quality standard for lossless images, PNG provides small files and rapid previews to enhance the responsiveness of Web pages, and to enable graphics to look the same on different platforms. Of particular value to designers who use variable transparency when creating images for print, PNG permits similar effects on the Web -- partially transparent overlaid images to be displayed over any background color or texture, eliminating white haloes.
The PNG format was created in December 1994 and has been stable since March 1995. Since then, the specification has become much more readable and has gained valuable tutorial and developer information. Simultaneously, acceptance of PNG has grown, viewers have appeared on many different platforms, content creation tools have appeared with PNG support, and browsers are starting to support PNG images inline.
Automatic gamma correction across platforms means that PNG files can be correctly displayed on Macs, PCs, SGI workstations, etc. (all of which have different gamma values) without appearing either too light and insipid or too dark and contrasty. Storing the source gamma used by the image author is the simplest way of helping interoperability with the biggest payoff. Chromaticity data can also be stored in PNG files and can be used by Color Management Systems on the viewing platform to compensate for differing monitor types; an important ability where precise color matching is required - brand recognition, product design, medical applications, fine art and on-line catalogs, for example.
In practice, indexed color PNG files average about 30% smaller than the equivalent GIF, and truecolor PNG files are 30-40% smaller than the equivalent (LZW compressed) RGB TIFF.
The advanced interlacing scheme gives a preview image after only 1/64th of the image data has been loaded (as opposed to 1/8th with GIF), and small details are recognizable much sooner than with an interlaced GIF image. In tests, it was found that five of the seven passes were retrieved and displayed in the same time as the first pass of interlaced GIF.
PNG files can contain keywords and text strings, which can be extracted by Web search tools and used to locate images on the Web. For example, a picture might have a description of "Sunset behind Edinburgh Castle, with Piper in foreground." A search on sunset and castle would locate this image; a search on Edinburgh and not piper would reject it, which is much easier than trying to guess based on the filename. Standard keywords include: Title, Author, Description, and Copyright. More specialized applications can define their own keywords. Lengthy text, such as legal disclaimers, can be compressed.
PNG is an extensible file format for the lossless, portable, well-compressed storage of single raster images. It is not a competitor for JPEG JFIF, which is an excellent format for lossy storage of suitable pictures.
PNG is a good choice for those truecolor images which are unsuited to JPEG compression, such as raytraced images.
PNG can also be used for greyscale images and for indexed (palette) images. PNG is a good choice for images that have transparency information. PNG can be used to store up to 16 bits of greyscale information, or 48 bits of color information, making it suitable as an output format from graphics arts scanners, medical equipment, and scientific applications.
Compression is asymmetric - decompression for viewing is quite fast, at the expense of longer compression time when the image is created - because images on the Web are read by many people, but only written once.