PNG

PNG (Portable Network Graphics), sometimes pronounced as "''ping''" ( in International Phonetic Alphabet for English), is a lossless bitmap Graphics file format that is popular on the World Wide Web and elsewhere. PNG was largely developed to deal with some of the shortcomings of the GIF format and allows storage of images with greater color depth and other important information. PNG is supported by the libpng reference Library (computer science), a platform independent library that contains C programming language functions for handling PNG images. ==History and development== The motivation for creating the PNG format came in early 1995, after Unisys Corporation announced that it would be enforcing software patents on the LZW data compression algorithm used for GIF. The algorithm is described in U.S. Patent 4,558,302 and other patents around the world. There were also other problems with the GIF format which made a replacement desirable, notably its limitation to 256 color at a time when computers capable of displaying far more than 256 colours were becoming common. Although GIF allows for computer animation, it was decided that PNG should be a single-image format. A companion format called MNG has been defined for animation. PNG gained some additional popularity in August 1999, after Unisys terminated its royalty-free patent licenses to developers of free software and non-commercial software. *Version 1.0 of the PNG specification was released on July 1 1996, and later appeared as RFC 2083. It became a World Wide Web Consortium Recommendation on October 1 1996. *Version 1.1, with some small changes and the addition of three new chunks, was released on December 31 1998. *Version 1.2, adding one extra chunk, was released on August 11 1999. *PNG is now an International Standard (International Organization for Standardization/International Electrotechnical Commission 15948:2003), also released as a W3C Recommendation on November 10 2003. This version of PNG differs only slightly from version 1.2 and adds no new chunks. ==Technical details==

A PNG image with an 8-bit transparency layer (top), overlayed onto a chequered background (bottom).

PNG uses a non-patented lossless data compression method known as DEFLATE (algorithm). This method is combined with prediction, where for each image line, a ''filter method'' is chosen that predicts the colour of each pixel based on the colours of previous pixels and subtracts the predicted colour of the pixel from the actual color. An image line filtered in this way is often more compressible than the raw image line would be. On most images, PNG can achieve greater compression than GIF, but some implementations make poor choices of filter methods and therefore produce unnecessarily large PNG files. The GIF file format has allowed portions of images to be transparency (optics), allowing them to appear to have a shape that is not rectangular. However, the GIF#Transparency was considered to be insufficient because it only allowed individual pixels to be fully transparent or fully opaque. The PNG format allows three different styles of transparency: one for indexed color images and two for greyscale or truecolor images. For indexed color images a table of 8 bit alpha values (numeric values representing levels of partial transparency from fully transparent to fully opaque) for palette entries is stored. If this table is smaller than the number of palette entries the remaining entries are considered to be fully opaque. For greyscale and truecolour images either a single colour value can be set as transparent or an alpha channel of the same bitdepth as the main channels can be added to the image. It should be noted that an alpha channel cannot be used with a 1 2 or 4 bit greyscale image though such an image could be stored in a png by defining it as palletted and putting the alpha values into the palette. Other image attributes that can be stored in PNG files include gamma correction values, background color, and textual information. The standard allows up to 16 bits per channel, though for most people, 8 bits per channel is usually enough. It should be noted that although PNG is designed in terms of bitdepth per channel people and image editing software usually talk about the total bits per pixel. So for example a 16 bit per channel RGBA image would have a total of 64 bits per pixel. PNG also supports color correction with the use of color management systems like sRGB color space.

The full range of color options
bitdepth per channel	1	2	4	8	16
indexed	1	2	4	8
greyscale	1	2	4	8	16
greyscale with alpha				16	32
truecolour (RGB)				24	48
truecolour with alpha (RGBA)				32	64

The table on the right shows the full range of color options: The green cells with numbers represent the supported formats, The numbers inside the total bits per pixel. The red cells represent unsupported combinations. The standard requires decoders can read all supported color formats but many image editors can only produce a small subset of them. A PNG file consists of an 8-byte magic number (programming) (89 50 4E 47 0D 0A 1A 0A in hexadecimal) followed by a number of ''chunks'', each of which conveys certain information about the image. Chunks declare themselves as ''critical'' or ''ancillary'', and a program encountering an ancillary chunk that it does not understand can safely ignore it. This chunk-based structure is designed to allow the PNG format to be extended while maintaining compatibility with older versions. The MIME media type for PNG is image/png (approved on October 14 1996). ==Support and usage== After the introduction of the PNG format, it took a long time for web browsers to fully support alpha channel transparency. Binary transparency is still more widely supported than alpha channel transparency as of 2005. For example, Microsoft Internet Explorer (for Windows) supports binary transparency but not alpha channel transparency. Version 7 of the browser, however, will finally support this missing feature [http://blogs.msdn.com/ie/archive/2005/04/22/410963.aspx]. Also in some versions of some web browsers assume some gamma correction values even when none are specified in the PNG. The images thus appear differently under different operating systems. PNG is rapidly replacing GIF for still images, mostly on web sites of computer enthusiasts protesting Unisys's license policies and on some cartoon sites that take advantage of PNG's tighter compression to save on Internet bandwidth charges. The web graphics package Macromedia Fireworks uses PNG as its native file format, but uses private ancillary chunks to store Fireworks-specific data, such as information about layers, frames (animation), text, and effects (allowing later re-editing). These PNG files have been reported to crash Internet Explorer 4.x and so PNGs for web-page display should be 'exported' from Fireworks, rather than just saved. [http://www.macromedia.com/cfusion/knowledgebase/index.cfm?id=tn_13871] ==Alternatives== In late 2004, an animation extension (APNG) was proposed which, while retaining the ability to render the first frame as a normal PNG in decoders that do not understand the APNG format, also contains additional chunks which allow it to act as an animated image similar to a GIF file in a decoder which does understand the extension. [http://www.vlad1.com/~vladimir/projects/apng/] ==See also== *Computer graphics *Adam7 algorithm *APNG *GIF *JNG *JPEG *MNG *TIFF *Truevision TGA *Comparison of layout engines (graphics) ==External links== ===libpng.org=== *[http://www.libpng.org/pub/png/ PNG Home Site] *[http://www.libpng.org/pub/png/libpng.html libpng Home Page] *[http://www.libpng.org/pub/png/pngapbr.html PNG-supporting browsers] - overview of PNG status in different web browsers. First Windows IE support in 4.0b1; Netscape 4.04 [http://oregon.usgs.gov/png_images.html]. *[http://www.libpng.org/pub/png/slashpng-1999.html ''The Story of PNG'' by Greg Roelofs] ===W3C=== *[http://www.w3.org/TR/2003/REC-PNG-20031110/ PNG Specification (Second Edition), W3C Recommendation 10 November 2003] *[http://www.w3.org/Graphics/PNG/Inline-img.html Test inline PNG images] ===Others=== *RFC 2083 *[http://entropymine.com/jason/testbed/pngtrans/ PNG transparency test] *[http://apps.everamber.com/alpha/ Transparent PNG Generator] *[http://php.net/gd The GD-library to generate dynamic PNG-files with PHP] *[http://www.cs.toronto.edu/~cosmin/pngtech/optipng.html A guide to PNG optimization] *[http://en.wikibooks.org/wiki/How_to_future_proof_electronic_information#Images_.26_Photos_.28Raster_Graphics.29 PNG as a future proof format] *[http://schaik.com/png/adam7.html PNG Adam7 interlacing] ===Browser incompatibilities=== *[http://support.microsoft.com/default.aspx?scid=kb;en-us;294714 PNG Files Do Not Show Transparency in Internet Explorer] *[http://homepage.ntlworld.com/bobosola/ The PNG problem in Windows Internet Explorer] *[http://www.nrkn.com/alpha/ png transparency in IE] Graphics file formats Graphics standards ISO standards W3C standards

PNG

Juuitchan wants to convert some BMP images to PNG for an article. Anyone willing to help out here? : For easily manipulating images in pre-determined ways (changing attributes, converting to other formats, resizing, etc.) my favourite toolset is [http://www.imagemagick.org/ ImageMagick]. It's free software and runs on most common operating systems. -- User:Bignose Most modern raster graphics tools understand PNG (those that don't aren't worth using), but some understand the format better than others. Also, people sometimes forget that PNG has got 8bpp and 16bpp modes, which are complete overkill for most maps and flags and such. [http://hackles.org/etc/png.html PNG Tips for Cartoonists] is an introduction to PNG for comics artists, but most of the article contains pretty useful information for Wikipedians too. The tool mentioned in the article, pngcrush, really does squeeze the last bits out of your PNG files and comes recommended.--user:Branko If he can put them somewhere I can access them (like FTPing them to ftp.piclab.com:/incoming), I'll do the job. If he wants to do it himself, I recommend Paint Shop Pro from http://www.jasc.com . --User:Lee Daniel Crocker Might need to disambiguate this from Papua New Guinea at some point —User:Mulad, May 29, 2003 ---- Does anyone know what license PNG is released under? For all the talk of the GIF proprietary format and Unisys's reneging on allowing opensource developers to use it, the article does not mention what licence PNG is. A document telling about the licence of PNG on the offical website ( http://www.libpng.org/pub/png/src/libpng-LICENSE.txt ) does not seem to indicate that its opensource. --user:ShaunMacPherson :I think this question confuses a few different concepts: it doesn't make sense to talk about the "license" of PNG. PNG is a specification, not software. In other words, it's not a program, it's just a list of guidelines of what a PNG file should look like. Terms like "open-source" and "closed-source" don't make sense. The important point for a specification is that it be open and not patented (which is the case for PNG). Now, once developers are given a specification, then they can write specific implementations of it, which can be either open- or closed-source. The link you gave refers not to PNG itself but to a specific implementation of the standard, "libpng". And in fact if you read the licence carefully, although it does not use the words "open source" as such, it says: ::''Permission is hereby granted to use, copy, modify, and distribute this source code, or portions hereof, for any purpose, without fee'' [...] :So it is open source. But not all implementations necessarily have to be (although right now in practice everybody just uses libpng). Hope this clears things up. --User:Shibboleth 04:00, 31 Aug 2004 (UTC) ::Actually, everyone very much doesn't use libpng. IE probably doesn't, since it doesn't do pngs properly. Photoshop doesn't either, since it even in newer versions uses some otherworldly bulky method which mostly gives crappy results. I'm sure everyone agrees that these two make "everyone" a too strong word. ; ) User:130.232.120.145 20:02, 22 Mar 2005 (UTC) :::that license that was linked (which is for libpng not the png spec itself) seems to be a fairly standard 3 clause BSD/MIT with a load of explanitory details and authorship information around it.User:Plugwash 23:41, 25 May 2005 (UTC) :The real problem with GIF has to do more with patents than with copyright and closed source. The patent is not on the GIF format per se, but on a compression algorithm (LZW) that the format optionally uses, which until recently required a license from Unisys to implement. This license cost what I remember as being a relatively nominal fee, but Unisys could have at any time changed the terms, and open source developers didn't like the idea of being beholden to Unisys for what they saw as no good reason. The patent has expired, but PNG is still considered a technically superior format in many ways, so that even if GIF hadn't been encumbered, it would still be worthwhile to switch to PNG. User:Grendelkhan|User_talk:Grendelkhan 14:32, 2005 May 26 (UTC) == Compare two PNGs == Does anyone know how to tell whether two PNG images are the same image, i.e. all pixels identical? If there's no transparency, one can convert to PNM and use pnmpsnr. But in general? User:Dbenbenn | User talk:Dbenbenn 21:37, 3 Jun 2005 (UTC) :Bah. Apparently I'll have to learn the PNG format and write my own program. User:Dbenbenn | User talk:Dbenbenn 21:40, 18 Jun 2005 (UTC) ::i've written a png writer but i've never attempted a reader, sorry. User:Plugwash 22:02, 18 Jun 2005 (UTC)

PNG

Papua New Guinea

Png

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PNG
PNG
PNG
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PNG_(disambiguation)