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PNG (Portable Network Graphics), sometimes pronounced as "ping", is a relatively new bitmap image format that is becoming 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, a platform independent library that contains C functions for handling PNG images.
The motivation for creating the PNG format came in early 1995, after Unisys 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 colours at a time when computers capable of displaying far more than 256 colours were becoming common. Although GIF allows for animation, it was decided that PNG should be a single-image format. A companion format called MNGMNG (pronounced ming is a public file format for animated images. The name stands for Multiple-image Network Graphics''. MNG is closely related to the PNG image format. When PNG development started in early 1995, developers decided not to incorporate supp has been defined for animation. PNG gained some additional popularity in August 1999For the album by Prince, see 1999 (album 1999 is a common year starting on Friday (see link for calendar), and was designated the International Year of Older Persons by the UN. Events Kosovo War Former child star Gary Coleman files for bankruptcy Y2K prep, after Unisys terminated its royalty-free patent licenses to developers of free softwareThis article refers to free software as defined by the Free Software Foundation. For software available free of charge, see Freeware. The term free software refers to software which, once obtained, can be used, copied, studied, modified and redistributed. and non-commercial software.
PNG uses a non-patented lossless data compressionLossless data compression is a class of data compression algorithms that allow the original data to be reconstructed exactly from the compressed data. Contrast with lossy data compression. Lossless data compression is used in software compression tools su method known as deflation. 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 transparentIn optics, transparency is the property of being transparent or allowing light to pass. The opposite property is opacity . Though transparency usually refers to visible light in common usage, it can actually refer to any type of radiation. For example, fl, allowing them to appear to have a shape that is not rectangular. However, the GIF method was considered to be rather poor because it only allowed individual pixels to be fully transparent or fully opaque. The PNG format improves upon this by also supporting an 8-bit alpha channel, so pixels in an image can have partial transparency. However, it took a long time for web browserA web browser is a software package that enables a user to display and interact with documents hosted by web servers. Popular browsers include Microsoft Internet Explorer and Mozilla Firefox. A browser is the most commonly used kind of user agent. The lars to fully support transparency, and some, notably Internet Explorer (for Windows), still do not.
Other image attributes that can be stored in PNG files include gamma values, background color, and textual information.
A PNG file consists of an 8- byte signature (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 14 October 1996).
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. [1]
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