Portable Network Graphics

Portable Network Graphics (PNG, pronounced /ˌpiːɛnˈdʒiː/ PEE-en-JEE or /pɪŋ/ PING) is a raster-graphics file-format that supports lossless data compression. PNG was developed as an improved, non-patented replacement for Graphics Interchange Format (GIF). PNG supports palette-based images (with palettes of 24-bit RGB or 32-bit RGBA colors), grayscale images (with or without alpha channel for transparency), and full-color non-palette-based RGB or RGBA images. The PNG working group designed the format for transferring images on the Internet, not for professional-quality print graphics, and therefore it does not support non-RGB color spaces such as CMYK. A PNG file contains a single image in an extensible structure of 'chunks', encoding the basic pixels and other information such as textual comments and integrity checks documented in RFC 2083. PNG files nearly always use the file extension PNG or png and are assigned MIME media type image/png.PNG was published as informational RFC 2083 in March 1997 and as an ISO/IEC standard in 2004. The motivation for creating the PNG format was the realization, in early 1995, that the Lempel–Ziv–Welch (LZW) data compression algorithm used in the Graphics Interchange Format (GIF) format was patented by Unisys. There were also other problems with the GIF format that made a replacement desirable, notably its limit of 256 colors at a time when computers able to display far more than 256 colors were becoming common. A January 1995 precursory discussion thread, on the usenet newsgroup 'comp.graphics' with the subject Thoughts on a GIF-replacement file format, had many propositions, which would later be part of the PNG file format. In this thread, Oliver Fromme, author of the popular DOS JPEG viewer QPEG, proposed the PING name, a recursive acronym meaning PING is not GIF, and also the PNG extension. Although GIF allows for animation, it was decided that PNG should be a single-image format. In 2001, the developers of PNG published the Multiple-image Network Graphics (MNG) format, with support for animation. MNG achieved moderate application support, but not enough among mainstream web browsers and no usage among web site designers or publishers. In 2008, certain Mozilla developers published the Animated Portable Network Graphics (APNG) format with similar goals. APNG is a format that is natively supported by Gecko- and Presto-based web browsers and is also commonly used for thumbnails on Sony's PlayStation Portable system (using the normal PNG file extension), and as of 2017, usage of APNG remains minimal despite being supported by all major browsers but Microsoft Edge. The original PNG specification was authored by an ad-hoc group of computer graphics experts and enthusiasts. Discussions and decisions about the format were conducted by email. The original authors listed on RFC 2083 are: A PNG file starts with an 8-byte signature (refer to hex editor image on the right):