Portable Network Graphics
Portable Network Graphics (PNG) is a raster graphics file format that supports lossless data compression. PNG was created as an improved, non-patented replacement for Graphics Interchange Format (GIF), and is the most widely used lossless image compression format on the Internet.
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/RGBA images (with or without alpha channel). PNG was designed for transferring images on the Internet, not for professional-quality print graphics, and therefore 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 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.
Portable Network Graphics (PNG) Specification (Second Edition)