png

PNG, or Portable Network Graphics, is a raster graphics file format that uses lossless compression. It was designed as an open, patent-free alternative to GIF and is widely used on the web for images that require sharp edges and transparency, such as logos and icons, as well as photographic content when lossless preservation is desired.

Development and history: The format was produced by the PNG Development Group in the 1990s and published as an international standard in 1996. It arose in part to provide a patent-unencumbered replacement for GIF, incorporating improvements in compression, color handling, and metadata. Since its release, PNG has become a core web image format alongside JPEG and WebP, with ongoing refinements through public updates to the specification.

Technical features: PNG employs lossless compression based on DEFLATE, preserving image data exactly. It supports various color types, including grayscale, truecolor, indexed color, and combinations that include an alpha channel for transparency. Bit depths range from 1 to 16 bits per sample, enabling both standard and high-fidelity images. PNG images can be interlaced using the Adam7 scheme, or stored non-interlaced. The format also allows extensive metadata and color management options through ancillary chunks, such as tEXt, zTXt, iTXt for text, and gAMA, sRGB, iCCP for color information. Transparency can be handled via a dedicated alpha channel or via a transparent index.

File structure: A PNG file starts with an 8-byte signature and consists of a series of chunks. Required chunks include IHDR (header), IDAT (image data), and IEND (end). Optional chunks include PLTE (palette), tRNS (transparency), and various metadata chunks. Each chunk carries a CRC for integrity.

Usage and variants: PNG is extensively supported across platforms and software. The standard PNG format is static, but the APNG extension enables simple animated images. PNG remains a prevalent choice for lossless web graphics and images requiring transparency.