PNGs

PNGs, or Portable Network Graphics, is a raster graphics file format that was created as an open, patent-free alternative to GIF. It was developed by the PNG Development Group and published as an international standard in 1996. The format became widely adopted for web images due to its lossless compression, support for transparency, and broad color depth.

PNG uses DEFLATE compression (a combination of LZ77 and Huffman coding) within IDAT chunks to reduce file size without quality loss. It supports five color types: grayscale (type 0); truecolor (type 2); indexed color (type 3); grayscale with alpha (type 4); and truecolor with alpha (type 6). Depending on color type, bit depth ranges from 1,2,4,8,16. PNG provides per-pixel alpha in truecolor/gray+alpha, and a single-color transparency option via a tRNS chunk for indexed or gray/truecolor images.

Interlacing is optional via the Adam7 method to progressively display images as data arrives. PNG also supports gamma, chromaticity, and sRGB color space information, as well as textual metadata in tEXt, zTXt, and iTXt chunks. It uses a standardized file structure with mandatory chunks such as IHDR, IDAT, and IEND, and optional chunks such as PLTE for palettes and tRNS for transparency.

Compared with JPEG, PNG is lossless and better for images with sharp edges and text, but typically larger for photographs. It competes with GIF for simple graphics and with newer formats like WebP and AVIF for web use. Its openness and wide support on devices and software have contributed to its long-term ubiquity in digital graphics.