Sustainability of Digital Formats: Planning for Library of Congress Collections

The PNG or Portable Network Graphics specification describes PNG as an "extensible file format for the lossless, portable, well-compressed storage of static and animated raster images. PNG provides a patent-free replacement for GIF and can also replace many common uses of TIFF. Indexed-color, grayscale, and truecolor images are supported, plus an optional alpha channel. Sample depths range from 1 to 16 bits." The specification is maintained and developed by the W3C Portable Network Graphics (PNG) Working Group.

PNG is pronounced "ping" and, according to The GIF Is Dead. Long Live the GIF, the acronym is a shortened version of PING for “PING Is Not GIF”.

The specification defines both a datastream and an associated file format for a lossless, portable, compressed, raster (bit-mapped) image. PNG is fully streamable with a progressive display option designed for "compressed individual computer graphics image or frame-based animation, transmitted across the Internet." Indexed color, grayscale, and RGB color (referred to as truecolor in the specification) are supported, with optional transparency (alpha channel).  PNG can store gamma and chromaticity data as well as a full ICC color profile for accurate color matching on heterogeneous platforms.  The PNG format was originally designed as an open standard to replace GIF_89a for use on the Internet, but is not limited to that use. It is often used for logos with transparent backgrounds. In addition, APNG, Animated Portal Network Graphics extends PNG to support support frame-based animated images through the addition of three more chunks.

Structurally, the PNG datastream is comprised of the PNG magic number followed by a sequence of chunks with each chunk having a chunk type which specifies its function. There are four critical chunk types: IHDR: image header, which is the first chunk in a PNG datastream; PLTE: palette table associated with indexed PNG images; IDAT: image data chunks; IEND: image trailer, which is the last chunk in a PNG datastream. These can be followed by a number of optional ancillary chunk types, which encoders may generate and decoders may interpret.In addition to the chunk type, each chunk consists of two or three fields: length (four-byte unsigned integer giving the number of bytes in the chunk's data field), chunk data (optional - may be empty) and chunk CRC/cyclic redundancy check (four-byte 32-bit CRC calculated on the preceding bytes in the chunk, including the chunk type field and chunk data fields, but not including the length field).

All compression in PNG is lossless. The standard defines only compression level 0: "DEFLATE compression with a sliding window (which is an upper bound on the distances appearing in the deflate stream) of at most 32768 bytes." See General for more information about DEFLATE.

APNG, Animated Portal Network Graphics extends PNG to support support frame-based animated images through the addition of three more chunks.

Adobe's PNG files lists several advantages of PNG files over other image formats (such as JPEG) including its use of lossless compression and its support for 16 million colors as well as its disadvantages such as larger file sizes and lack of "support for CMYK color modes so transferring them to print can prove difficult."

PNG (Portable Network Graphics) Specification Version 1.0, W3C Recommendation 01-October-1996

PNG (Portable Network Graphics) Specification, Version 1.1 (no date listed)

PNG (Portable Network Graphics) Specification, Version 1.2 (at http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html)

ISO/IEC 15948:2004 Information technology -- Computer graphics and image processing -- Portable Network Graphics (PNG): Functional specification.

W3C Portable Network Graphics (PNG) Specification (Second Edition), same text as ISO/IEC 15948:2004, at https://www.w3.org/TR/PNG/

Portable Network Graphics (PNG) Specification (Third Edition - 24 June, 2025) - Recommendation

As of 2005, PNG was supported by most browsers. However, Internet Explorer 6.x for Windows did not support the transparency feature. Slow deployment of full browser support delayed, or even prevented widespread adoption. In 2025, CanIUse reports widespread support of PNG across all major browsers (with the continued exception of IE 6). With the expiration of the LZW patent, the original objective, a patent-free standard to replace GIF, is no longer significant.

The Open Preservation Foundation's International Comparison of Recommended File Formats (updated last in 2025) lists PNG version 1.2 as a preferred or acceptable format for a wide range of international national and federal archives (Norway, Netherlands, Finland, Sweden, Australia, Bibliothèque et Archives Nationales du Québec, Archives New Zealand) and scientific libraries and repositories (Digital Repository of Ireland, Switzerland) just to name a few.

PNG is widely supported in image software systems including Photoshop, IrfanView, and Canva. The site titled PNG includes a long list of PNG supporting applications, programming resources, images and animations.

The IANA media type filing (and the specification note the potential for security risks from the chunk-based format: "The gIFx chunk type is used to encapsulate Application Extension data, and some use of that data might present security risks, though no risks are known. Likewise, the security risks associated with future chunk types cannot be evaluated, particularly unregistered chunks. However, it is the intention of the PNG Working Group to disallow chunks containing "executable" data to become registered chunks. The text chunks, tEXt, iTXT and zTXt, contain data that can be displayed in the form of comments, etc. Some operating systems or terminals might allow the display of textual data with embedded control characters to perform operations such as re-mapping of keys, creation of files, etc. For this reason, the specification recommends that the text chunks be filtered for control characters before direct display."

The PNG specification allows labeled text (ASCII or UTF-8) elements to be embedded in text chunks and predefines a few standard keywords (element labels): Title, Author, Description, Copyright, Creation Time, Software, Disclaimer, Warning, Source, Comment.

It is possible to embed XMP metadata in PNG files, according to the XMP specification. However, the documentation for ExifTool for PNG tags suggests that practices for storing XMP or EXIF metadata in PNG images have not been consistent.

Accessibility Features

Accessibility for still image content is often supported by the use of alt text when displayed on the web. The carriage of this data is typically not embedded in the file itself but rather in the HTML code. See W3C's Images Tutorial for the Web Accessibility Initiative. PNG files may have limited support for web accessibility because of its ability to store text strings associated with the image, including image description. However, this information may be not be accessible to screen readers.

As described in GZIP, the "DEFLATE algorithm is a combination of LZ77 and Huffman coding lossless compression algorithms. DEFLATE was developed as a replacement for Lempel-Ziv-Welch (LZW) lossless compression algorithm and other patented compression algorithms that limited the usability of other popular archivers. This choice aimed to overcome patent restrictions and enhance compression efficiency.

The DEFLATE compressed data format used by GZIP offers improved compression compared to Unix compress, along with fast decompression. Additionally, it includes a CRC-32 as an integrity check for data. The header format in GZIP allows for the storage of more information than the compress format, such as the original file name and file modification time.

The zlib library supports DEFLATE compression and decompression, offering three types of wrapping around deflate streams: raw deflate, zlib wrapping (used in portable network graphic (PNG) format data blocks), and GZIP wrapping. Zlib wrapping is more compact (six bytes) compared to GZIP (a minimum of 18 bytes), and its integrity check (Adler-32) is faster than the CRC-32 used by GZIP. Raw deflate is utilized by programs handling the .zip format, another format that wraps around deflate compressed data.

The original specification for PNG, version 1.0, was developed by the independent PNG development group and released under the auspices of the World Wide Web Consortium (W3C) on 1 October 1996 as its first Recommendation. On 15 January 1997 it was released by the IETF as RFC 2083. The PNG specification was updated to version 1.1 on 31 December 1998. It included new chunks for cross-platform color correction (sRGB and iCCP), a revised and much more sensible description of gamma correction, and a number of other minor improvements and clarifications (all fully backward compatible, of course!). A second, more minor update (version 1.2) was released in August 1999; its only change was the addition of the iTXt chunk (international text).

Version 1.2 was submitted to ISO/IEC as a proposed standard in 1999. The ISO/IEC standard was published in March 2004 as ISO/IEC 15948:2004. W3C published equivalent text as Portable Network Graphics (PNG) Specification (Second Edition) at https://www.w3.org/TR/PNG/ in November 2003.

Version 2 was released on 10 November 2003.

The W3C recommendation for Version 3 was released on 24 June, 2025. Prior to this recommendation, there were several drafts and candidate recommendations documented on Portable Network Graphics (PNG) Specification (Third Edition) publication history. The specification notes that "this specification is intended to become an International Standard, but is not yet one. It is inappropriate to refer to this specification as an International Standard."

For a list of publication dates and changes between Version 2 and Version 3, see the Third Edition Change List. For changes between Version 1 and Version 2, see the Second Edition Change List. W3C also makes available the pre-release drafts for the Second Edition and the First Edition.

The PNG Working Group Charter mentions a forthcoming Fourth Edition, potentially in 2025 but this is all to be confirmed as work progresses.