Sustainability of Digital Formats: Planning for Library of Congress Collections
Sustainability of Digital Formats: Planning for Library of Congress Collections |
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| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| Full name | PNG (Portable Network Graphics) |
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| Description | The PNG 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. 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 heterogenous 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. |
| Production phase | May be an initial-state or middle-state format; more often used as final-state format. |
| Relationship to other formats | |
| Has subtype | Has versions not separately described. |
| LC experience or existing holdings | As of August 2023, the Library of Congress has approximately 10 TB (over 31 million) PNG files in its collections across many divisions. |
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| LC preference | The Library of Congress Recommended Formats Statement (RFS) includes PNG as a preferred format for photographs in digital form, other graphic images in digital form and 2D and 3D Computer Aided Design raster images. The RFS does not specify a version of PNG. |
| Disclosure | Open standard. |
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| Documentation |
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/ |
| Adoption |
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. With the expiration of the LZW patent, the original objective, a patent-free standard to replace GIF, is no longer significant. According to a webpage from May 2020 now available via Internet Archive, the National Archives of Australia preferred PNG as a "preservation format" for bit-mapped images and normalized at-risk image formats to PNG. The National Archives of Australia currently lists PNG as an acceptable preservation format for born-digital files and recommends lossless PNG as a minimum specification for business-as-usual digitization processes at government agencies. Library and Archives Canada has adopted PNG as a recommended format for still images. |
| Licensing and patents | None. |
| Transparency | Depends upon algorithms and tools for decompression to read; requires sophistication to build tools based on documentation. |
| Self-documentation |
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. The compilers of this resource are not able to assess the degree to which such metadata is found in practice or whether other keywords are in common use. An attempt in 2000 to develop open source tools to convert EXIF images (including EXIF metadata) to PNG seems to have been abandoned. See https://pmt.sourceforge.io/exif/drafts/d020.html. Without such tools and agreed practices, PNG can not rank highly for self-documentation. 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. |
| External dependencies | None |
| Technical protection considerations | None |
| Still Image | |
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| Normal rendering | Good support. |
| Clarity (high image resolution) | Excellent support, with support for progressive display for images retrieved over the Internet. The standard is flexible as to color space and bit depth, supporting indexed color, grayscale, and RGB color. RGB color data is often 8 bits-per-channel (24-bit RGB) but may be extended to 16 bits (48-bit RGB). The term truecolor is often used to refer to RGB color images with 24-bit or greater data. |
| Color maintenance | A PNG image can include chunks for gamma and chromaticity data and for a full ICC color profile. |
| Support for vector graphics, including graphic effects and typography | An alpha channel, representing transparency information on a per-pixel basis, can be included in grayscale and color PNG images. When transparency data is included in color images, the color space is sometimes called RGBA. |
| Functionality beyond normal rendering | None. Related formats, MNG and JNG, have been defined to support multi-page images and animation. |
| Tag | Value | Note |
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| Filename extension | png |
The PNG standard recommends the use of png as extension. |
| Internet Media Type | image/png |
See registration at IANA. |
| Magic numbers | Hex: 89 50 4e 47 0d 0a 1a 0a ASCII: \211 P N G \r \n \032 \n |
Documented in PNG standard. |
| Mac OS file type | PNGf |
Documented in PNG standard. |
| Pronom PUID | fmt/11 |
See https://www.nationalarchives.gov.uk/PRONOM/fmt/11 for PNG 1.0. |
| Pronom PUID | fmt/12 |
See https://www.nationalarchives.gov.uk/PRONOM/fmt/12 for PNG 1.1. |
| Pronom PUID | fmt/13 |
See https://www.nationalarchives.gov.uk/PRONOM/fmt/13 for PNG 1.2. |
| Wikidata Title ID | Q178051 |
See https://www.wikidata.org/wiki/Q178051. No version information. |
| General | |
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| History |
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. |
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