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PNG, Portable Network Graphics
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Format Description Properties
• ID: fdd000153
• Short name: PNG
• Content categories: still image
• Format category: file format
• Last significant update: 2006-10-04
• Draft status: Partial
Identification and description
| Full name | PNG (Portable Network Graphics) |
| 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, greyscale, 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 | |
Local use
| LC experience or existing holdings | None |
| LC preference | One of the acceptable formats for bitmapped images. |
Sustainability factors
| Disclosure | Open standard. |
| Documentation |
ISO/IEC 15948:2004
Information technology -- Computer graphics and image processing -- Portable Network Graphics (PNG): Functional specification. [Not in hand as of February 2005.]
PNG (Portable Network Graphics) Specification, Version 1.2 (at http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html)
|
| Adoption | As of 2005, PNG is supported by most browsers. However, Internet Explorer 6.x for Windows does not support the transparency feature. Slow deployment of full browser support has 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.
PNG is a preferred data format in the list of FCLA
recommended formats. Library and Archives Canada has also adopted PNG as a recommended format for still images.
|
| Licensing and patent claims | 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
http://pmt.sourceforge.net/exif/drafts/d020.html. Without such tools and agreed practices, PNG can not rank highly for self-documentation. |
| External dependencies | None |
| Technical protection considerations | None |
Quality and functionality factors (still image)
| Normal rendering for still images | Good support. |
| Clarity (support for 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 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 image rendering | None. Related formats, MNG and JNG, have been defined to support multi-page images and animation. |
File type signifiers
| Tag type | Value | Note |
| Filename Extension | png | The PNG standard recommends the use of png as extension. |
| Internet Media Type | image/png | See
registration from IANA. |
| Mac OS file type | PNGf | Documented in PNG standard. |
| Magic numbers | Hex: 89 50 4e 47 0d 0a 1a 0a
ASCII: \211 P N G \r \n \032 \n | Documented in PNG standard.
|
Notes
| General | |
| History | The original specification for PNG, version 1.0, was developed under the auspices of the World Wide Web Consortium (W3C). On 1 October 1996 it was released by the W3C as its first Recommendation, and on 15 January 1997 it was released by the IETF as RFC 2083. Version 1.2 was released in July 1999. This version of PNG (1.2) was published in March 2004 as an ISO/IEC standard. |
Format specifications
URLs
•
PNG (Portable Network Graphics) Specification, Version 1.2 [http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html]
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• ISO/IEC 15948:2004. Information technology -- Computer graphics and image processing -- Portable Network Graphics (PNG): Functional specification
Useful references
URLs
• http://www.rfc-editor.org/rfc/rfc2083.txt
• http://www.libpng.org/pub/png/
• http://www.w3.org/Graphics/PNG/
• http://vds.cnes.fr/pin/presentations/2003/format_png.pdf
• FCLA
recommended formats (www.fcla.edu/digitalArchive/pdfs/recFormats.pdf)
• Library and Archives Canada recommended formats (http://www.collectionscanada.ca/information-management/002/007002-3017-e.html)
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Last Updated: 03/ 7/2007
