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 | Graphics Interchange Format, Version 89a |
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| Description | Commonly known as GIF, this is a bitmapped image format widely used on the Web. Options include "progressive display" in which the rendering exploits interlaced lines, permitting recognizable images to appear before the whole file has downloaded; and short animations that exploit multiple images and control data within a single file. GIF uses LZW compression and palette-based color (256 or fewer shades). |
| Production phase | Generally used for middle- and final-state (end-user delivery) purposes. |
| Relationship to other formats | |
| Has earlier version | Graphics Interchange Format, version 87a, not documented here |
| Contains | LZW, LZW Compression Encoding |
| LC experience or existing holdings | None (or very little) |
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| LC preference | TIFF_G4 (for documents) and TIFF_UNC (for documents or pictorial images) are preferred as master images. JPEG 2000 is also a preferred format for photographs in digital form, other graphic images in digital form, and 2D and 3D Computer Aided Design raster images. The Library of Congress Recommended Formats Statement (RFS) includes GIF as an acceptable format for photographs in digital form and other graphic images in digital form. It is also a preferred format for 2D and 3D Computer Aided Design raster images. The RFS does not specify a version of GIF. |
| Disclosure | Proprietary standard, openly described. Originally developed by CompuServe Incorporated, Columbus, Ohio. |
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| Documentation | The specification is available from more than one third party, e.g., W3C (the World Wide Web Consortium). |
| Adoption | Widely adopted. Many software tools exist for encoding and decoding. Natively supported by Web browsers. |
| Licensing and patents | GIF became notorious in 1994, when UniSys began charging fees to license the LZW compression algorithm. Unisys's US patent expired in June 2003, and its European and Japanese patents expired in June 2004. |
| Transparency | Relatively transparent but depends upon algorithms and tools to read. |
| Self-documentation |
None 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. W3 provides guidance on a technique to set the duration of animated images, which is a component of W3 Accessibility Guideline 2.2.2. Setting the duration of animated images allows users with disabilities more time to read and use content. Comments welcome. |
| External dependencies | None |
| Technical protection considerations | None |
| Still Image | |
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| Normal rendering | Good support. |
| Clarity (high image resolution) | Good, within the limits of palette-based color, discussed in the GIF entry in the Wikipedia (as of September 14, 2003). LZW compression is lossless, which supports clarity. |
| Color maintenance | Excellent, since GIF files carry palette information (in terms of RGB values), as internal data. ICC Profile version 4.2.0.0 (Specification ICC.1:2004-10, page 70) provides guidance for embedding ICC profiles in GIF files as Application Extension blocks: "The Application Identifier for an embedded profile shall be the following 8 bytes: 'ICCRGBG1.'" |
| Support for vector graphics, including graphic effects and typography | Not applicable. |
| Functionality beyond normal rendering | GIF 89a supports "alpha channel" information relating to the transparency of colors, although not all browsers support this feature. |
| Tag | Value | Note |
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| Filename extension | gif |
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| Internet Media Type | image/gif |
From IANA Image Media Types list and RFC 2046, Internet Engineering Task Force. |
| Magic numbers | Hex: 47 49 46 38 39 61 ASCII: GIF89a |
From Gary Kessler's File Signature Page. Kessler's page also reports a GIF "trailer" as Hex: 00 3B ASCII: {NUL} ;. The 00 (NUL) marks the end of the image data (content stream) and the semicolon the end of file. See What's In A GIF - Bit by Byte for diagrams of the structure of a GIF file. |
| Pronom PUID | fmt/4 |
See https://www.nationalarchives.gov.uk/PRONOM/fmt/4. |
| Wikidata Title ID | Q27526739 |
See https://www.wikidata.org/wiki/Q27526739. |
| General | According to Wikipedia (as of August 29, 2008), "The creators of the format pronounced GIF with a soft "g", /ˈdʒɪf/, as in "George". However, many people pronounce GIF with a hard "G", as in a 'gift' IPA: /ˈɡɪf/, reflecting the way it is pronounced in its own acronym (Graphics Interchange Format)." |
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| History |
According to Wikipedia (as of August 29, 2008), "CompuServe introduced the GIF format in 1987 to provide a color image format for their file downloading areas, replacing their earlier run-length encoding (RLE) format, which was black and white only. GIF became popular because it used LZW data compression, which was more efficient than the run-length encoding that formats such as PCX and MacPaint used, and fairly large images could therefore be downloaded in a reasonably short time, even with very slow modems. ... The optional interlacing feature, which stores image scan lines out of order in such a fashion that even a partially downloaded image was somewhat recognizable, also helped GIF's popularity, as a user could abort the download if it was not what was required." The ironically titled "Sad day . . . GIF patent dead at 20" includes a useful chronology compiled by an individual unhappy with Unisys's patent protections as applied to LZW compression. The Wikipedia article also includes information about PNG (a format specifically designed to succeed GIF and to avoid patent problems) and MNG (a variant of PNG that supports animation). PNG was never widely adopted, probably because it arrived coincident with the availability of browser support for JFIF (the file format for JPEG_ENC encoding), and because the LZW US patent was scheduled to expire in 2003. MNG was never directly supported in browsers. |
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