Sustainability of Digital Formats
 Planning for Library of Congress Collections

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Identification and description

Full name	ISO/IEC 10918. Information technology -- Digital compression and coding of continuous-tone still images(formal name) DCT-based lossy compression is one of two broad classes specified JPEG lossy compression (common name)
Description	Compression encoding generally used for full color and grayscale continuous-tone pictorial images; does not work well with bitonal or palette-color images. Compression is variable and governed by a number of parameters; typical settings provide from 10:1 to 20:1 reductions in file size. The ISO/IEC standard covers both lossy and lossless images. This document concerns lossy compression, which employs discrete cosine transforms (DCT) and other processes. The encoding process is described in the specification and on a number of web sites, including http://www.faqs.org/faqs/compression-faq/part2/ (Subject [75] on this lengthy page).
Production phase	May be applied in initial-state picture creation; often used for middle- and final-state archiving or end-user delivery.
Relationship to other formats
Has subtype	JPEG_DCT_BL, JPEG DCT Compression Encoding, Baseline
Has subtype	JPEG_DCT_EXT, JPEG DCT Compression Encoding, Extensions

Local use

LC experience or existing holdings

American Memory and other Library of Congress activities have created and archived extensive numbers of JPEG_DCT images. These are generally JPEG_DCT_BL (baseline) and are generally reduced-data derivatives of uncompressed master images.

LC preference

The Library's general preference for still image "masters" is for uncompressed bitstreams, and rich metadata is always welcome. Thus TIFF_UNC_EXIF and TIFF_UNC are preferred. For images only available in lossy compressed form, JPEG_DCT is acceptable. Preferred file formats for JPEG_DCT include JPEG_EXIF (rich metadata) and JFIF; raw JPEG data (see Notes) is only acceptable if no other format is available. Meanwhile, J2K_C_LSY (JPEG 2000 Part 1, Core Coding, Lossy Compression) is an emerging preference for compressed bitmapped still images.

Sustainability factors

Disclosure	Fully disclosed. Developed by the Joint Photographic Experts Group (JPEG), formally known as ISO/IEC JTC 1/SC 29/WG 10.
Documentation	ISO/IEC 10918. Four parts have been published; parts 1 and 3 are central; both of these parts include exensions. See list of ISO documents in Format specifications below. See Notes for comments regarding JPEG lossless compression.
Adoption	Very widely adopted in various devices (scanners, cameras, etc.) and supported by a number of image software applications. Baseline is most widely adopted; conventional wisdom holds that, of the extensions, progressive JPEG is the most widely adopted.
Licensing and patent claims	For many years, it was thought that there were none. In 2002, however, Forgent Networks began actions to exploit a 1987 patent held by a company purchased by Forgent and, in 2004, Forgent brought patent infringement suits against 31 software and device-manufacturing companies (not corporate or individual end-users; reportedly, an additional 11 lawsuits were subsequently brought). Forgent has argued that the patent will not expire until 2006; news reports state that 95 percent of the company's revenues derive from licensing fees.
Transparency	Depends upon algorithms and tools to read; will require sophistication to build tools.
Self-documentation	See the file format descriptions for JFIF, SPIFF, and JPEG_EXIF.
External dependencies	None.
Technical protection considerations	None.

Quality and functionality factors

Normal rendering for still images	Good support.
Clarity (support for high image resolution)	See JPEG_DCT_BL and JPEG_DCT_EXT.
Color maintenance	See JPEG_DCT_BL and JPEG_DCT_EXT.
Support for graphic effects and typography	No support for vector graphics.
Functionality beyond normal image rendering	Various features supported in JPEG_DCT_EXT.

File type signifiers

Tag type	Value	Note
Filename Extension	n/a	See the file format descriptions for JFIF, SPIFF, and JPEG_EXIF.
Internet Media Type	n/a	See the file format descriptions for JFIF, SPIFF, and JPEG_EXIF.
Magic numbers	n/a	See the file format descriptions for JFIF, SPIFF, and JPEG_EXIF.

Notes

General

Paraphrased from Vanryper Murray's Encylopedia of Graphics File Formats (O'Reilly & Assoc, 1994, ISBN 1565920589): The JPEG bitstream stores 16-bit word values in big-endian format. JPEG data in general is stored as a stream of blocks, and each block is identified by a marker value. The first two bytes of every JPEG stream are the Start Of Image (SOI) marker values FFh D8h. In a JFIF-compliant file there is a JFIF APP0 (Application) marker, immediately following the SOI, which consists of the marker code values FFh E0h and the characters JFIF in the marker data, as described in the next section. In addition to the JFIF marker segment, there may be one or more optional JFIF extension marker segments, followed by the actual image data. JPEG images transform RGB color space to YCrCb (luminance-chrominance) color space before compression, and then transform back to RGB for display. JPEG bitstreams are sometimes transmitted or exchanged as "raw" files. "Raw" is one of the JPEG profiles covered in the documentation from Harvard University's JHOVE (pronounced "jove") activity. (JHOVE stands for the JSTOR/Harvard Object Validation Environment; JSTOR stands for Journal STORage.) ISO/IEC 10918 part 1 includes discussion of lossless (non-DCT) encoding. However, the JPEG organization offers this comment at their Web site: "After creating the JPEG standard described above, the [JPEG] committee started to look at some of the criticisms of the existing standard. High amongst these was the poor quality (and lack of integration) of lossless coding in the standard. With this in mind, the committee developed the JPEG-LL (lossless) standard - ISO/IEC IS 14495-1 | ITU-T Recommendation T.87."

History

Format specifications

URLs
Print
• ISO/IEC 10918-1:1994. Information technology -- Digital compression and coding of continuous-tone still images: Requirements and guidelines
• ISO/IEC 10918-2:1995. Information technology -- Digital compression and coding of continuous-tone still images: Compliance testing
• ISO/IEC 10918-3:1997. Information technology -- Digital compression and coding of continuous-tone still images: Extensions
  • ISO/IEC 10918-3:1997/Amd 1:1999. Provisions to allow registration of new compression types and versions in the SPIFF header
• ISO/IEC 10918-4:1999. Information technology -- Digital compression and coding of continuous-tone still images: Registration of JPEG profiles, SPIFF profiles, SPIFF tags, SPIFF colour spaces, APPn markers, SPIFF compression types and Registration Authorities (REGAUT)

Useful references

URLs
• www.jpeg.org
• http://www.faqs.org/faqs/compression-faq/part2/, Subject [75] on this lengthy page is devoted to JPEG.
• JHOVE's JPEG page (http://hul.harvard.edu/jhove/jpeg-hul.html)
• Forgent Networks patent information (http://www.forgent.com/ip/672patent.shtml) Print