Sustainability of Digital Formats
 Planning for Library of Congress Collections

Format Description Categories >> Browse Alphabetical List >> Back

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 (including extensions) is one of two broad classes specified JPEG lossy compression, baseline (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. The ISO/IEC standard covers both lossy and lossless images. This document concerns extensions to JPEG's lossy compression encoding, which employs discrete cosine transforms (DCT) and other processes. One common extension is called progressive encoding, in which multiple "scans" are made. When a progressive image is decoded, "every other line" (figuratively speaking) appears, displaying a top-to-bottom image relatively quickly; following this, the missing lines are filled in. Other less common extensions include variable quantization within segments of an image, selective refinement of portions of an image, image tiling, the SPIFF format extension, and others. The encoding processes are described in the specifications 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
Is subtype of	JPEG_DCT, JPEG Lossy (DCT) Compression Encoding
Is used by	JFIF, JPEG File Interchange Format
Is used by	SPIFF, Still Picture Interchange File Format
Is used by	JTIP, JPEG Tiled Image Pyramid Format

Local use

LC experience or existing holdings

American Memory and other Library of Congress activities have created and archived progressive JPEG_DCT_EXT images, generally for AM collection home pages and related interpretive Web documents.

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_EXT is acceptable. Preferred file formats for JPEG_DCT_EXT 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 10918-1:1994. Information technology -- Digital compression and coding of continuous-tone still images: Requirements and guidelines. Includes the specification for progressive JPEG. 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 (amendment). Provisions to allow registration of new compression types and versions in the SPIFF header
Adoption	Progressive JPEG is widely adopted in various devices (scanners, cameras, etc.) and supported by a number of image software applications. The extent of adoption of other JPEG_DCT extensions is not known to the compiler of this document.
Licensing and patent claims	See JPEG_DCT.
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)	Good. Quality varies according to the sophistication of the encoding and the degree of compression applied. The extensions generally appear to affect how an image is segmented (e.g., tiling) or to allow for differential "refinement" of parts of an image, and do not seem to change the fundamentals of how encoding affects clarity, as compared to baseline JPEG images. [Comments from readers of this web page welcome!] Most commentators agree that, at the same compression ratios, discrete cosine tranforms (DCT) produce less clarity than discrete wavelet transforms (DWT), used by J2K (JPEG 2000) and other formats.
Color maintenance	See file format descriptions: JPEG_EXIF, JFIF, and SPIFF.
Support for graphic effects and typography	No support for vector graphics.
Functionality beyond normal image rendering	None.

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	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.)
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 10918-3:1997. 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

Useful references

URLs
• 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) Print