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JPEG DCT Compression Encoding, Baseline
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Format Description Properties
• ID: fdd000149
• Short name: JPEG_DCT_BL
• Content categories: still image
• Format category: bitstream encoding
• Last significant update: 2004-10-12
• Draft status: Full
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 baseline) is one of two broad classes specified
JPEG lossy compression, extensions (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 baseline lossy compression, which employs discrete cosine transforms (DCT) and other processes. In baseline encoding, the 8 pixel by 8 pixel blocks that comprise the image are input sequentially, from left to right and top to bottom. When decoded, the image appears from top to bottom like a windowshade rolling down.
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 | |
| 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 | JPEG_EXIF, JPEG Encoded File with Exif Metadata |
Local use
| LC experience or existing holdings | American Memory and other Library of Congress activities have created and archived extensive numbers of JPEG_DCT_BL images. These 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_BL is acceptable. Preferred file formats for JPEG_DCT_BL 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. |
| Adoption | Very widely adopted in various devices (scanners, cameras, etc.) and supported by a number of image software applications. |
| 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. It is not clear to the compiler of this document whether some extensions (see JPEG_DCT_EXT) do or do not offer better quality than baseline.1 Most commentators agree that, at the same compression ratios, discrete cosine tranforms (DCT) produce less clarity than discrete wavelet tranforms (DWT), used by J2K (JPEG 2000) and other compression algorithms. |
| 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
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)
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Last Updated: 03/ 7/2007
