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 |
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.
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External dependencies | None. |
Technical protection considerations | None. |
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.
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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. |
, Subject [75] on this lengthy page is devoted to JPEG.