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Ultimedia Services Version 2 for AIX: Programmer's Guide and Reference

Overview of JPEG Codec

JPEG is an international standard for the compression of digital continuous-tone images. It was developed under the auspices of the International Organization for Standardization (ISO) and the International Telecommunications Union (ITU). JPEG is an acronym for Joint Photographic Experts Group, which is the name of the committee in the ISO that defined the algorithms.

While the JPEG standard represents 16 algorithms, the simplest, or baseline, algorithm is the most popular. Ultimedia Services supports only the baseline algorithm. (In this document, "JPEG" refers to the baseline algorithm, unless specified otherwise.)

JPEG produces a good tradeoff between compression and retained image quality, and it works with any image dimensions. For color images of natural scenes displayed on a computer monitor (1k x 1k or smaller) that have been compressed by 10:1 to 15:1 and decompressed, the distortion due to JPEG is hard to detect. As the compression ratio rises to 30:1 or 40:1, the decompressed images still appear good. For compression ratios above 50:1, however, artifacts such as blockiness and contouring, degradation of texture and fine patterns, and waviness in smooth areas and around edges are easily detectable. For this reason, JPEG is not used to compress black-and-white images of printed text, or halftone images.

Advantages of JPEG include:

JPEG's basic disadvantage is its computational complexity. Also, JPEG was not designed to compress digital video; it cannot take advantage of interframe correlations in a digitized video clip.

However, JPEG is often used to compress video clips one frame at a time; when used in this way, it is called Motion JPEG (MJPEG). Although other algorithms achieve better compression performance for video, MJPEG offers faster compression and allows for easier editing of sequences, since each frame is compressed independently.

Note: MJPEG is not an ISO standard; there is no single standard version of MJPEG.

Generally speaking, JPEG produces very good quality still images and video. At high compression settings, it produces aliasing and blockiness. The blockiness is obvious in smooth areas, whereas the other artifacts are most visible around edges and in regions of texture and detail.

For introductory information, see Comparison of Video Codecs.

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