J

JPEG
Jy

JPEG

Joint Photographic Experts Group: The original name of the committee that designed the JPEG image compression algorithm and file interchange format (JFIF). JPEG uses Huffmann compression and is designed for compressing either full-color or gray scale digital images of natural, i.e., real-world, scenes. JPEG compression is lossy, i.e. the decompressed image will not be identical to the original. (JPEG can achieve much greater compression than is possible with lossless methods.) JPEG is designed to exploit known limitations of the human eye, notably the fact that small color changes are perceived less accurately than small changes in brightness. Thus, JPEG is intended for compressing images that will be looked at by humans, and should not be used on images that will be subjected to further machine-analysis.

A useful property of JPEG is that the degree of lossiness can be varied by adjusting compression parameters. File size can thus be traded against output image quality. JPEG decoders can also trade off decoding speed against image quality, by using fast but inaccurate approximations to the required calculations. Until recently, most publicly available JPEG code has adopted a best-possible-quality philosophy, but decoders are now appearing that give up some image quality in order to obtain significant speedups.

Because of the information loss, JPEG is less suitable for compressing non-realistic images, such as color graphics or line drawings. JPEG does not handle compression of black-and-white (1-bit-per-pixel) images or moving pictures. Standards for compressing those types of images are being worked on by other committees, named JBIG and MPEG respectively.

Jy

Jansky: The unit of flux density adopted by the IAU in 1973, equal to . Named after Karl Jansky, who discovered the galactic radio emission in 1931-32.

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abridle@nrao.edu, 15 August 1996, 16:41 EDT