Glossary definition of Joint Photographic Experts Group

Also known as: JPEG

JPEG (Joint Photographic Experts Group) is an independant organisation formed in the mid eighties with the aim of creating true colour computer image standards. The initiative to develop a photographic image format was initially taken on by the ISO but other groups were merged (including the ITU-T) in order to take advantage of their experience.

The standard most commonly referred to by the term JPEG is ISO / IEC IS 10918-1, which defines techniques for digitally coding photographic images. There is an extension of this standard called JFIF (JPEG File Interchange Format) which is the standard commonly used for almost all JPEG image files.

JPEG can be used to store either truecolour (24-bit) or greyscale images. It uses a lossy compression algorithm called DCT (Discrete Cosine Transform) which is a lot more complex than normal bitmap encodings and so requires more computing power to encode or decode; the benefit is that good image quality can be acheived with a small file. DCT in JPEGs works best on photographic images; graphics that contain sharp contrasts such as straight edges dont compress so well. Being a lossy compression method, it is possible to have manual control over the balanace between the quality and file size of JPEG-coded images.

There has been demand for JPEG images that employ lossless compression as the basic standard does not define it very clearly or thoroughly. The JPEG-LS standard (ISO / IEC IS 14495-1) has been developed by the Joint Photographic Experts Group to help increase the quality and integration of lossless compression.

The JPEG standard is heavily supported by the open source movement, and the good accessibility of JPEG coding and decoding packages has helped its rapid adoption into the mainstream in the mid nineties.

You can find the Joint Photographic Experts Group website at www.jpeg.org