What is JPEG?

JPEG (pronounced "jay-peg") is a standardized image compression mechanism. JPEG stands for Joint Photographic Experts Group, the original name of the committee that wrote the standard. JPEG is designed for compressing either full-color or gray-scale digital images of "natural", real-world scenes. It does not work very well on non-realistic images, such as cartoons or line drawings.

JPEG does not handle black-and-white (1-bit-per-pixel) images, nor does it handle motion picture compression. Related standards for compressing those types of images exist, and are called JBIG and MPEG respectively.

Regular JPEG is "lossy", meaning that the image you get out of decompression isn't quite identical to what you originally put in. The algorithm achieves much of its compression by exploiting known limitations of the human eye, notably the fact that small color details aren't perceived as well as small details of light-and-dark. Thus, JPEG is intended for compressing images that will be looked at by humans. If you plan to machine-analyze your images, the small errors introduced by JPEG may be a problem for you, even if they are invisible to the eye. The JPEG standard includes a separate lossless mode, but it is rarely used and does not give nearly as much compression as the lossy mode.

Question 75 "Introduction to JPEG" (in part 2 of this FAQ) gives an overview of how JPEG works and provides references for further reading. Also see the JPEG FAQ article, which covers JPEG software and usage hints. The JPEG FAQ is posted regularly in news.answers by Tom Lane . (See question 53 "Where are FAQ lists archived" if this posting has expired at your site.)

For JPEG software, see item 15 above.

For JPEG hardware, see item 85 in part 3 of this FAQ.