| | | | Joint Photographic Experts Group. Named after the committee that created it, JPEG is a picture file format (also refers to the method of compression used to create the file) particularly used for photographic images, in preference to the GIF format. The details are specified in ISO 10918-1. The usual file extension for JPEG files is .jpg however .jpeg, .jfif, .jpe, .jpe, and .jif are also used. When creating (or converting another file format to) a JPEG file, the degree of compression required by the encoding algorithm is specified by the user. High quality pictures demand that little compression is used, resulting in a large file size. More compressed files are smaller, but there is a trade-off between this and the image quality. The data transformation when a JPEG file is created is termed "lossy" because in compressing the file, some data is actually lost as the blocks of pixels the picture is divided into by the algorithm are successively halved and halved again until the file size required is achieved. Compare this with the GIF file format which is "lossless" when using only a 256 distinct colour palette - picture quality is unaffected but no compression is possible. Compression ratios of up to 100:1 are possible with JPEG. 10, 20 or 30:1 are usually indistinguishable from the original by the naked eye however, as with any lossy file compression method, there will always be "compression artifacts" present - the issue is to what degree these are acceptable and detectable. Colours as perceived by the human eye suffer less than luminance (important for distinguishing contours) with JPEG transformation. | | | |
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