Jpeg Compression


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Introduction to Jpeg

As yet nobody has managed to slap a license on its use, camera manufactures and photo editing programs can freely enjoy the benefits of this picture image format. In 1982 the ISO formed the Photographic Expert Group to research methods of transmitting Video and still images over data links. It's Goal was to produce a set of industry standards. In 1986 the CCITT set up a group to work on Compression methods required for colour facsimile machine transmission. In 1987 the two groups were combined to form a joint committee (Joint Photographic Experts Group ( JPEG )) that would research and produce a single standard.

JPEG unlike other compression methods is not a single algorithm but maybe thought of as a toolkit of image compression methods to suit the users needs.

JPEG uses a lossy compression method that throws useless data away during encoding. This is why lossy schemes manage to obtain superior compression ratios over most loss-less schemes. JPEG is designed to discard information the human eye cannot easily see. The eye barely notices slight changes in colour but will pick out slight changes in brightness or contrast. Therefore Jpeg's lossy encoding tends to be frugal with the gray-scale part of an picture and frivolous with the colour component. A typical photographic quality image maybe compressed by 20:1 without experiencing any noticeable degradation in quality.

In the later part of this article will attempt to measure the degree of image degradation by comparing the pixel data from a Jpeg against the original pixel data in a Tiff file. We will also examine the degree of image degradation with different sorts of image data after Jpeg compression and decompression. The effects of multiple Jpeg processing are also taken into account.