JPEG

JPEG definition - computer

(Joint Photographic Experts Group) An ISO/ITU standard for compressing still images. Pronounced "jay-peg," the JPEG format is very popular due to its variable compression range. JPEGs are saved on a sliding resolution scale based on the quality desired. For example, an image can be saved in high quality for photo printing, in medium quality for the Web and in low quality for attaching to e-mails, the latter providing the smallest file size for fastest transmission over dial-up connections.

Not Great for Text
JPEGs are not suitable for graphs, charts and explanatory illustrations because the text appears fuzzy, especially at low resolutions. Compressing images in the GIF format is much better for such material (see GIF).

JPEGs Are Lossy
Using discrete cosine transform, JPEG is a lossy compression method, wherein some data from the original image is lost. It depends on the image, but ratios of 10:1 to 20:1 may provide little noticeable loss. The more the loss can be tolerated, the more the image can be compressed.

Compression is achieved by dividing the picture into tiny pixel blocks, which are halved over and over until the desired amount of compression is achieved. JPEGs can be created in software or hardware, the latter providing sufficient speed for real-time, on-the-fly compression. C-Cube Microsystems introduced the first JPEG chip. See JPEG2000, JPE file and GIF.

File Extensions
JPEGs use the JPEG File Interchange Format (JFIF), and file extensions are .JPG or .JFF. M-JPEG and MPEG are variations of JPEG used for full-motion digital video (see MPEG). See graphics formats.


_JPG1.GIF


_JPG2.GIF


Saving JPEGs

At first glance, these two dialogs seem contradictory because the larger number means high quality in one and low quality in the other. Photoshop (top) uses a 1-to-12 scale for quality, while Epson (bottom) uses a percentage for the amount of compression. The more compression, the more loss in the result. Standard is the original JPEG format, while Optimized compresses more effectively. Progressive causes the image to display incrementally from top to bottom. Web users with slow connections see the image form on screen right away rather than wait for the entire file to download first.






JPEG++ definition - computer

An extension to the JPEG format from Storm Technology, Mountain View, CA, that allowed picture areas to be selectable for different ratios. For example, the background could be compressed higher than the foreground image. Areas containing text could be compressed very little, while the rest of the image could be compressed a lot. Storm Technology closed its doors in 1998. See JPEG.



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