Joint Photographic Expert Group (JPEG)


(pronounced “JAY-peg”) (n.) 1. An international consortium of hardware, software, and publishing interests. Under the auspices of the International Organization for Standardization (ISO), JPEG was defined as a universal standard for the digital compression and decompression of still images for use in computer systems. 2. The format that the group defined. JPEG is a lossy compression scheme. It uses discrete cosine transform (DCT) and quantization to encode still images. The technique may be used to compress images with moderate detail up to about a 15:1 ratio before visible degradation occurs. An 800-kilobyte (KB) file may be compressed to approximately 80 KB with the application of JPEG compression, and relatively little necessary data will be lost in this 10:1 ratio. When the same 800-KB file is reduced to 40 KB, which is a 20:1 ratio, artifacts may be noticeable. In the process, a single numeric chroma descriptor is used to identify a block of pixels that are all approximately the same color. That descriptor is multiplied by a factor that defines the area of the block. High compression ratios produce a “blocky,” or pixelated, look. Simple images with little detail and few gradients lend themselves to higher compression ratios.

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