Today's top choice for still image compression is JPEG. Just as a celebrity is someone who is well known for being well known, JPEG is popular because of its popularity. It's nearly universal, exactly what it was designed to be. One reason is that JPEG has something for everyone-it can deliver high compression ratios, completely lossless image quality, and layered transmission that lets you quickly send rough images that progressively fill in with detail. Of course, JPEG is no panacea, and gaining one advantage means losing another. Specifically, when you opt for high compression you lose image quality. The point is, however, that JPEG gives you the choice. When it does compress, it minimizes the apparent loss in image quality.
The standard gains these advantage by being a non-standard, the kind of thing you'd expect to be crafted by a committee. In the case of JPEG, however, the variability translates into the versatility that's the standard's strongest point. JPEG allows many options in the image processing used for compression, maintaining order by giving a standard system for documenting the steps taken so that they can be reversed to reconstitute the image. All the compression details are listed in the header of the image file or at the beginning of a transmitted data stream.
JPEG is at its best compressing color images because it relies on psycho-visual perception effects to discard image data that you might not be able to perceive. It also works on gray scale images but yields lower compression ratios at a given quality level. It does not work well on monochrome (two tone or black and white) images and requires that color mapped images be converted to a conventional, continuous tone color format before processing-which, of course, loses the compression effect of the color mapping.
JPEG processing involves several steps, some of which are optional. Several of these steps may reduce the amount of detail in the image, and thus its quality. The JPEG standard allows you to select the amount of information that's thrown away in these steps so you can control how well an image reconstructed from the compressed data will resemble the original. One option is lossless, which throws away no information other than that which would be redundant. This typically compresses an image file to 50 percent of its original size. Even invoking lossy compression, you can reconstruct an image visually indistinguishable from the original with a reduction to 33 percent of the original. The loss become apparent somewhere around reductions at 5 to 10 percent of the original data size. You can brute force the image data down to 1 percent of the original size, although the results will resemble more a new work of computer art than whatever masterpiece you started with.
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