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20.2. JPEG Compression

JPEG uses what is known as a "lossy" compression scheme, meaning that some image information is actually thrown out in the compression process. Fortunately, for photographic images at most compression levels, this loss is not discernible to the human eye, particularly when the image is displayed on a monitor at screen resolution (and even less so for images saved at print resolutions).

Using "lossy" compression algorithms, JPEG is able to achieve 10:1 to 20:1 data-compression ratios without visible loss in quality. Of course, the savings in file size at any given compression is dependent on the content of the specific image, and results vary. If maintaining high image quality is not a priority, these ratios can go even higher.

The efficiency of JPEG compression is based on the spatial frequency, or concentration of detail, of the image. Image areas with low frequency (smooth gradients, like a blue sky) are compressed much further than areas with higher frequency (lots of detail, like blades of grass). Even a single sharp color boundary, although not giving "lots of detail," represents a surge in spatial frequency and therefore poses problems for JPEG compression.

The compression algorithm samples the image in 8 8-pixel squares and then translates the relative color and brightness information into mathematical formulas. These sampling squares may become visible when images are compressed with the highest compression ratios (lowest quality settings).

It is perhaps most meaningful to compare JPEG and GIF compression on photographic images. A detail-rich photographic image that takes up 85K of disk space as a GIF image would require only 35K as a JPEG. Again, the rate of compression depends on the specific image, but in general, a JPEG compresses a photographic image two to three times smaller than GIF. For flat-color graphics, however, GIF is far more efficient than JPEG.

20.2.1. Image Loss

Be aware that once image quality is lost in JPEG compression, you can never get it back again. Loss in image quality is also cumulative, meaning you lose a little bit more information each time you decompress and compress an image. Each time you open a JPEG and resave it, you degrade the image further. Not only that, you may introduce new artifacts to the image that prevent the second compression from working as efficiently as the first, resulting in higher file sizes.

It is a good idea to hang on to one copy of the original digital image if you anticipate having to make changes, so your final image only goes through the compression process once. You should also start from an original image each time to experiment with different compression levels. The new web graphics tools (Fireworks, Photoshop 5.5 and higher, and ImageReady) make this easy because they always retain the original and allow you to export graphics with your chosen settings.

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