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.
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.
Variable Compression Levels
One advantage to JPEGs is that you can control the degree to which the image is compressed. The higher the quality, the larger the file. The goal is to find the smallest file size that still maintains acceptable image quality.
The quality of a JPEG image is denoted by its "Q" setting, usually on a scale from 0 to 100. In nearly all programs, the lower numbers represent lower image quality but better compression rates (and smaller files). The higher numbers result in better image quality and larger files. For the most part, the Q setting is an arbitrary value with no specific mathematical significance. It is just a way to specify the image quality level you'd like to maintain.
When JPEG compression goes to work, it compresses as much as it can while maintaining the targeted Q setting. The actual compression ratio depends on the content of the individual image. The scales for specifying Q-settings (or "Quality") vary among tools that create JPEGs. Most current web tools use a scale from 0 to 100; however, you will still find some that use a scale from 0 to 10 or 0 to 12. The numbers themselves are not significant (a 30 in one program may be radically different than 30 in another); what matters is the way the image looks and its resulting file size.
JPEGs need to be decompressed before they can be displayed; therefore, it takes a browser longer to decode and assemble a JPEG than a GIF of the same file size. Bear in mind that a small portion of the download time-savings gained by using a JPEG instead of a GIF is lost to the added time it takes to display. (Not much though, so don't sweat it.)
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