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JPEG 图像压缩 FAQ, part 1
Section - [2] Why use JPEG?
There are two good reasons: to make your image files smaller, and to store 24-bit-per-pixel color
data instead of 8-bit-per-pixel data.
Making image files smaller is a win for transmitting files across networks and for archiving libraries
of images. Being able to compress a 2 Mbyte full-color file down to, say, 100 Kbytes makes a big
difference in disk space and transmission time! And JPEG can easily provide 20:1 compression of
full-color data. If you are comparing GIF and JPEG, the size ratio is usually more like 4:1 (see "[4]
How well does JPEG compress images?").
Now, it takes longer to decode and view a JPEG image than to view an image of a simpler format
such as GIF. Thus using JPEG is essentially a time/space tradeoff: you give up some time in order
to store or transmit an image more cheaply. But it's worth noting that when network transmission
is involved, the time savings from transferring a shorter file can be greater than the time needed
to decompress the file.
The second fundamental advantage of JPEG is that it stores full color information: 24 bits/pixel (16
million colors). GIF, the other image format widely used on the net, can only store 8 bits/pixel (256
or fewer colors). GIF is reasonably well matched to inexpensive computer displays --- most
run-of-the-mill PCs can't display more than 256 distinct colors at once. But full-color hardware
is getting cheaper all the time, and JPEG photos look *much* better than GIFs on such hardware.
Within a couple of years, GIF will probably seem as obsolete as black-and-white MacPaint
format does today. Furthermore, JPEG is far more useful than GIF for exchanging images
among people with widely varying display hardware, because it avoids prejudging how many
colors to use (see "[8] What is color quantization?"). Hence JPEG is considerably more appropriate
than GIF for use as a Usenet and World Wide Web standard photo format.
A lot of people are scared off by the term "lossy compression". But when it comes to representing
real-world scenes, *no* digital image format can retain all the information that impinges on your
eyeball. By comparison with the real-world scene, JPEG loses far less information than GIF.
The real disadvantage of lossy compression is that if you repeatedly compress and decompress
an image, you lose a little more quality each time (see "[10] Does loss accumulate with repeated
compression/decompression?"). This is a serious objection for some applications but matters not
at all for many others. |
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