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Section - [5] What are good "quality" settings for JPEG?
Most JPEG compressors let you pick a file size vs. image quality tradeoff by selecting a quality setting. There seems
to be widespread confusion about the meaning of these settings. "Quality 95" does NOT mean "keep 95% of the
information", as some have claimed. The quality scale is purely arbitrary; it's not a percentage of anything.
In fact, quality scales aren't even standardized across JPEG programs. The quality settings discussed in this article
apply to the free IJG JPEG software (see part 2, item 15), and to many programs based on it. Some other JPEG
implementations use completely different quality scales. For example:
* Apple used to use a scale running from 0 to 4, not 0 to 100.
* Recent Apple software uses an 0-100 scale that has nothing to do with the IJG scale (their Q 50 is about the
same as Q 80 on the IJG scale).
* Paint Shop Pro's scale is the exact opposite of the IJG scale, PSP setting N = IJG 100-N; thus lower numbers
are higher quality in PSP.
* Adobe Photoshop doesn't use a numeric scale at all, it just gives you "high"/"medium"/"low" choices. (But I
hear this is changing in 4.0.)
Fortunately, this confusion doesn't prevent different implementations from exchanging JPEG files. But you do need
to keep in mind that quality scales vary considerably from one JPEG-creating program to another, and that just
saying "I saved this at Q 75" doesn't mean a thing if you don't say which program you used.
In most cases the user's goal is to pick the lowest quality setting, or smallest file size, that decompresses into an image
indistinguishable from the original. This setting will vary from one image to another and from one observer to another,
but here are some rules of thumb.
For good-quality, full-color source images, the default IJG quality setting (Q 75) is very often the best choice. This
setting is about the lowest you can go without expecting to see defects in a typical image. Try Q 75 first; if you see
defects, then go up.
If the image was less than perfect quality to begin with, you might be able to drop down to Q 50 without objectionable
degradation. On the other hand, you might need to go to a *higher* quality setting to avoid further loss. This is often
necessary if the image contains dithering or moire patterns (see "[9] What are some rules of thumb for converting GIF
images to JPEG?").
Except for experimental purposes, never go above about Q 95; using Q 100 will produce a file two or three times as
large as Q 95, but of hardly any better quality. Q 100 is a mathematical limit rather than a useful setting. If you see
a file made with Q 100, it's a pretty sure sign that the maker didn't know what he/she was doing.
If you want a very small file (say for preview or indexing purposes) and are prepared to tolerate large defects, a Q
setting in the range of 5 to 10 is about right. Q 2 or so may be amusing as "op art". (It's worth mentioning that the
current IJG software is not optimized for such low quality factors. Future versions may achieve better image quality
for the same file size at low quality settings.)
If your image contains sharp colored edges, you may notice slight fuzziness or jagginess around such edges no matter
how high you make the quality setting. This can be suppressed, at a price in file size, by turning off chroma
downsampling in the compressor. The IJG encoder regards downsampling as a separate option which you can turn
on or off independently of the Q setting. With the "cjpeg" program, the command line switch "-sample 1x1" turns
off downsampling; other programs based on the IJG library may have checkboxes or other controls for downsampling.
Other JPEG implementations may or may not provide user control of downsampling. Adobe Photoshop, for example,
automatically switches off downsampling at its higher quality settings. On most photographic images, we recommend
leaving downsampling on, because it saves a significant amount of space at little or no visual penalty.
For images being used on the World Wide Web, it's often a good idea to give up a small amount of image quality in order
to reduce download time. Quality settings around 50 are often perfectly acceptable on the Web. In fact, a user viewing
such an image on a browser with a 256-color display is unlikely to be able to see any difference from a higher quality
setting, because the browser's color quantization artifacts will swamp any imperfections in the JPEG image itself. It's
also worth knowing that current progressive-JPEG-making programs use default progression sequences that are
tuned for quality settings around 50-75: much below 50, the early scans will look really bad, while much above 75,
the later scans won't contribute anything noticeable to the picture. |
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发表于 2015-4-13 11:28:33
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