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Section - [9] What are some rules of thumb for
converting GIF images to JPEG?
Converting GIF files to JPEG is a tricky business --- you are piling one set of limitations atop a quite different set,
and the results can be awful. Certainly a JPEG made from a GIF will never be as good as a JPEG made from true
24-bit color data. But if what you've got is GIFs, and you need to save space, here are some hints for getting the
best results.
With care and a clean source image, it's often possible to make a JPEG of quality equivalent to the GIF. This does
not mean that the JPEG looks pixel-for-pixel identical to the GIF --- it won't. Especially not on an 8-bit display,
because the color quantization process used to display the JPEG probably won't quite match the quantization
process used to make the GIF from the original data (see "[8] What is color quantization?"). But remember that
the GIF itself is not all that faithful to the full-color original, if you look at individual pixels. Looking at the overall
image, a converted JPEG can look as good as its GIF source. Some people claim that on 24-bit displays, a carefully
converted JPEG can actually look better than the GIF source, because dither patterns have been eliminated. (More
about dithering in a moment.)
On the other hand, JPEG conversion absolutely *will* degrade an unsuitable image or one that is converted carelessly.
If you are not willing to take the amount of trouble suggested below, you're much better off leaving your GIF images
alone. Simply cranking the JPEG quality setting up to a very high value wastes space (which defeats the whole point
of the exercise, no?) and some images will be degraded anyway.
The first rule is never to convert an image that's not appropriate for JPEG (see "[3] When should I use JPEG, and
when should I stick with GIF?"). Large, high-visual-quality photographic images are usually the best source material.
And they take up lots of space in GIF form, so they offer significant potential space savings. (A good rule of thumb is
not to bother converting any GIF that's much under 100 Kbytes; the potential savings isn't worth the hassle.)
The second rule is to know where the image came from. Repeated GIF<=>JPEG conversions are guaranteed to turn
an image into mush, because you pay a steep quality price on each round trip. Don't reconvert images that have been
converted before.
The third rule is to get rid of the border. Many people have developed an odd habit of putting a large single-color
border around a GIF image. While useless, this is nearly free in terms of storage cost in GIF files. It is *not* free in
JPEG files, either in storage space or in decoding time. Worse, the sharp border boundary can create visible artifacts
(ghost edges). Furthermore, when viewing a bordered JPEG on an 8-bit display, the quantizer will think the border
color is important because there's so much of it, and hence will waste color palette entries on the border, thus actually
reducing the displayed quality of the main part of the image! So do yourself a favor and crop off any border before
JPEGing.
The final rule is to look at each JPEG, to make sure you are happy with it, before throwing away the corresponding
GIF. This will give you a chance to re-do the conversion with a higher quality setting if necessary. Also compare
the file sizes --- if the image isn't suitable JPEG material, a JPEG file of reasonable quality may come out *larger*
than the GIF.
Gray-scale photos usually convert without much problem. When using cjpeg, be sure to use the -gray switch.
(Otherwise, cjpeg treats a GIF as color data; this works, but it wastes space and time if the image is really only
gray-scale.) Quality settings around the default (75) are usually fine.
Color images are much trickier. Color GIFs of photographic images are usually "dithered" to fool your eye into
seeing more than the 256 colors that GIF can actually store. If you enlarge the image, you will find that adjacent
pixels are often of significantly different colors; at normal size the eye averages these pixels together to produce
the illusion of an intermediate color value. The trouble with dithering is that, to JPEG, it looks like high-spatial-
frequency color noise; and JPEG can't compress noise very well. The resulting JPEG file is both larger and of lower
image quality than what you would have gotten from JPEGing the original full color image (if you had it). To get
around this, you need to "smooth" the GIF image before compression. Smoothing averages together nearby pixels,
thus approximating the color that you thought you saw anyway, and in the process getting rid of the rapid color
changes that give JPEG trouble. Proper use of smoothing will both reduce the size of the compressed file and give
you a better-looking output image than you'd get without smoothing.
With the IJG JPEG software (cjpeg or derived programs), a simple smoothing capability is built in. Try "-smooth
10" or so when converting GIFs. Values of 10 to 25 seem to work well for high-quality GIFs. GIFs with heavy-
handed dithering may require larger smoothing factors. (If you can see regular fine-scale patterns on the GIF
image even without enlargement, then strong smoothing is definitely called for.) Too large a smoothing factor
will blur the output image, which you don't want. If you are an image processing wizard, you can also do smoothing
with a separate filtering program, but appropriate use of such tools is beyond the scope of this FAQ.
Quality settings around 85 (a bit higher than default) usually work well when converting color GIFs, assuming that
you've picked a good smoothing factor. You may need still higher quality settings if you can't hide the dithering
pattern with a reasonable smoothing factor. Really badly dithered GIFs are best left as GIFs.
Don't expect JPEG files converted from GIFs to be as small as those created directly from full-color originals.
The dithering noise wastes space, but you won't be able to smooth away all the noise without blurring the image.
Typically, a good-quality converted JPEG will be one-half to one-third the size of the GIF file, not one-fourth as
suggested in section 4. If the JPEG comes out much more than half the size of the GIF, this is a good sign that
the image shouldn't be converted at all.
The upshot of all this is that "cjpeg -quality 85 -smooth 10" is probably a good starting point for converting
color GIFs. But if you care about the image, you'll want to check the results and maybe try a few other settings.
Blindly converting a large GIF library at this or any other setting is a recipe for disaster. |
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发表于 2015-4-21 16:14:41
楼主
发表于 2020-4-9 01:06:08
