|
|
Section - [3] When should I use JPEG,
and when should I stick with GIF?
JPEG is *not* going to displace GIF entirely; for some types of images, GIF is superior in image quality,
file size, or both. One of the first things to learn about JPEG is which kinds of images to apply it to.
Generally speaking, JPEG is superior to GIF for storing full-color or gray-scale images of "realistic" scenes;
that means scanned photographs, continuous-tone artwork, and similar material. Any smooth variation in
color, such as occurs in highlighted or shaded areas, will be represented more faithfully and in less space by
JPEG than by GIF.
GIF does significantly better on images with only a few distinct colors, such as line drawings and simple carto
-ons. Not only is GIF lossless for such images, but it often compresses them more than JPEG can. For
example, large areas of pixels that are all *exactly*the same color are compressed very efficiently indeed by
GIF. JPEG can't squeeze such data as much as GIF does without introducing visible defects. (One implication
of this is that large single-color borders are quite cheap in GIF files, while they are best avoided in JPEG files.)
Computer-drawn images, such as ray-traced scenes, usually fall between photographs and cartoons in terms
of complexity.The more complex and subtly rendered the image, the more likely that JPEG will do well on it.
The same goes for semi-realistic artwork (fantasy drawings and such). But icons that use only a few colors are
handled better by GIF.
JPEG has a hard time with very sharp edges: a row of pure-black pixels adjacent to a row of pure-white pixels,
for example. Sharp edges tend to come out blurred unless you use a very high quality setting. Edges this sharp
are rare in scanned photographs, but are fairly common in GIF files: consider borders, overlaid text, etc. The
blurriness is particularly objectionable with text that's only a few pixels high. If you have a GIF with a lot of small
-size overlaid text, don't JPEG it. (If you want to attach descriptive text to a JPEG image, put it in as a comment
rather than trying to overlay it on the image. Most recent JPEG software can deal with textual comments in a
JPEG file, although older viewers may just ignore the comments.)
Plain black-and-white (two level) images should never be converted to JPEG; they violate all of the conditions
given above. You need at least about 16 gray levels before JPEG is useful for gray-scale images. It should also
be noted that GIF is lossless for gray-scale images of up to 256 levels, while JPEG is not.
If you have a large library of GIF images, you may want to save space by converting the GIFs to JPEG. This
is trickier than it may seem --- even when the GIFs contain photographic images, they are actually very poor
source material for JPEG, because the images have been color-reduced. Non-photographic images should
generally be left in GIF form. Good-quality photographic GIFs can often be converted with no visible quality
loss, but only if you know what you are doing and you take the time to work on each image individually.
Otherwise you're likely to lose a lot of image quality or waste a lot of disk space ... quite possibly both. Read
sections 8 and 9 if you want to convert GIFs to JPEG. |
|
发表于 2015-4-7 17:48:30
楼主
