it-e-70 Why You should Not Use JPEGs for Image Storage
If I were king, I'd banish JPEGs forever. JPEGs (pronounced "jay-pegs", and sometimes
spelled "JPGs") are the images you see all over the Web. Pictures stored as JPEGs are highly
compressed so they can be sent across the Internet faster. A picture that takes up 1 million bytes
can be squeezed down to 1/10th of that size if you turn it into a JPEG.
That would seem like a good idea. But it's actually a good idea gone bad. JPEGs are like
jigsaw puzzles that can never be put back together again right. Pieces will always be missing.
Every time you change a normal image into a JPEG, you lose part of the picture. You can never
get it back.
Remember that. I'll repeat it in a slightly different way to help out: When you create or save
a JPEG image, you lose part of the picture and you simply can't get it back, ever.
JPEGs are losers. They use what's called "lossy" compression. To squeeze an image's file
size down to something really small, the JPEG process tosses out 80 or 90 percent of the
information and fakes a lot of the rest. JPEGs take advantage of a trick of our eyesight. When we
see part of a small, familiar object, we automatically fill in the rest of it. JPEG processing breaks
an image up into a lot of small pieces and strips out a great deal of detail from each small piece.
A picture of your grandmother holding a shawl might show every detail ordinarily, but a
JPEG version might not show the way her fingers wrap around the fabric. Instead, they might be
depicted as areas of light and dark contrast that suggest an old woman's fingers. Your eyes and
brain do the rest. They turn what is only a suggestion of fingers into what your brain thinks is the
real thing.
This might be fine for some uses. But anyone who cares about digital photography should
consider JPEG as an enemy, not a friend. I'll bet you don't want that picture of grandma turned
into someone who looks sorta like grandma. You want the real thing.
JPEGs are one-way trips to the Badlands. Here's a familiar scenario. You have a normal,
non-compressed image. It's a Windows bitmap, maybe. (They're BMP files.) You open this
bitmap in your image-editing software and save it as a JPEG. The file is a lot smaller. You
celebrate a little. What a trick you just pulled!
Later, you realize you need to tweak the picture a little. Maybe you need to crop it. Maybe
you just want to make the colors more vivid.
So you open the JPEG in your image editor, make your changes and save the file as a JPEG
again.
The image you cropped or tweaked can't be the same as the one you started with. It was
already chopped up a lot by the JPEG processing, and when you saved it again it got chopped and
diced some more.
The PNG image format is the best thing since the wheel. PNG (pronounced "Ping," not "Pea
En Gee") means "Portable Network Graphics." When you save an image as a PNG, nothing gets
sliced or diced. A PNG is almost surely going to be smaller than a standard uncompressed image,
but it won't have any wounds. Cut it up all you want, slice it hither and yon. It will be all there,
even though it's smaller.
This miracle is actually just a trick of counting. We all do the same thing that PNGs do. We
learned this trick in grade school.
Here's how it works. Suppose you have a lot of sheep to count. You could count every
single one of them. Boring, right? Or you could count them by twos. Or maybe, if your eye is
quick, you could count them by threes.
Let's say you've done thismaybe you were trying the old technique of counting sheep to
get to sleep but it didn't workand now it's time to come up with the real number. If you counted
by twos, you just multiply your total by two. If you counted by threes, you multiply your number
by three.
Cool, right? You're storing something like 31 in your brain, but you can quickly turn that
into the real number62 or 93.
It's all shorthand. It's a perfectly sane mathematical trick. PNG uses the same approach. It
squeezes the data in an image file using tricks of math. Does the image have 61 pixels of nothing
but azure blue in one area? PNG stores that information as an instruction to make an azure blue
pixel 61 times. That takes up a lot less space than something like this: azure blue pixel, azure
blue pixel, azure blue pixel, ... you get the point.
The best part of this trick is the way it can be totally reversed without losses. the sky looks
exactly the way it was supposed to in the image with all those azure blue pixels. Storing
information by listing it as "1 pixel times 61" is the same as storing 61 pixels. Except that it
doesn't take up all that wasted space.
PNGs can't quite squeeze images as much as JPEGs can. There's no way to do it the lossless
way and get the same reduction in file size. But PNGs can cut an image down to anywhere from
one-half to one-third of its normal file size without any loss of data. Some images can be reduced
even more.
I took a few hundred uncompressed digital images and changed them from BMP to PNG to
find out how much space I could save. Most of them were 12 megabyte files stored as BMPs.
PNG squeezed these scans down to a little less than half their uncompressed size on average.
Some scans, especially ones that did not have much detail, compressed a lot more. But since I
started storing most of my digital images as PNGs, I've come to expect about a 50 percent to 60
percent savings in file space.
That might not seem like much compared to the way JPEG squeezes images, but it's a huge
bonus when you realize that PNG is harmless. Images are not changed in any way.
Good image viewers and editors know how to deal with PNG. If you don't see PNG listed as
an option in your image editor's "Save As" menu (under the "File" menu), you have old software
and should get a newer version.
If you don't have an image viewer of any kind, buy ACDSee, available for both Windows
PCs and Macintoshes.
1, shawl [ʃɔ:l]
n. 披肩,围巾
2, sorta ['sɔ:tə]
adv. 近似，有几分；可以说是
3, vivid ['vivid]
a. 生动的,栩栩如生的,鲜艳的
4, dice [dais]
vt. 切成方块
n. 骰子
vi. 掷骰子
5, sliced [slaist]
adj. （食物）已切成薄片的
v. 切成薄片；分配（slice的过去式和过去分词）
6, yon [jɔn]
adj. 那边的，彼处的
adv. 在那边，在远处
n. 那边，远处
pron. 彼处之人或物
7, chopped
v. 斩碎(开裂,中断,多变,开路)
8, sane [sein]
a. 神智健全的,理智的
9, azure
n. 天蓝色,碧空
a. 蔚蓝的
10, squeeze [skwi:z]
n. 压榨,挤
v. 紧握,挤
PNG原来是无损压缩位图
GIF也是无损压缩位图,只是由于压缩算法专利问题，逐渐被PNG取代。
