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The .jpg file format is "lossy." This does not make it a "virus." I'm
wondering how long it will be now, before we get warnings, a la Good
Times, about not downloading graphics. .Jpg is not a virus. It will not
seek out and destroy your images. It just isn't perfect for every image
or situation.
A small amount of degradation occurs when you load an image and save it
in .jpg. If you open the resulting .jpg and save it again, the image is
degraded a little more. This means that .jpg is not a good format for
storing a graphic to work on later - after a series of editing sessions,
the image can get pretty bad. However, most graphics software (including
Paint Shop Pro, which is a very good package) doesn't reload the file it
just saved. So if you work on a picture, save it to disk, and keep
working on it, the copy save to disk is degraded, but the copy in
memory, which you are working on, is not.
There is nothing magic about saving a .jpg over the same file name - it
just means you don't have the old file to go back to. The tip makes
sense when it suggests saving to a new file name, but deleting the old
file doesn't help.
To those of us who have worked in analog media like videotape and film,
the idea of each copy being a little less perfect than the last is
nothing new. It's kind of like what happens when a document is a copy of
a copy of a fax of a copy.
Michelle Nolan mentions that the loss is due to roundoff. Part of it is,
but part of it is due to compression, which you can usualy control. Most
software that saves to .jpg allows you to set the quality or
compression. They are usualy expressed as percentages, and are opposite
ends of a spectrum. Even at the highest quality (lowwest compression)
there is some loss. The only way to know the appropriate compression
level for your application is to try a few and look at the results.
By the way: .jpg is not a virus.
The Bottom Line:
-----------------------
.Jpg is a good format for delivering images where a huge palette is
important. Photographs and graphics that have smooth grades of colors
are examples. The format provides good compression, reasonable image
quality, and a huge palette. It is supported by both the major web
browsers, and almost every image processing software package.
For graphics where 256 colors are enough, consider .gif. As the person
who started this thread found, .gif images don't always look right - 256
colors just isn't enough for subtle shading.
.Jpg is not a good format for storing images between editing sessions.
When you come back to it, the image won't be quite as clear as when you
left it. The difference probably won't be noticable (unless you got
stingy in your quality/compression setting) but it adds up. Use the
format that your image editing software suggests.
Did I mention that .jpg is not a virus?
-----Original Message-----
From: Ann Mackenzie [SMTP:AnnMacknz -at- AOL -dot- COM]
...
The .jpg file is just about worthless except as a "one time" shot to
reduce
file
size.
Every time a .jpg is saved, it degrades. [Never save files from a
digital
camera in .jpg format unless you don't value the images.]
The first in a series of .jpg images is the best. It is one step less
clear
than
the original. Consequently, every time you save a .jpg over the same
file
name,
it gets less and less clear.
Tip: Always save a .jpg to a new filename OR delete the old version
before
saving to the same filename.
The best way to describe the behavior of a .jpg picture is as an image
virus.
...
>-----Original Message-----
>From: Michelle Nolan [SMTP:nolanmj -at- HOTMAIL -dot- COM]
>
>This is pure and utter nonsense. Before we send everyone into a panic
>over how many times they've saved their JPEG files.....
>
>1) Any graphics package worth its weight will not degrade a JPEG image
>each time it is saved. BTW, for anyone looking for a nice, cheap
>alternative to Photoshop, I suggest you go to http://www.jasc.com and
>download Paint Shop Pro. PSP can easily address the problems your
>friend is having.
>
>2) I would hardly call the JPEG file format "worthless". It is one of
>the few formats supported by web browsers and is arguably the most
>popular form of image compression in the world. JPEG has become the
>format of choice because it maintains a high quality in the image while
>achieving a high compression ratio.
>
>3) The JPEG format does cause a loss of information in an image, but
>most of this is due to rounding error (e.g., 128.127345 becomes 128).
>For visual use, nothing to get all worked up about. If you show someone
>a raw, uncompressed image and it's JPEG counterpart, I highly doubt they
>would be able to tell them apart.
>
