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神经网络昆斯林的新闻组分类2006

program, you definitely should replace it with JPEG Convert.

Apple's free program PictPixie can view images in JFIF, QuickTime JPEG, and
GIF format, and can convert between these formats.  You can get PictPixie
from ftp.apple.com, file dts/mac/quicktime/qt.1.0.stuff/pictpixie.hqx.
Requires QuickTime.  PictPixie was intended as a developer's tool, and it's
really not the best choice unless you like to fool around with QuickTime.
Some of its drawbacks are that it requires lots of memory, it produces
relatively poor color image quality on anything less than a 24-bit display,
and it has a relatively unfriendly user interface.  Worse, PictPixie is an
unsupported program, meaning it has some minor bugs that Apple does not
intend to fix.  (There is an old version of PictPixie, called
PICTCompressor, floating around the net.  If you have this you should trash
it, as it's even buggier.  Also, the QuickTime Starter Kit includes a much
cleaned-up descendant of PictPixie called Picture Compressor.  Note that
Picture Compressor is NOT free and may not be distributed on the net.)

Storm Technology's Picture Decompress is a free JPEG viewer/converter.
This rather old program is inferior to the above programs in many ways, but
it will run without System 7 or QuickTime, so you may be forced to use it on
older systems.  (It does need 32-bit QuickDraw, so really old machines can't
use it.)  You can get it from sumex-aim.stanford.edu, file
/info-mac/app/picture-decompress-201.hqx.  You must set the file type of a
downloaded image file to 'JPEG' to allow Picture Decompress to open it.

If your machine is too old to run 32-bit QuickDraw (a Mac Plus for instance),
GIFConverter is your only choice for single-program JPEG viewing.  If you
don't want to pay for GIFConverter, use JPEG Convert and a free GIF viewer.

More and more commercial Mac applications are supporting JPEG, although not
all can deal with the Usenet-standard JFIF format.  Adobe Photoshop, version
2.0.1 or later, can read and write JFIF-format JPEG files (use the JPEG
plug-in from the Acquire menu).  You must set the file type of a downloaded
JPEG file to 'JPEG' to allow Photoshop to recognize it.

Amiga:

(Most programs listed in this section are stored in the AmiNet archive at
amiga.physik.unizh.ch (130.60.80.80).  There are many mirror sites of this
archive and you should try to use the closest one.  In the USA, a good
choice is wuarchive.wustl.edu; look under /mirrors/amiga.physik.unizh.ch/...)

HamLab Plus is an excellent JPEG viewer/converter, as well as being a
general image manipulation tool.  It's cheap (shareware, $20) and can read
several formats besides JPEG.  The current version is 2.0.8.  A demo version
is available from amiga.physik.unizh.ch (and mirror sites), file
amiga/gfx/edit/hamlab208d.lha.  The demo version will crop images larger
than 512x512, but it is otherwise fully functional.

Rend24 (shareware, $30) is an image renderer that can display JPEG, ILBM,
and GIF images.  The program can be used to create animations, even
capturing frames on-the-fly from rendering packages like Lightwave.  The
current version is 1.05, available from amiga.physik.unizh.ch (and mirror
sites), file amiga/os30/gfx/rend105.lha.  (Note: although this directory is
supposedly for AmigaDOS 3.0 programs, the program will also run under
AmigaDOS 1.3, 2.04 or 2.1.)

Viewtek is a free JPEG/ILBM/GIF/ANIM viewer.  The current version is 1.04,
available from amiga.physik.unizh.ch (and mirror sites), file
amiga/gfx/show/ViewTek104.lha.

If you're willing to spend real money, there are several commercial packages
that support JPEG.  Two are written by Thomas Krehbiel, the author of Rend24
and Viewtek.  These are CineMorph, a standalone image morphing package, and
ImageFX, an impressive 24-bit image capture, conversion, editing, painting,
effects and prepress package that also includes CineMorph.  Both are
distributed by Great Valley Products.  Art Department Professional (ADPro),
from ASDG Inc, is the most widely used commercial image manipulation
software for Amigas.  ImageMaster, from Black Belt Systems, is another
well-regarded commercial graphics package with JPEG support.

The free IJG JPEG software is available compiled for Amigas from
amiga.physik.unizh.ch (and mirror sites) in directory amiga/gfx/conv, file
AmigaJPEGV4.lha.  These programs convert JPEG to/from PPM,GIF,Targa formats.

The Amiga world is heavily infested with quick-and-dirty JPEG programs, many
based on an ancient beta-test version of the free IJG JPEG software (thanks
to a certain magazine that published same on its disk-of-the-month, without
so much as notifying the authors).  Among these are "AugJPEG", "NewAmyJPEG",
"VJPEG", and probably others I have not even heard of.  In my opinion,
anything older than IJG version 3 (March 1992) is not worth the disk space
it's stored on; if you have such a program, trash it and get something newer.

Atari ST:

The free IJG JPEG software is available compiled for Atari ST, TT, etc,
from atari.archive.umich.edu, file /atari/Graphics/jpeg4bin.zoo.
These programs convert JPEG to/from PPM, GIF, Targa formats.

For monochrome ST monitors, try MGIF, which manages to achieve four-level
grayscale effect by flickering.  Version 4.1 reads JPEG files.  Available
from atari.archive.umich.edu, file /atari/Graphics/mgif41b.zoo.

I have not heard of any other free or shareware JPEG-capable viewers for
Ataris, but surely there must be some by now?  Pointers appreciated.

Acorn Archimedes:

!ChangeFSI, supplied with RISC OS 3 version 3.10, can convert from and view
JPEG JFIF format.  Provision is also made to convert images to JPEG,
although this must be done from the CLI rather than by double-clicking.

Recent versions (since 7.11) of the shareware program Translator can handle
JPEG, along with about 30 other image formats.  While older versions can be
found on some Archimedes bboards, the current version is only available by
registering with the author, John Kortink, Nutterbrink 31, 7544 WJ, Enschede,
The Netherlands.  Price 35 Dutch guilders (about $22 or 10 pounds).

There's also a commercial product called !JPEG which provides JPEG read/write
functionality and direct JPEG viewing, as well as a host of other image
format conversion and processing options.  This is more expensive but not
necessarily better than the above programs.  Contact: DT Software, FREEPOST,
Cambridge, UK.  Tel: [phone].


Portable software for almost any system:

If none of the above fits your situation, you can obtain and compile the free
JPEG conversion software described in 6B.  You'll also need a viewer program.
If your display is 8 bits or less, any GIF viewer will do fine; if you have a
display with more color capability, try to find a viewer that can read Targa
or PPM 24-bit image files.

There are numerous commercial JPEG offerings, with more popping up every
day.  I recommend that you not spend money on one of these unless you find
the available free or shareware software vastly too slow.  In that case,
purchase a hardware-assisted product.  Ask pointed questions about whether
the product complies with the final JPEG standard and about whether it can
handle the JFIF file format; many of the earliest commercial releases are
not and never will be compatible with anyone else's files.


[6B]  If you are looking for source code to work with:

Free, portable C code for JPEG compression is available from the Independent
JPEG Group, which I lead.  A package containing our source code,
documentation, and some small test files is available from several places.
The "official" archive site for this source code is ftp.uu.net (137.39.1.9
or 192.48.96.9).  Look under directory /graphics/jpeg; the current release
is jpegsrc.v4.tar.Z.  (This is a compressed TAR file; don't forget to
retrieve in binary mode.)  You can retrieve this file by FTP or UUCP.
If you are on a PC and don't know how to cope with .tar.Z format, you may
prefer ZIP format, which you can find at Simtel20 and mirror sites (see NOTE
above), file msdos/graphics/jpegsrc4.zip.  This file will also be available on
CompuServe, in the GRAPHSUPPORT forum (GO PICS), library 15, as jpsrc4.zip.
If you have no FTP access, you can retrieve the source from your nearest
comp.sources.misc archive; version 4 appeared as issues 55-72 of volume 34.
(If you don't know how to retrieve comp.sources.misc postings, see the FAQ
article "How to find sources", referred to at the top of section 6.)

The free JPEG code provides conversion between JPEG "JFIF" format and image
files in GIF, PBMPLUS PPM/PGM, Utah RLE, and Truevision Targa file formats.
The core compression and decompression modules can easily be reused in other
programs, such as image viewers.  The package is highly portable; we have
tested it on many machines ranging from PCs to Crays.

We have released this software for both noncommercial and commercial use.
Companies are welcome to use it as the basis for JPEG-related products.
We do not ask a royalty, although we do ask for an acknowledgement in
product literature (see the README file in the distribution for details).
We hope to make this software industrial-quality --- although, as with
anything that's free, we offer no warranty and accept no liability.

The Independent JPEG Group is a volunteer organization; if you'd like to
contribute to improving our software, you are welcome to join.


[7]  What's all this hoopla about color quantization?

Most people don't have full-color (24 bit per pixel) display hardware.
Typical display hardware stores 8 or fewer bits per pixel, so it can display
256 or fewer distinct colors at a time.  To display a full-color image, the
computer must map the image into an appropriate set of representative
colors.  This process is called "color quantization".  (This is something
of a misnomer, "color selection" would be a better term.  We're stuck with
the standard usage though.)

Clearly, color quantization is a lossy process.  It turns out that for most
images, the details of the color quantization algorithm have MUCH more impact
on the final image quality than do any errors introduced by JPEG (except at
the very lowest JPEG quality settings).

Since JPEG is a full-color format, converting a color JPEG image for display
on 8-bit-or-less hardware requires color quantization.  This is true for
*all* color JPEGs: even if you feed a 256-or-less-color GIF into JPEG, what
comes out of the decompressor is *not* 256 colors, but thousands of colors.
This happens because JPEG's lossiness affects each pixel a little
differently, so two pixels that started with identical colors will probably
come out with slightly different colors.  Each original color gets "smeared"
into a group of nearby colors.  Therefore quantization is always required to
display a color JPEG on a colormapped display, regardless of the image
source.  The only way to avoid quantization is to ask for gray-scale output.

(Incidentally, because of this effect it's nearly meaningless to talk about
the number of colors used by a JPEG image.  Even if you attempted to count
the number of distinct pixel values, different JPEG decoders would give you
different results because of roundoff error differences.  I occasionally see
posted images described as "256-color JPEG".  This tells me that the poster
(a) hasn't read this FAQ and (b) probably converted the JPEG from a GIF.
JPEGs can be classified as color or gray-scale (just like photographs), but
number of colors just isn't a useful concept for JPEG.)

On the other hand, a GIF image by definition has already been quantized to
256 or fewer colors.  (A GIF *does* have a definite number of colors in its
palette, and the format doesn't allow more than 256 palette entries.)
For purposes of Usenet picture distribution, GIF has the advantage that the
sender precomputes the color quantization, so recipients don't have to.
This is also the *disadvantage* of GIF: you're stuck with the sender's
quantization.  If the sender quantized to a different number of colors than
what you can display, you have to re-quantize, resulting in much poorer
image quality than if you had quantized once from a full-color image.
Furthermore, if the sender didn't use a high-quality color quantization
algorithm, you're out of luck.

For this reason, JPEG offers the promise of significantly better image quality
for all users whose machines don't match the sender's display hardware.
JPEG's full color image can be quantized to precisely match the user's display
hardware.  Furthermore, you will be able to take advantage of future
improvements in quantization algorithms (there is a lot of active research in
this area), or purchase better display hardware, to get a better view of JPEG
images you already have.  With a GIF, you're stuck forevermore with what was
sent.

It's also worth mentioning that many GIF-viewing programs include rather
shoddy quantization routines.  If you view a 256-color GIF on a 16-color EGA
display, for example, you are probably getting a much worse image than you
need to.  This is partly an inevitable consequence of doing two color
quantizations (one to create the GIF, one to display it), but often it's
also due to sloppiness.  JPEG conversion programs will be forced to use
high quality quantizers in order to get acceptable results at all, and in
normal use they will quantize directly to the number of colors to be
displayed.  Thus, JPEG is likely to provide better results than the average
GIF program for low-color-resolution displays as well as high-resolution ones!

Finally, an ever-growing number of people have better-than-8-bit display
hardware already: 15-bit "hi-color" PC displays, true 24-bit displays on
workstations and Macintoshes, etc.  For these people, GIF is already
obsolete, as it cannot represent an image to the full capabilities of their
display.  JPEG images can drive these displays much more effectively.
Thus, JPEG is an all-around better choice than GIF for representing images
in a machine-independent fashion.


[8]  How does JPEG work?

The buzz-words to know are chrominance subsampling, discrete cosine
transforms, coefficient quantization, and Huffman or arithmetic entropy
coding.  This article's long enough already, so I'm not going to say more
than that here.  For technical information, see the comp.compression FAQ.
This is available from the news.answers archive at rtfm.mit.edu, in files
/pub/usenet/news.answers/compression-faq/part[1-3].  If you need help in
using the news.answers archive, see the top of this article.


[9]  What about lossless JPEG?

There's a great deal of confusion on this subject.  The JPEG committee did
define a truly lossless compression algorithm, i.e., one that guarantees the
final output is bit-for-bit identical to the original input.  However, this
lossless mode has almost nothing in common with the regular, lossy JPEG
algorithm, and it offers much less compression.  At present, very few
implementations of lossless JPEG exist, and all of them are commercial.

Saying "-Q 100" to the free JPEG software DOES NOT get you a lossless image.