Archive-name: jpeg-faq Last-modified: 16 May 1993  This FAQ article discusses JPEG image compression.  Suggestions for additions and clarifications are welcome.  New since version of 2 May 1993:   * Added info on ImageViewer for NeXT.   This article includes the following sections:  [1]  What is JPEG? [2]  Why use JPEG? [3]  When should I use JPEG, and when should I stick with GIF? [4]  How well does JPEG compress images? [5]  What are good "quality" settings for JPEG? [6]  Where can I get JPEG software?     [6A] "canned" software, viewers, etc.     [6B] source code [7]  What's all this hoopla about color quantization? [8]  How does JPEG work? [9]  What about lossless JPEG? [10]  Why all the argument about file formats? [11]  How do I recognize which file format I have, and what do I do about it? [12]  What about arithmetic coding? [13]  Does loss accumulate with repeated compression/decompression? [14]  What are some rules of thumb for converting GIF images to JPEG?  Sections 1-6 are basic info that every JPEG user needs to know; sections 7-14 are advanced info for the curious.  This article is posted every 2 weeks.  You can always find the latest version in the news.answers archive at rtfm.mit.edu (18.70.0.226).  By FTP, fetch /pub/usenet/news.answers/jpeg-faq; or if you don't have FTP, send e-mail to mail-server@rtfm.mit.edu with body "send usenet/news.answers/jpeg-faq". Many other FAQ articles are also stored in this archive.  For more instructions on use of the archive, send e-mail to the same address with the words "help" and "index" (no quotes) on separate lines.  If you don't get a reply, the server may be misreading your return address; add a line such as "path myname@mysite" to specify your correct e-mail address to reply to.   ----------   [1]  What is JPEG?  JPEG (pronounced "jay-peg") is a standardized image compression mechanism. JPEG stands for Joint Photographic Experts Group, the original name of the committee that wrote the standard.  JPEG is designed for compressing either full-color or gray-scale digital images of "natural", real-world scenes. It does not work so well on non-realistic images, such as cartoons or line drawings.  JPEG does not handle black-and-white (1-bit-per-pixel) images, nor does it handle motion picture compression.  Standards for compressing those types of images are being worked on by other committees, named JBIG and MPEG respectively.  JPEG is "lossy", meaning that the image you get out of decompression isn't quite identical to what you originally put in.  The algorithm achieves much of its compression by exploiting known limitations of the human eye, notably the fact that small color details aren't perceived as well as small details of light-and-dark.  Thus, JPEG is intended for compressing images that will be looked at by humans.  If you plan to machine-analyze your images, the small errors introduced by JPEG may be a problem for you, even if they are invisible to the eye.  A useful property of JPEG is that the degree of lossiness can be varied by adjusting compression parameters.  This means that the image maker can trade off file size against output image quality.  You can make *extremely* small files if you don't mind poor quality; this is useful for indexing image archives, making thumbnail views or icons, etc. etc.  Conversely, if you aren't happy with the output quality at the default compression setting, you can jack up the quality until you are satisfied, and accept lesser compression.   [2]  Why use JPEG?  There are two good reasons: to make your image files smaller, and to store 24-bit-per-pixel color data instead of 8-bit-per-pixel data.  Making image files smaller is a big win for transmitting files across networks and for archiving libraries of images.  Being able to compress a 2 Mbyte full-color file down to 100 Kbytes or so makes a big difference in disk space and transmission time!  (If you are comparing GIF and JPEG, the size ratio is more like four to one.  More details below.)  If your viewing software doesn't support JPEG directly, you'll have to convert JPEG to some other format for viewing or manipulating images.  Even with a JPEG-capable viewer, it takes longer to decode and view a JPEG image than to view an image of a simpler format (GIF, for instance).  Thus, using JPEG is essentially a time/space tradeoff: you give up some time in order to store or transmit an image more cheaply.  It's worth noting that when network or phone transmission is involved, the time savings from transferring a shorter file can be much greater than the extra time to decompress the file.  I'll let you do the arithmetic yourself.  The other reason why JPEG will gradually replace GIF as a standard Usenet posting format is that JPEG can store full color information: 24 bits/pixel (16 million colors) instead of 8 or less (256 or fewer colors).  If you have only 8-bit display hardware then this may not seem like much of an advantage to you.  Within a couple of years, though, 8-bit GIF will look as obsolete as black-and-white MacPaint format does today.  Furthermore, for reasons detailed in section 7, JPEG is far more useful than GIF for exchanging images among people with widely varying color display hardware.  Hence JPEG is considerably more appropriate than GIF for use as a Usenet posting standard.   [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.  As a rule of thumb, JPEG is superior to GIF for storing full-color or gray-scale images of "realistic" scenes; that means scanned photographs and similar material.  JPEG is superior even if you don't have 24-bit display hardware, and it is a LOT superior if you do.  (See section 7 for details.)  GIF does significantly better on images with only a few distinct colors, such as cartoons and line drawings.  In particular, large areas of pixels that are all *exactly* the same color are compressed very efficiently indeed by GIF.  JPEG can't squeeze these files as much as GIF does without introducing visible defects.  This sort of image is best kept in GIF form. (In particular, single-color borders are quite cheap in GIF files, but they should be avoided in JPEG files.)  JPEG also 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.  Again, this sort of thing is not found in scanned photographs, but it shows up fairly often in GIF files: 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.  Computer-drawn images (ray-traced scenes, for instance) usually fall between scanned images 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).  Plain black-and-white (two level) images should never be converted to JPEG. 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 an existing library of GIF images, you may wonder whether you should convert them to JPEG.  You will lose a little image quality if you do. (Section 7, which argues that JPEG image quality is superior to GIF, only applies if both formats start from a full-color original.  If you start from a GIF, you've already irretrievably lost a great deal of information; JPEG can only make things worse.)  However, the disk space savings may justify converting anyway.  This is a decision you'll have to make for yourself. If you do convert a GIF library to JPEG, see section 14 for hints.  Be prepared to leave some images in GIF format, since some GIFs will not convert well.   [4]  How well does JPEG compress images?  Pretty darn well.  Here are some sample file sizes for an image I have handy, a 727x525 full-color image of a ship in a harbor.  The first three files are for comparison purposes; the rest were created with the free JPEG software described in section 6B.  File	   Size in bytes		Comments  ship.ppm	1145040  Original file in PPM format (no compression; 24 bits 			 or 3 bytes per pixel, plus a few bytes overhead) ship.ppm.Z	 963829  PPM file passed through Unix compress 			 compress doesn't accomplish a lot, you'll note. 			 Other text-oriented compressors give similar results. ship.gif	 240438  Converted to GIF with ppmquant -fs 256 | ppmtogif 			 Most of the savings is the result of losing color 			 info: GIF saves 8 bits/pixel, not 24.  (See sec. 7.)  ship.jpg95	 155622  cjpeg -Q 95    (highest useful quality setting) 			 This is indistinguishable from the 24-bit original, 			 at least to my nonprofessional eyeballs. ship.jpg75	  58009  cjpeg -Q 75    (default setting) 			 You have to look mighty darn close to distinguish this 			 from the original, even with both on-screen at once. ship.jpg50	  38406  cjpeg -Q 50 			 This has slight defects; if you know what to look 			 for, you could tell it's been JPEGed without seeing 			 the original.  Still as good image quality as many 			 recent postings in Usenet pictures groups. ship.jpg25	  25192  cjpeg -Q 25 			 JPEG's characteristic "blockiness" becomes apparent 			 at this setting (djpeg -blocksmooth helps some). 			 Still, I've seen plenty of Usenet postings that were 			 of poorer image quality than this. ship.jpg5o	   6587  cjpeg -Q 5 -optimize  (-optimize cuts table overhead) 			 Blocky, but perfectly satisfactory for preview or 			 indexing purposes.  Note that this file is TINY: 			 the compression ratio from the original is 173:1 !  In this case JPEG can make a file that's a factor of four or five smaller than a GIF of comparable quality (the -Q 75 file is every bit as good as the GIF, better if you have a full-color display).  This seems to be a typical ratio for real-world scenes.   [5]  What are good "quality" settings for JPEG?  Most JPEG compressors let you pick a file size vs. image quality tradeoff by selecting a quality setting.  There seems to be widespread confusion about the meaning of these settings.  "Quality 95" does NOT mean "keep 95% of the information", as some have claimed.  The quality scale is purely arbitrary; it's not a percentage of anything.  The name of the game in using JPEG is to pick the lowest quality setting (smallest file size) that decompresses into an image indistinguishable from the original.  This setting will vary from one image to another and from one observer to another, but here are some rules of thumb.  The default quality setting (-Q 75) is very often the best choice.  This setting is about the lowest you can go without expecting to see defects in a typical image.  Try -Q 75 first; if you see defects, then go up.  Except for experimental purposes, never go above -Q 95; saying -Q 100 will produce a file two or three times as large as -Q 95, but of hardly any better quality.  If the image was less than perfect quality to begin with, you might be able to go down to -Q 50 without objectionable degradation.  On the other hand, you might need to go to a HIGHER quality setting to avoid further degradation. The second case seems to apply much of the time when converting GIFs to JPEG. The default -Q 75 is about right for compressing 24-bit images, but -Q 85 to 95 is usually better for converting GIFs (see section 14 for more info).  If you want a very small file (say for preview or indexing purposes) and are prepared to tolerate large defects, a -Q setting in the range of 5 to 10 is about right.  -Q 2 or so may be amusing as "op art".  (Note: the quality settings discussed in this article apply to the free JPEG software described in section 6B, and to many programs based on it.  Other JPEG implementations, such as Image Alchemy, may use a completely different quality scale.  Some programs don't even provide a numeric scale, just "high"/"medium"/"low"-style choices.)   [6]  Where can I get JPEG software?  Most of the programs described in this section are available by FTP. If you don't know how to use FTP, see the FAQ article "How to find sources". (If you don't have direct access to FTP, read about ftpmail servers in the same article.)  That article appears regularly in news.answers, or you can get it by sending e-mail to mail-server@rtfm.mit.edu with "send usenet/news.answers/finding-sources" in the body.  The "Anonymous FTP List FAQ" may also be helpful --- it's usenet/news.answers/ftp-list/faq in the news.answers archive.  NOTE: this list changes constantly.  If you have a copy more than a couple months old, get the latest JPEG FAQ from the news.answers archive.   [6A]  If you are looking for "canned" software, viewers, etc:  The first part of this list is system-specific programs that only run on one kind of system.  If you don't see what you want for your machine, check out the portable JPEG software described at the end of the list.  Note that this list concentrates on free and shareware programs that you can obtain over Internet; but some commercial programs are listed too.  X Windows:  XV (shareware, $25) is an excellent viewer for JPEG, GIF, and many other image formats.  It can also do format conversion and some simple image manipulations.  It's available for FTP from export.lcs.mit.edu (18.24.0.12), file contrib/xv-3.00.tar.Z.  Version 3.00 is a major upgrade with support for 24-bit displays and many other improvements; however, it is brand new and still has some bugs lurking.  If you prefer not to be on the bleeding edge, stick with version 2.21, also available from export.  Note that version 2.21 is not a good choice if you have a 24-bit display (you'll get only 8-bit color), nor for converting 24-bit images to JPEG.  But 2.21 works fine for converting GIF and other 8-bit images to JPEG.  CAUTION: there is a glitch in version 2.21: be sure to check the "save at normal size" checkbox when saving a JPEG file, or the file will be blurry.  Another good choice for X Windows is John Cristy's free ImageMagick package, also available from export.lcs.mit.edu, file contrib/ImageMagick.tar.Z. This package handles many image processing and conversion tasks.  The ImageMagick viewer handles 24-bit displays correctly; for colormapped displays, it does better (though slower) color quantization than XV or the basic free JPEG software.  Both of the above are large, complex packages.  If you just want a simple image viewer, try xloadimage or xli.  xloadimage supports JPEG in its latest release, 3.03.  xloadimage is free and available from export.lcs.mit.edu, file contrib/xloadimage-3.03.tar.Z.  xli is a variant version of xloadimage, said by its fans to be somewhat faster and more robust than the original. (The current xli is indeed faster and more robust than the current xloadimage, at least with respect to JPEG files, because it has the IJG v4 decoder while xloadimage 3.03 is using a hacked-over v1.  The next xloadimage release will fix this.)  xli is also free and available from export.lcs.mit.edu, file contrib/xli.1.14.tar.Z.  Both programs are said to do the right thing with 24-bit displays.   MS-DOS:  This covers plain DOS; for Windows or OS/2 programs, see the next headings.  One good choice is Eric Praetzel's free DVPEG, which views JPEG and GIF files. The current version, 2.5, is available by FTP from sunee.uwaterloo.ca (129.97.50.50), file pub/jpeg/viewers/dvpeg25.zip.  This is a good basic viewer that works on either 286 or 386/486 machines.  The user interface is not flashy, but it's functional.  Another freeware JPEG/GIF/TGA viewer is Mohammad Rezaei's Hiview.  The current version, 1.2, is available from Simtel20 and mirror sites (see NOTE below), file msdos/graphics/hv12.zip.  Hiview requires a 386 or better CPU and a VCPI-compatible memory manager (QEMM386 and 386MAX work; Windows and OS/2 do not).  Hiview is currently the fastest viewer for images that are no bigger than your screen.  For larger images, it scales the image down to fit on the screen (rather than using panning/scrolling as most viewers do). You may or may not prefer this approach, but there's no denying that it slows down loading of large images considerably.  Note: installation is a bit tricky; read the directions carefully!  A shareware alternative is ColorView for DOS ($30).  This is easier to install than either of the two freeware alternatives.  Its user interface is also much spiffier-looking, although personally I find it harder to use --- more keystrokes, inconsistent behavior.  It is faster than DVPEG but a little slower than Hiview, at least on my hardware.  (For images larger than screen size, DVPEG and ColorView seem to be about the same speed, and both are faster than Hiview.)  The current version is 2.1, available from Simtel20 and mirror sites (see NOTE below), file msdos/graphics/dcview21.zip. Requires a VESA graphics driver; if you don't have one, look in vesadrv2.zip or vesa-tsr.zip from the same directory.  (Many recent PCs have a built-in VESA driver, so don't try to load a VESA driver unless ColorView complains that the driver is missing.)  A second shareware alternative is Fullview, which has been kicking around the net for a while, but I don't know any stable archive location for it. The current (rather old) version is inferior to the above viewers anyway. The author tells me that a new version of Fullview will be out shortly and it will be submitted to the Simtel20 archives at that time.  The well-known GIF viewer CompuShow (CSHOW) supports JPEG in its latest revision, 8.60a.  However, CSHOW's JPEG implementation isn't very good: it's slow (about half the speed of the above viewers) and image quality is poor except on hi-color displays.  Too bad ... it'd have been nice to see a good JPEG capability in CSHOW.  Shareware, $25.  Available from Simtel20 and mirror sites (see NOTE below), file msdos/gif/cshw860a.zip.  Due to the remarkable variety of PC graphics hardware, any one of these viewers might not work on your particular machine.  If you can't get *any* of them to work, you'll need to use one of the following conversion programs to convert JPEG to GIF, then view with your favorite GIF viewer.  (If you have hi-color hardware, don't use GIF as the intermediate format; try to find a TARGA-capable viewer instead.  VPIC5.0 is reputed to do the right thing with hi-color displays.)  The Independent JPEG Group's free JPEG converters are FTPable from Simtel20 and mirror sites (see NOTE below), file msdos/graphics/jpeg4.zip (or jpeg4386.zip if you have a 386 and extended memory).  These files are DOS compilations of the free source code described in section 6B; they will convert JPEG to and from GIF, Targa, and PPM formats.  Handmade Software offers free JPEG<=>GIF conversion tools, GIF2JPG/JPG2GIF. These are slow and are limited to conversion to and from GIF format; in particular, you can't get 24-bit color output from a JPEG.  The major advantage of these tools is that they will read and write HSI's proprietary JPEG format as well as the Usenet-standard JFIF format.  Since HSI-format files are rather widespread on BBSes, this is a useful capability.  Version 2.0 of these tools is free (prior versions were shareware).  Get it from Simtel20 and mirror sites (see NOTE below), file msdos/graphics/gif2jpg2.zip. NOTE: do not use HSI format for files to be posted on Internet, since it is not readable on non-PC platforms.  Handmade Software also has a shareware image conversion and manipulation package, Image Alchemy.  This will translate JPEG files (both JFIF and HSI formats) to and from many other image formats.  It can also display images. A demo version of Image Alchemy version 1.6.2 is available from Simtel20 and mirror sites (see NOTE below), file msdos/graphics/alch162.zip.  NOTE ABOUT SIMTEL20: The Internet's key archive site for PC-related programs is Simtel20, full name wsmr-simtel20.army.mil (192.88.110.20).  Simtel20 runs a non-Unix system with weird directory names; where this document refers to directory (eg) "msdos/graphics" at Simtel20, that really means "pd1:<msdos.graphics>".  If you are not physically on MILnet, you should expect rather slow FTP transfer rates from Simtel20.  There are several Internet sites that maintain copies (mirrors) of the Simtel20 archives; most FTP users should go to one of the mirror sites instead.  A popular USA mirror site is oak.oakland.edu (141.210.10.117), which keeps Simtel20 files in (eg) "/pub/msdos/graphics".  If you have no FTP capability, you can retrieve files from Simtel20 by e-mail; see informational postings in comp.archives.msdos.announce to find out how.  If you are outside the USA, consult the same newsgroup to learn where your nearest Simtel20 mirror is.  Microsoft Windows:  There are several Windows programs capable of displaying JPEG images. (Windows viewers are generally slower than DOS viewers on the same hardware, due to Windows' system overhead.  Note that you can run the DOS conversion programs described above inside a Windows DOS window.)  The newest entry is WinECJ, which is free and EXTREMELY fast.  Version 1.0 is available from ftp.rahul.net, file /pub/bryanw/pc/jpeg/wecj.zip. Requires Windows 3.1 and 256-or-more-colors mode.  This is a no-frills viewer with the bad habit of hogging the machine completely while it decodes; and the image quality is noticeably worse than other viewers. But it's so fast you'll use it anyway, at least for previewing...  JView is freeware, fairly fast, has good on-line help, and can write out the decompressed image in Windows BMP format; but it can't create new JPEG files, and it doesn't view GIFs.  JView also lacks some other useful features of the shareware viewers (such as brightness adjustment), but it's an excellent basic viewer.  The current version, 0.9, is available from ftp.cica.indiana.edu (129.79.20.84), file pub/pc/win3/desktop/jview090.zip. (Mirrors of this archive can be found at some other Internet sites, including wuarchive.wustl.edu.)  WinJPEG (shareware, $20) displays JPEG,GIF,Targa,TIFF, and BMP image files; it can write all of these formats too, so it can be used as a converter. It has some other nifty features including color-balance adjustment and slideshow.  The current version is 2.1, available from Simtel20 and mirror sites (see NOTE above), file msdos/windows3/winjp210.zip.  (This is a slow 286-compatible version; if you register, you'll get the 386-only version, which is roughly 25% faster.)  ColorView is another shareware entry ($30).  This was an early and promising contender, but it has not been updated in some time, and at this point it has no real advantages over WinJPEG.  If you want to try it anyway, the current version is 0.97, available from ftp.cica.indiana.edu, file pub/pc/win3/desktop/cview097.zip.  (I understand that a new version will be appearing once the authors are finished with ColorView for DOS.)  DVPEG (see DOS heading) also works under Windows, but only in full-screen mode, not in a window.  OS/2:  The following files are available from hobbes.nmsu.edu (128.123.35.151). Note: check /pub/uploads for more recent versions --- the hobbes moderator is not very fast about moving uploads into their permanent directories. /pub/os2/2.x/graphics/jpegv4.zip     32-bit version of free IJG conversion programs, version 4. /pub/os2/all/graphics/jpeg4-16.zip     16-bit version of same, for OS/2 1.x. /pub/os2/2.x/graphics/imgarc12.zip     Image Archiver 1.02: image conversion/viewing with PM graphical interface.     Strong on conversion functions, viewing is a bit weaker.  Shareware, $15. /pub/os2/2.x/graphics/pmjpeg11.zip     PMJPEG 1.1: OS/2 2.x port of WinJPEG, a popular viewer for Windows     (see description in Windows section).  Shareware, $20. /pub/os2/2.x/graphics/pmview85.zip     PMView 0.85: JPEG/GIF/BMP/Targa/PCX viewer.  GIF viewing very fast,     JPEG viewing roughly the same speed as the above two programs.  Has     image manipulation & slideshow functions.  Shareware, $20.  Macintosh:  Most Mac JPEG programs rely on Apple's JPEG implementation, which is part of the QuickTime system extension; so you need to have QuickTime installed. To use QuickTime, you need a 68020 or better CPU and you need to be running System 6.0.7 or later.  (If you're running System 6, you must also install the 32-bit QuickDraw extension; this is built-in on System 7.)  You can get QuickTime by FTP from ftp.apple.com, file dts/mac/quicktime/quicktime.hqx. (As of 11/92, this file contains QuickTime 1.5, which is better than QT 1.0 in several ways.  With respect to JPEG, it is marginally faster and considerably less prone to crash when fed a corrupt JPEG file.  However, some applications seem to have compatibility problems with QT 1.5.)  Mac users should keep in mind that QuickTime's JPEG format, PICT/JPEG, is not the same as the Usenet-standard JFIF JPEG format.  (See section 10 for details.)  If you post images on Usenet, make sure they are in JFIF format. Most of the programs mentioned below can generate either format.  The first choice is probably JPEGView, a free program for viewing images that are in JFIF format, PICT/JPEG format, or GIF format.  It also can convert between the two JPEG formats.  The current version, 2.0, is a big improvement over prior versions.  Get it from sumex-aim.stanford.edu (36.44.0.6), file /info-mac/app/jpeg-view-20.hqx.  Requires System 7 and QuickTime.  On 8-bit displays, JPEGView usually produces the best color image quality of all the currently available Mac JPEG viewers.  JPEGView can view large images in much less memory than other Mac viewers; in fact, it's the only one that can deal with JPEG images much over 640x480 pixels on a typical 4MB Mac.  Given a large image, JPEGView automatically scales it down to fit on the screen, rather than presenting scroll bars like most other viewers.  (You can zoom in on any desired portion, though.)  Some people like this behavior, some don't.  Overall, JPEGView's user interface is very well thought out.  GIFConverter, a shareware ($40) image viewer/converter, supports JFIF and PICT/JPEG, as well as GIF and several other image formats.  The latest version is 2.3.2.  Get it from sumex-aim.stanford.edu, file /info-mac/art/gif/gif-converter-232.hqx.  Requires System 6.0.5 or later. GIFConverter is not better than JPEGView as a plain JPEG/GIF viewer, but it has much more extensive image manipulation and format conversion capabilities, so you may find it worth its shareware fee if you do a lot of playing around with images.  Also, the newest version of GIFConverter can load and save JFIF images *without* QuickTime, so it is your best bet if your machine is too old to run QuickTime.  (But it's faster with QuickTime.) Note: If GIFConverter runs out of memory trying to load a large JPEG, try converting the file to GIF with JPEG Convert, then viewing the GIF version.  JPEG Convert, a Mac version of the free IJG JPEG conversion utilities, is available from sumex-aim.stanford.edu, file /info-mac/app/jpeg-convert-10.hqx. This will run on any Mac, but it only does file conversion, not viewing. You can use it in conjunction with any GIF viewer.  Previous versions of this FAQ recommended Imagery JPEG v0.6, a JPEG<=>GIF converter based on an old version of the IJG code.  If you are using this 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: 0223 841099.  NeXT:  ImageViewer is a PD utility that displays images and can do some format conversions.  The current version reads JPEG but does not write it. ImageViewer is available from the standard NeXT archives at sonata.cc.purdue.edu and cs.orst.edu, somewhere in /pub/next (both are currently being re-organized, so it's hard to point to specific sub-directories).  Note that there is an older version floating around that does not support JPEG.   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. What it does get rid of is deliberate information loss in the coefficient quantization step.  There is still a good deal of information loss in the color subsampling step.  (With the V4 free JPEG code, you can also say "-sample 1x1" to turn off subsampling.  Keep in mind that many commercial JPEG implementations cannot cope with the resulting file.)  Even with both quantization and subsampling turned off, the regular JPEG algorithm is not lossless, because it is subject to roundoff errors in various calculations.  The maximum error is a few counts in any one pixel value; it's highly unlikely that this could be perceived by the human eye, but it might be a concern if you are doing machine processing of an image.  At this minimum-loss setting, regular JPEG produces files that are perhaps half the size of an uncompressed 24-bit-per-pixel image.  True lossless JPEG provides roughly the same amount of compression, but it guarantees bit-for-bit accuracy.  If you have an application requiring lossless storage of images with less than 6 bits per pixel (per color component), you may want to look into the JBIG bilevel image compression standard.  This performs better than JPEG lossless on such images.  JPEG lossless is superior to JBIG on images with 6 or more bits per pixel; furthermore, JPEG is public domain (at least with a Huffman back end), while the JBIG techniques are heavily covered by patents.   [10]  Why all the argument about file formats?  Strictly speaking, JPEG refers only to a family of compression algorithms; it does *not* refer to a specific image file format.  The JPEG committee was prevented from defining a file format by turf wars within the international standards organizations.  Since we can't actually exchange images with anyone else unless we agree on a common file format, this leaves us with a problem.  In the absence of official standards, a number of JPEG program writers have just gone off to "do their own thing", and as a result their programs aren't compatible with anybody else's.  The closest thing we have to a de-facto standard JPEG format is some work that's been coordinated by people at C-Cube Microsystems.  They have defined two JPEG-based file formats:   * JFIF (JPEG File Interchange Format), a "low-end" format that transports     pixels and not much else.   * TIFF/JPEG, aka TIFF 6.0, an extension of the Aldus TIFF format.  TIFF is     a "high-end" format that will let you record just about everything you     ever wanted to know about an image, and a lot more besides :-).  TIFF is     a lot more complex than JFIF, and may well prove less transportable,     because different vendors have historically implemented slightly different     and incompatible subsets of TIFF.  It's not likely that adding JPEG to the     mix will do anything to improve this situation. Both of these formats were developed with input from all the major vendors of JPEG-related products; it's reasonably likely that future commercial products will adhere to one or both standards.  I believe that Usenet should adopt JFIF as the replacement for GIF in picture postings.  JFIF is simpler than TIFF and is available now; the TIFF 6.0 spec has only recently been officially adopted, and it is still unusably vague on some crucial details.  Even when TIFF/JPEG is well defined, the JFIF format is likely to be a widely supported "lowest common denominator"; TIFF/JPEG files may never be as transportable.  A particular case that people may be interested in is Apple's QuickTime software for the Macintosh.  QuickTime uses a JFIF-compatible format wrapped inside the Mac-specific PICT structure.  Conversion between JFIF and QuickTime JPEG is pretty straightforward, and several Mac programs are available to do it (see Mac portion of section 6A).  If you have an editor that handles binary files, you can strip a QuickTime JPEG PICT down to JFIF by hand; see section 11 for details.  Another particular case is Handmade Software's programs (GIF2JPG/JPG2GIF and Image Alchemy).  These programs are capable of reading and writing JFIF format.  By default, though, they write a proprietary format developed by HSI.  This format is NOT readable by any non-HSI programs and should not be used for Usenet postings.  Use the -j switch to get JFIF output.  (This applies to old versions of these programs; the current releases emit JFIF format by default.  You still should be careful not to post HSI-format files, unless you want to get flamed by people on non-PC platforms.)   [11]  How do I recognize which file format I have, and what do I do about it?  If you have an alleged JPEG file that your software won't read, it's likely to be HSI format or some other proprietary JPEG-based format.  You can tell what you have by inspecting the first few bytes of the file:  1.  A JFIF-standard file will start with the characters (hex) FF D8 FF E0,     followed by two variable bytes (often hex 00 10), followed by 'JFIF'.  2.  If you see FF D8 at the start, but not the rest of it, you may have a     "raw JPEG" file.  This is probably decodable as-is by JFIF software ---     it's worth a try, anyway.  3.  HSI files start with 'hsi1'.  You're out of luck unless you have HSI     software.  Portions of the file may look like plain JPEG data, but they     won't decompress properly with non-HSI programs.  4.  A Macintosh PICT file, if JPEG-compressed, will have a couple hundred     bytes of header followed by a JFIF header (scan for 'JFIF').  Strip off     everything before the FF D8 and you should be able to read it.  5.  Anything else: it's a proprietary format, or not JPEG at all.  If you are     lucky, the file may consist of a header and a raw JPEG data stream.     If you can identify the start of the JPEG data stream (look for FF D8),     try stripping off everything before that.  In uuencoded Usenet postings, the characteristic JFIF pattern is  	"begin" line 	M_]C_X ...  whereas uuencoded HSI files will start with  	"begin" line 	M:'-I ...  If you learn to check for the former, you can save yourself the trouble of downloading non-JFIF files.   [12]  What about arithmetic coding?  The JPEG spec defines two different "back end" modules for the final output of compressed data: either Huffman coding or arithmetic coding is allowed. The choice has no impact on image quality, but arithmetic coding usually produces a smaller compressed file.  On typical images, arithmetic coding produces a file 5 or 10 percent smaller than Huffman coding.  (All the file-size numbers previously cited are for Huffman coding.)  Unfortunately, the particular variant of arithmetic coding specified by the JPEG standard is subject to patents owned by IBM, AT&T, and Mitsubishi. Thus *you cannot legally use arithmetic coding* unless you obtain licenses from these companies.  (The "fair use" doctrine allows people to implement and test the algorithm, but actually storing any images with it is dubious at best.)  At least in the short run, I recommend that people not worry about arithmetic coding; the space savings isn't great enough to justify the potential legal hassles.  In particular, arithmetic coding *should not* be used for any images to be exchanged on Usenet.  There is some small chance that the legal situation may change in the future.  Stay tuned for further details.   [13]  Does loss accumulate with repeated compression/decompression?  It would be nice if, having compressed an image with JPEG, you could decompress it, manipulate it (crop off a border, say), and recompress it without any further image degradation beyond what you lost initially. Unfortunately THIS IS NOT THE CASE.  In general, recompressing an altered image loses more information, though usually not as much as was lost the first time around.  The next best thing would be that if you decompress an image and recompress it *without changing it* then there is no further loss, i.e., you get an identical JPEG file.  Even this is not true; at least, not with the current free JPEG software.  It's essentially a problem of accumulation of roundoff error.  If you repeatedly compress and decompress, the image will eventually degrade to where you can see visible changes from the first-generation output.  (It usually takes many such cycles to get visible change.) One of the things on our to-do list is to see if accumulation of error can be avoided or limited, but I am not optimistic about it.  In any case, the most that could possibly be guaranteed would be that compressing the unmodified full-color output of djpeg, at the original quality setting, would introduce no further loss.  Even such simple changes as cropping off a border could cause further roundoff-error degradation. (If you're wondering why, it's because the pixel-block boundaries move. If you cropped off only multiples of 16 pixels, you might be safe, but that's a mighty limited capability!)  The bottom line is that JPEG is a useful format for archival storage and transmission of images, but you don't want to use it as an intermediate format for sequences of image manipulation steps.  Use a lossless format (PPM, RLE, TIFF, etc) while working on the image, then JPEG it when you are ready to file it away.  Aside from avoiding degradation, you will save a lot of compression/decompression time this way :-).   [14]  What are some rules of thumb for converting GIF images to JPEG?  As stated earlier, you *will* lose some amount of image information if you convert an existing GIF image to JPEG.  If you can obtain the original full-color data the GIF was made from, it's far better to make a JPEG from that.  But if you need to save space and have only the GIF to work from, here are some suggestions for getting maximum space savings with minimum loss of quality.  The first rule when converting a GIF library is to look at each JPEG, to make sure you are happy with it, before throwing away the corresponding GIF; that will give you a chance to re-do the conversion with a higher quality setting if necessary.  Some GIFs may be better left as GIFs, as explained in section 3; in particular, cartoon-type GIFs with sixteen or fewer colors don't convert well.  You may find that a JPEG file of reasonable quality will be *larger* than the GIF.  (So check the sizes too.)  Experience to date suggests that large, high-visual-quality GIFs are the best candidates for conversion to JPEG.  They chew up the most storage so offer the most potential savings, and they convert to JPEG with least degradation. Don't waste your time converting any GIF much under 100 Kbytes.  Also, don't expect JPEG files converted from GIFs to be as small as those created directly from full-color originals.  To maintain image quality you may have to let the converted files be as much as twice as big as straight-through JPEG files would be (i.e., shoot for 1/2 or 1/3rd the size of the GIF file, not 1/4th as suggested in earlier comparisons).  Many people have developed an odd habit of putting a large constant-color border around a GIF image.  While useless, this was nearly free in terms of storage cost in GIF files.  It is NOT free in JPEG files, and the sharp border boundary can create visible artifacts ("ghost" edges).  Do yourself a favor and crop off any border before JPEGing.  (If you are on an X Windows system, XV's manual and automatic cropping functions are a very painless way to do this.)  cjpeg's default Q setting of 75 is appropriate for full-color input, but for GIF inputs, Q settings of 85 to 95 often seem to be necessary to avoid image degradation.  (If you apply smoothing as suggested below, the higher Q setting may not be necessary.)  Color GIFs of photographs or complex artwork 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 see 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 want 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.  Appropriate use of smoothing will often let you avoid using a high Q factor, thus further reducing the size of the compressed file, while still obtaining a better-looking output image than you'd get without smoothing.  With the V4 free JPEG software (or products based on it), 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.  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, such as pnmconvol from the PBMPLUS package.  However, cjpeg's built-in smoother is a LOT faster than pnmconvol...  The upshot of all this is that "cjpeg -quality 85 -smooth 10" is probably a good starting point for converting GIFs.  But if you really care about the image, you'll want to check the results and maybe try a few other settings.   ---------------------  For more information about JPEG in general or the free JPEG software in particular, contact the Independent JPEG Group at jpeg-info@uunet.uu.net.