Subject: JPEG image compression: Frequently Asked Questions Summary: Useful info about JPEG (JPG) image files and programs Keywords: JPEG, image compression, FAQ Archive-name: jpeg-faq Last-modified: 13 May 1992 This FAQ article discusses JPEG image compression. Suggestions for additions and clarifications are welcome. This article includes the following sections: 1) What is JPEG? 2) Why use JPEG? 3) How well does it work? 4) What are good "quality" settings for JPEG? 5) When should I use JPEG, and when should I stick with GIF? 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) And what about arithmetic coding? 12) Does loss accumulate with repeated compression/decompression? ---------- 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 as 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.) Unless your viewing software supports 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 the 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) How well does it work? 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; 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. 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 -b helps some). Still, I've seen plenty of Usenet postings that were of poorer image quality than this. ship.jpg5o 6587 cjpeg -Q 5 -o (-o reduces 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. 4) What are good "quality" settings for JPEG? (Note: the -Q settings discussed in this article apply to the free JPEG software described in section 6B. Other JPEG implementations, such as Image Alchemy, may use a completely different quality scale.) 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 most 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 next section 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". Another recommendation: when you are making a final version of an image for posting on Usenet or archiving, specify "-o" to cjpeg. This will make the file a little smaller without affecting image quality; it will take longer to do the compression, but not any longer to decompress. 5) When should I use JPEG, and when should I stick with GIF? 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. (Incidentally, many people have developed an odd habit of putting a large constant-color border around a GIF image. This was nearly free in terms of storage cost in GIF files. It is NOT free in JPEG files. Do yourself a favor and don't add a border; let your viewer blank out unused screen area. If you're converting a GIF to JPEG, crop off any border first.) 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. If you have an existing library of GIF images, you may wonder whether you should convert them to JPEG. You will lose some 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, I recommend looking 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. Experience to date suggests that large, high-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 shown in the earlier comparisons). A -Q setting of 85 to 95 will minimize the additional degradation caused by converting a GIF to JPEG. 6) Where can I get JPEG software? 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. X Windows: John Bradley's free XV (version 2.00 and up) is an excellent viewer for JPEG, GIF, and other image formats. It's available for FTP from export.lcs.mit.edu or ftp.cis.upenn.edu. The file is called 'xv-???.tar.Z' (where ??? is the version number, currently 2.21) and is located in the 'contrib' directory on export or the 'pub/xv' directory at upenn. XV reduces all images to 8 bits internally, which means it's not a real good choice if you have a 24-bit display (you still get only 8-bit color). Also, you shouldn't use XV to convert full-color images to JPEG, because they'll get color-quantized first. "xloadimage" is less featureful than XV, but it will do the right thing on 24-bit displays. xloadimage is free and available from export.lcs.mit.edu, file contrib/xloadimage.3.01.tar.Z. Version 3.01 does not read JPEG files, but it will read the PPM files put out by the free JPEG software described in 6B. There is also a patched version called "xli" (see files xli.* in same directory) that does read JPEG directly. However, xli is a quick hack rather than an official release; caveat user. Another good choice for X Windows is John Cristy's free ImageMagick package, also available from export, file contrib/ImageMagick.tar.Z. The viewer included in this package handles 24-bit displays correctly; for colormapped displays, it does better (though slower) color quantization than XV or the basic free JPEG software. MS-DOS: The Independent JPEG Group's free JPEG converters are FTPable from wuarchive.wustl.edu (128.252.135.4), directory mirrors/msdos/graphics, file jpeg3.zip (or jpeg3386.zip if you have a 386 and extended memory); also from SIMTEL20 and its other mirror sites. These are DOS compilations of the free source code described in section 6B. Handmade Software offers two rather pricy shareware programs: Image Alchemy and GIF2JPG/JPG2GIF (contact hsi@netcom.com for details). The PC versions of these programs are available for FTP from wuarchive.wustl.edu, directory mirrors/msdos/graphics, files alchmy15.zip and gif2jpg5.zip; also from SIMTEL20 and its other mirror sites. GIF2JPG/JPG2GIF only perform JPEG<=>GIF format conversion. Image Alchemy converts files between these and many other formats, and can also display images on some types of hardware. The display option is limited and not very high quality, so you'll still want a separate viewer program. (CAUTION: GIF2JPG produces a proprietary file format unless you specify -j. Be sure to use -j if you want to exchange JPEG files with other Usenet users. For that matter, it's not real clear that you should be posting JPEG files made from GIFs; see section 5.) In my biased opinion, the free JPEG software is a better choice than GIF2JPG/JPG2GIF; it's faster, as good or better image quality, and free :-). On the other hand, Image Alchemy may be worth its price, if you need the additional image manipulation capabilities it provides. Currently there are no DOS JPEG viewers that I care to recommend (I expect this will change soon). Use one of the above programs to convert JPEG to GIF, then view with your favorite GIF viewer. Microsoft Windows: There are pre-release versions of a couple of shareware JPEG viewers floating around the net. I can't recommend these in their current state, but if you've gotta have one now, look in pub/MSDOS_UPLOADS at wuarchive.wustl.edu for cview.zip (cview386.zip for 386 and better CPUs) or winjpeg2.zip. At least two freeware Windows JPEG viewers are also undergoing beta test and should be out shortly; watch this space... The DOS conversion programs described above can be used inside a Windows DOS window. Macintosh: Storm Technology has released a free program that can decode and view JPEG images (though not create them). This is called Picture Decompress. Make sure you get version 2.0.1 or later; earlier versions are not compatible with JFIF file format. This program can be FTPed from sumex-aim.stanford.edu, directory /info-mac/app, file picture-decompress-201.hqx. You'll need to be running 32-bit QuickDraw to use it. You'll also need a tool for adjusting file type codes; set the type of a downloaded image file to 'JPEG' to allow Picture Decompress to open it. Finally, you need enough memory to store the whole image in 24-bit format; for large images this can be several megabytes. (System 7 virtual memory helps.) Apple has released a program called PictPixie (formerly known as PictCompressor) that can convert the Usenet-standard JFIF JPEG format to and from QuickTime's internal JPEG format. So if you have QuickTime installed, you can use PictPixie to convert posted files and then use any QuickTime-compatible application to view them. I believe PictPixie can also be used as a viewer by itself. You can get PictPixie from ftp.apple.com, file dts/mac/quicktime/pictpixie.hqx. PictPixie is reputedly faster and less buggy than Picture Decompress. On the minus side, you need both QuickTime and 32-bit QuickDraw to run PictPixie. PictPixie is considered a developer's tool; you may not care for its user interface. There is a simplified version of this program in the QuickTime Starter Kit. There is also a new free program called JPEGView, available from sumex-aim.stanford.edu, file /info-mac/app/jpeg-view-09.hqx. This requires System 7 and QuickTime. The initial release (version 0.9) has some limitations and rough edges; for instance, it can't zoom. For what it does, though, it's a good program. On 8-bit-color Macs, neither Picture Decompress nor JPEGView produce really high quality displays. I hear that PictPixie does better than the other two programs. (The free JPEG software, described later, does an even better job of reducing JPEG images to 8-bit color. Unfortunately, no one has gotten around to releasing a precompiled version of the free software for Macintoshes.) Amiga: I'm told the shareware program HAMLab is about the best tool for viewing/ converting JPEG files. This should be available from most big Amiga archive sites. 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. If you are not reasonably handy at configuring and installing portable C programs, you may have some difficulty installing the free source code. Steve Davis (strat@cis.ksu.edu) has volunteered to maintain an archive of pre-built executable versions of the free JPEG code for various machines. His FTP archive is at ftp.cis.ksu.edu (129.130.10.80); look under /pub/JPEG to see what he currently has. (The administrators of this system ask that FTP traffic be limited to non-prime hours.) This archive is not maintained by the Independent JPEG Group, and files in it may not represent the latest free source code. 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.v3.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. Folks in Europe may find it easier to FTP from nic.funet.fi (see directory pub/graphics/programs/jpeg). The source code is also available on CompuServe, in the GRAPHSUPPORT forum (GO PICS), library 10, as jpsrc3.zip. If you have no FTP access, you can retrieve the source from your nearest comp.sources.misc archive; version 3 appeared as issues 1-18 of volume 29. (See the "How to find sources" FAQ article, which appears regularly in news.answers, if you don't know how to retrieve comp.sources.misc postings.) 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". 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 inherently a full-color format, converting a JPEG image for display on 8-bit-or-less hardware requires color quantization. On the other hand, a GIF image by definition has already been quantized to 256 or fewer colors. 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! 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. For a good technical introduction, see: Wallace, Gregory K. "The JPEG Still Picture Compression Standard", Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44. (Adjacent articles in that issue discuss MPEG motion picture compression, applications of JPEG, and related topics.) If you don't have the CACM issue handy, a PostScript file containing a revised version of this article is available at ftp.uu.net, graphics/jpeg/wallace.ps.Z. The file (actually a preprint for an article to appear in IEEE Trans. Consum. Elect.) omits the sample images that appeared in CACM, but it includes corrections and some added material. Note: the Wallace article is copyright ACM and IEEE, and it may not be used for commercial purposes. 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. 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. (There should be a command line switch to disable subsampling, but as of today, there isn't one.) 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 8 or more bits per pixel; furthermore, it is public domain, 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. 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; in fact Apple has released a utility program for the purpose (see PictPixie in section 6A). 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/JPEG spec is still not officially adopted. Even when TIFF/JPEG is available, the JFIF format is likely to be a widely supported "lowest common denominator"; TIFF/JPEG files may never be as transportable. 11) And 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. 12) 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 :-). --------------------- 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. -- tom lane organizer, Independent JPEG Group Internet: tgl@cs.cmu.edu BITNET: tgl%cs.cmu.edu@carnegie