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This article includes the following sections: Basic questions: .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? .7. How do I view JPEG images posted on Usenet? More advanced questions: .8. What is color quantization? .9. What are some rules of thumb for converting GIF images to JPEG? .10. Does loss accumulate with repeated compression/decompression? .11. What is progressive JPEG? .12. Isn't there a lossless JPEG? .13. Why all the argument about file formats? .14. How do I recognize which file format I have, and what do I do about it? .15. How does JPEG work? .16. What about arithmetic coding? .17. Could an FPU speed up JPEG? How about a DSP chip? .18. Isn't there an M-JPEG standard for motion pictures? Miscellaneous: .19. Where are FAQ lists archived? This article and its companion are posted every 2 weeks. If you can't find part 2, you can get it from the news.answers archive at rtfm.mit.edu (see ".19. Where are FAQ lists archived?"). Part 2 changes very frequently; get a new copy if the one you are reading is more than a couple months old. ------------------------------ Subject: .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 images of natural, real-world scenes. It works well on photographs, naturalistic artwork, and similar material; not so well on lettering, simple cartoons, or line drawings. JPEG handles only still images, but there is a related standard called MPEG for motion pictures. JPEG is "lossy," meaning that the decompressed image isn't quite the same as the one you started with. (There are lossless image compression algorithms, but JPEG achieves much greater compression than is possible with lossless methods.) JPEG is designed to exploit known limitations of the human eye, notably the fact that small color changes are perceived less accurately than small changes in brightness. 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 applications such as indexing image archives. 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. Another important aspect of JPEG is that decoders can trade off decoding speed against image quality, by using fast but inaccurate approximations to the required calculations. Some viewers obtain remarkable speedups in this way. ------------------------------ Subject: .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 win for transmitting files across networks and for archiving libraries of images. Being able to compress a 2 Mbyte full-color file down to, say, 100 Kbytes makes a big difference in disk space and transmission time! And JPEG can easily provide 20:1 compression of full-color data. If you are comparing GIF and JPEG, the size ratio is usually more like 4:1 (see ".4. How well does JPEG compress images?"). If your viewing software doesn't support JPEG directly, you'll have to convert JPEG to some other format to view the image. 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 such as GIF. Thus, using JPEG is essentially a time/space tradeoff: you give up some time in order to store or transmit an image more cheaply. But it's worth noting that when network or telephone transmission is involved, the time savings from transferring a shorter file can be greater than the time needed to decompress the file. The second fundamental advantage of JPEG is that it stores full color information: 24 bits/pixel (16 million colors). GIF, the other image format widely used on Usenet, can only store 8 bits/pixel (256 or fewer colors). GIF is reasonably well matched to inexpensive computer displays --- most run-of-the-mill PCs can't display more than 256 distinct colors at once. But full-color hardware is getting cheaper all the time, and JPEG images look *much* better than GIFs on such hardware. Within a couple of years, GIF will probably seem as obsolete as black-and-white MacPaint format does today. Furthermore, JPEG is far more useful than GIF for exchanging images among people with widely varying display hardware, because it avoids prejudging how many colors to use (see ".8. What is color quantization?"). Hence JPEG is considerably more appropriate than GIF for use as a Usenet posting standard. A lot of people are scared off by the term "lossy compression". But when it comes to representing real-world scenes, *no* digital image format can retain all the information that impinges on your eyeball. By comparison with the real-world scene, JPEG loses far less information than GIF. The real disadvantage of lossy compression is that if you repeatedly compress and decompress an image, you lose a little quality each time (see ".10. Does loss accumulate with repeated compression/decompression?"). This is a serious objection for some applications but matters not at all for many others. ------------------------------ Subject: .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. Generally speaking, JPEG is superior to GIF for storing full-color or gray-scale images of "realistic" scenes; that means scanned photographs and similar material. Any continuous variation in color, such as occurs in highlighted or shaded areas, will be represented more faithfully and in less space by JPEG than by GIF. GIF does significantly better on images with only a few distinct colors, such as line drawings and simple cartoons. Not only is GIF lossless for such images, but it often compresses them more than JPEG can. For example, large areas of pixels that are all *exactly* the same color are compressed very efficiently indeed by GIF. JPEG can't squeeze such data as much as GIF does without introducing visible defects. (One implication of this is that large single-color borders are quite cheap in GIF files, while they are best avoided in JPEG files.) Computer-drawn images, such as ray-traced scenes, usually fall between photographs 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). But icons that use only a few colors are handled better by GIF. JPEG 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. Edges this sharp are rare in scanned photographs, but are fairly common in GIF files: consider 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. (If you want to attach descriptive text to a JPEG image, put it in as a comment rather than trying to overlay it on the image. Most recent JPEG software can deal with textual comments in a JPEG file, although older viewers may just ignore the comments.) Plain black-and-white (two level) images should never be converted to JPEG; they violate all of the conditions given above. 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 a large library of GIF images, you may want to save space by converting the GIFs to JPEG. This is trickier than it may seem --- even when the GIFs contain photographic images, they are actually very poor source material for JPEG, because the images have been color-reduced. Non-photographic images should generally be left in GIF form. Good-quality photographic GIFs can often be converted with no visible quality loss, but only if you know what you are doing and you take the time to work on each image individually. Otherwise you're likely to lose a lot of image quality or waste a lot of disk space ... quite possibly both. Read sections 8 and 9 if you want to convert GIFs to JPEG. ------------------------------ Subject: .4. How well does JPEG compress images? Very well indeed, when working with its intended type of image (photographs and suchlike). For full-color images, the uncompressed data is normally 24 bits/pixel. The best known lossless compression methods can compress such data about 2:1 on average. JPEG can typically achieve 10:1 to 20:1 compression without visible loss, bringing the effective storage requirement down to 1 to 2 bits/pixel. 30:1 to 50:1 compression is possible with small to moderate defects, while for very-low-quality purposes such as previews or archive indexes, 100:1 compression is quite feasible. An image compressed 100:1 with JPEG takes up the same space as a full-color one-tenth-scale thumbnail image, yet it retains much more detail than such a thumbnail. For comparison, a GIF version of the same image would start out by sacrificing most of the color information to reduce the image to 256 colors (8 bits/pixel). This provides 3:1 compression. GIF has additional "LZW" compression built in, but LZW doesn't work very well on typical photographic data; at most you may get 5:1 compression overall, and it's not at all uncommon for LZW to be a net loss (i.e., less than 3:1 overall compression). LZW *does* work well on simpler images such as line drawings, which is why GIF handles that sort of image so well. When a JPEG file is made from full-color photographic data, using a quality setting just high enough to prevent visible loss, the JPEG will typically be a factor of four or five smaller than a GIF file made from the same data. Gray-scale images do not compress by such large factors. Because the human eye is much more sensitive to brightness variations than to hue variations, JPEG can compress hue data more heavily than brightness (gray-scale) data. A gray-scale JPEG file is generally only about 10%-25% smaller than a full-color JPEG file of similar visual quality. But the uncompressed gray-scale data is only 8 bits/pixel, or one-third the size of the color data, so the calculated compression ratio is much lower. The threshold of visible loss is often around 5:1 compression for gray-scale images. The exact threshold at which errors become visible depends on your viewing conditions. The smaller an individual pixel, the harder it is to see an error; so errors are more visible on a computer screen (at maybe 70 dots/inch) than on a high-quality color printout (300 or more dots/inch). Thus a higher-resolution image can tolerate more compression ... which is fortunate considering it's much bigger to start with. The numbers quoted above are typical for screen viewing. Also note that the threshold of visible error varies considerably across images. ------------------------------ Subject: .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. In fact, quality scales aren't even standardized across JPEG programs. The quality settings discussed in this article apply to the free IJG JPEG software (described in part 2), and to many programs based on it. Other JPEG implementations, notably Apple's and HSI's, use completely different quality scales; for instance, Apple's scale runs from 0-4, not 0-100. Some programs don't even provide a numeric scale, just "high"/"medium"/"low" style choices. (Fortunately, this doesn't prevent different implementations from exchanging compressed files.) In most cases the user's goal is to pick the lowest quality setting, or 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. For good-quality, full-color source images, the default IJG 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. If the image was less than perfect quality to begin with, you might be able to drop down to Q 50 without objectionable degradation. On the other hand, you might need to go to a *higher* quality setting to avoid further loss. This is often necessary if the image contains dithering or moire patterns (see ".9. What are some rules of thumb for converting GIF images to JPEG?"). Except for experimental purposes, never go above about Q 95; using Q 100 will produce a file two or three times as large as Q 95, but of hardly any better quality. If you see a file made with Q 100, it's a pretty sure sign that the maker didn't know what he/she was doing. 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". (It's worth mentioning that the current IJG software is not optimized for such low quality factors. Future versions may achieve better image quality for the same file size at low quality settings.) ------------------------------ Subject: .6. Where can I get JPEG software? See part 2 of this FAQ for recommendations about programs for particular systems. Part 2 also tells where to find free C source code for implementing JPEG, in case you want to write your own programs using JPEG. The comp.graphics FAQs and the alt.binaries.pictures FAQ are more general sources of information about graphics programs available on the Internet (see ".19. Where are FAQ lists archived?"). ------------------------------ Subject: .7. How do I view JPEG images posted on Usenet? Image files posted on the alt.binaries.pictures.* newsgroups are usually "uuencoded". Uuencoding converts binary image data into text that can safely be posted. Most posters also divide large posts into multiple parts, since some news software can't cope with big articles. Before your viewer will recognize the image, you must combine the parts into one file and run the text through a uudecode program. (This is all true for GIF as well as JPEG, by the way.) There are programs available to automate this process. For more info see the alt.binaries.pictures FAQ, which is available from rtfm.mit.edu:/pub/usenet/news.answers/pictures-faq/part.1-3., or on WWW at http://www.cis.ohio-state.edu/hypertext/faq/usenet/pictures-faq/top.html (see also ".19. Where are FAQ lists archived?"). ------------------------------ Subject: .8. What is 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 choose an appropriate set of representative colors and map the image into these colors. This process is called "color quantization". (This is something of a misnomer; "color selection" or "color reduction" would be a better term. But we're stuck with the standard usage.) 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 itself (except at the very lowest JPEG quality settings). Making a good color quantization method is a black art, and no single algorithm is best for all images. Since JPEG is a full-color format, displaying a color JPEG image on 8-bit-or-less hardware requires color quantization. The speed and image quality of a JPEG viewer running on such hardware are largely determined by its quantization algorithm. Depending on whether a quick-and-dirty or good-but-slow method is used, you'll see great variation in image quality among viewers on 8-bit displays, much more than occurs on 24-bit displays. On the other hand, a GIF image has already been quantized to 256 or fewer colors. (A GIF always has a specific number of colors in its palette, and the format doesn't allow more than 256 palette entries.) GIF has the advantage that the image maker precomputes the color quantization, so viewers don't have to; this is one of the things that make GIF viewers faster than JPEG viewers. But this is also the *disadvantage* of GIF: you're stuck with the image maker's quantization. If the maker quantized to a different number of colors than what you can display, you'll either waste display capability or else have to requantize to reduce the number of colors (which usually results in much poorer image quality than quantizing once from a full-color image). Furthermore, if the maker didn't use a high-quality color quantization algorithm, you're out of luck --- the image is ruined. For this reason, JPEG promises significantly better image quality than GIF for all users whose machines don't match the image maker's display hardware. JPEG's full color image can be quantized to precisely match the viewer's display hardware. Furthermore, you will be able to take advantage of future improvements in quantization algorithms, 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. A 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. In short, JPEG is an all-around better choice than GIF for representing photographic images in a machine-independent fashion. It's sometimes thought that a JPEG converted from a GIF shouldn't require color quantization. This is false: even when 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 usually come out with slightly different colors. Considering the whole image, 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 same effect makes it nearly meaningless to talk about the number of colors used by a JPEG image. Even if you tried 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, but number of colors just isn't a useful concept for JPEG, any more than it is for a real photograph. ------------------------------ Subject: .9. What are some rules of thumb for converting GIF images to JPEG? Converting GIF files to JPEG is a tricky business --- you are piling one set of limitations atop a quite different set, and the results can be awful. Certainly a JPEG made from a GIF will never be as good as a JPEG made from true 24-bit color data. But if what you've got is GIFs, and you need to save space, here are some hints for getting the best results. With care and a clean source image, it's often possible to make a JPEG of quality equivalent to the GIF. This does not mean that the JPEG looks pixel-for-pixel identical to the GIF --- it won't. Especially not on an 8-bit display, because the color quantization process used to display the JPEG probably won't quite match the quantization process used to make the GIF from the original data (see ".8. What is color quantization?"). But remember that the GIF itself is not all that faithful to the full-color original, if you look at individual pixels. Looking at the overall image, a converted JPEG can look as good as its GIF source. Some people claim that on 24-bit displays, a carefully converted JPEG can actually look better than the GIF source, because dither patterns have been eliminated. (More about dithering in a moment.) On the other hand, JPEG conversion absolutely *will* degrade an unsuitable image or one that is converted carelessly. If you are not willing to take the amount of trouble suggested below, you're much better off leaving your GIF images alone. Simply cranking the JPEG quality setting up to a very high value wastes space (which defeats the whole point of the exercise, no?) and some images will be degraded anyway. The first rule is never to convert an image that's not appropriate for JPEG (see ".3. When should I use JPEG, and when should I stick with GIF?"). Large, high-visual-quality photographic images are usually the best source material. And they take up lots of space in GIF form, so they offer significant potential space savings. (A good rule of thumb is not to bother converting any GIF that's much under 100 Kbytes; the potential savings isn't worth the hassle.) The second rule is to look at each JPEG, to make sure you are happy with it, before throwing away the corresponding GIF. This will give you a chance to re-do the conversion with a higher quality setting if necessary. Also compare the file sizes --- if the image isn't suitable JPEG material, a JPEG file of reasonable quality may come out *larger* than the GIF. The third rule is to get rid of the border. Many people have developed an odd habit of putting a large single-color border around a GIF image. While useless, this is nearly free in terms of storage cost in GIF files. It is NOT free in JPEG files, either in storage space or in decoding time; and the sharp border boundary can create visible artifacts ("ghost" edges). Furthermore, when viewing a bordered JPEG on an 8-bit display, the quantizer will think the border color is important because there's so much of it, and hence will waste color palette entries on the border, thus actually reducing the displayed quality of the main part of the image! So do yourself a favor and crop off any border before JPEGing. The fourth rule is to know where the image came from. Repeated GIF<=>JPEG conversions are guaranteed to turn an image into mush, because you pay a quality-loss price on each round trip. Don't reconvert images that have been converted before. Gray-scale images usually convert without much problem. When using cjpeg, be sure to use the -gray switch. (Otherwise, cjpeg treats a GIF as color data; this works, but it wastes space and time if the image is really only gray-scale.) Quality settings around the default (75) are usually fine. Color images are much trickier. Color GIFs of photographic images 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 find 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 need 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. Proper use of smoothing will both reduce the size of the compressed file and give you a better-looking output image than you'd get without smoothing. With the IJG JPEG software (cjpeg or derived programs), 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. GIFs with 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, but appropriate use of such tools is beyond the scope of this FAQ. Quality settings around 85 (a bit higher than default) usually work well when converting color GIFs, assuming that you've picked a good smoothing factor. You may need still higher quality settings if you can't hide the dithering pattern with a reasonable smoothing factor. Really badly dithered GIFs are best left as GIFs. Don't expect JPEG files converted from GIFs to be as small as those created directly from full-color originals. The dithering noise wastes space, but you won't be able to smooth away all the noise without blurring the image. Typically, a good-quality converted JPEG will be one-half to one-third the size of the GIF file, not one-fourth as suggested in section 4. If the JPEG comes out much more than half the size of the GIF, this is a good sign that the image shouldn't be converted at all. The upshot of all this is that "cjpeg -quality 85 -smooth 10" is probably a good starting point for converting color GIFs. But if you care about the image, you'll want to check the results and maybe try a few other settings. Blindly converting a large GIF library at this or any other setting is a recipe for disaster. ------------------------------ Subject: .10. 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. Hence it's important to minimize the number of generations of JPEG compression between initial and final versions of an image. It turns out that if you decompress and recompress an image at the same quality setting first used, little or no further degradation occurs. This means that you can make local modifications to a JPEG image without material degradation of other areas of the image. The areas you change will degrade, however. Counterintuitively, this works better the lower the quality setting. But you must use *exactly* the same setting, or all bets are off. Also, the decompressed image must be saved in a full-color format; if you do JPEG=>GIF=>JPEG, the color quantization step loses lots of information. Unfortunately, cropping doesn't count as a local change! JPEG processes the image in small blocks, and cropping usually moves the block boundaries, so that the image looks completely different to JPEG. You can take advantage of the low-degradation behavior if you are careful to crop the top and left margins only by a multiple of the block size (typically 16 pixels), so that the remaining blocks start in the same places. 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 24-bit 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 :-). ------------------------------ Subject: .11. What is progressive JPEG? A simple or "baseline" JPEG file is stored as one top-to-bottom scan of the image. Progressive JPEG allows the image to be stored as a series of scans of increasing quality. The first scan shows the image at the equivalent of a very low quality setting, and therefore it takes very little space. Successive scans improve the quality. Each scan adds to the data already provided, so that the total storage requirement is about the same as for a baseline JPEG image of the same quality as the final scan. (Basically, progressive JPEG is just a rearrangement of the same data into a more complicated order.) The advantage of progressive JPEG is that if an image is being viewed on-the-fly as it is transmitted, the viewer sees an approximation to the whole image very quickly, with gradual improvement of quality as one waits longer; this is much nicer than a slow top-to-bottom display of the image. The disadvantage is that each scan takes about the same amount of computation to display as a whole baseline JPEG file would. So progressive JPEG only makes sense if one has a decoder that's much faster than the communication link. Up until recently, there weren't many applications in which progressive JPEG looked attractive, so hardly any implementations of it are available. But with the popularity of WWW browsers running over rather slow modem links, and with the ever-increasing horsepower of personal computers, progressive JPEG is starting to look like a win for WWW. I expect that progressive JPEG capability will start to appear in WWW browsers during 1995. ------------------------------ Subject: .12. Isn't there a lossless JPEG? There's a great deal of confusion on this subject. The JPEG standard does include a truly lossless compression algorithm, i.e., one that guarantees its decompressed 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. Lossless JPEG typically compresses full-color data by around 2:1. Lossless JPEG works well only on continuous-tone images; it does not provide useful compression of palette-color images or low-bit-depth images. (The JBIG standard is considered superior to lossless JPEG for images of less than about 6 bits/sample.) At present, very few implementations of lossless JPEG exist. The PVRG code mentioned in part 2 handles lossless JPEG. Another free implementation is available from ftp.cs.cornell.edu:/pub/multimed/ljpg.tar.Z; this is a smaller program that handles *only* lossless JPEG. Cranking a regular JPEG implementation up to its maximum quality setting