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Image Compression Information

This document is from November of 1995 (!), so it is presented mostly for archive purposes.


Table of contents



WHY THIS PAGE ?

SITO is a place for communication of ideas and images. As with any form of communication, it is best to be concise, to communicate your ideas efficiently. With respects to images on SITO, effeciency is a matter of choosing an appropriate physical dimension for an image, and choosing the compression method or format that yeilds the best results.

SITO is intended to be a forum and a meeting place, not merely a massive archive of images. The exchange of images should be as seemless as possible and tax the physical resources (ie. disk space and bandwidth) as little as possible.

This is an active place for you to expose yourself, to see other people's work, to critique and be critiqued. Image files tend to get quite large, and many times this can impair the exchange of ideas. It is difficult for a person connecting via modem to browse the images as one would in a gallery. Even with an ethernet connection " browsing " still may not be as fluid as one would like. The larger the size of an image, the longer it takes to download, which means a person gets to see a relatively small number of images if they are too large.

You may submit images in the size and format of your choice. This page is simply to encourage artists to submit their work in an efficient format. SITO accepts submissions in both GIF and JPEG format. These two formats are explained below, and links are provided for further investigation. Certain types of images often look better and save more space when compressed in the appropriate format.

IMAGE DIMENSIONS

Does your image really need to stretch indefinitely off the screen, only to be rescaled for viewing? No. . . unless you are distributing your images for printing purposes, or if you feel that the fine details in the image really merit distribution of a large file.

If you do wish to make your images available for people to print in publications. . . Consider providing a lower resolution image for distribution, and include a note in the credits saying that the high-res version is available. Include similar comments in your SITO artist info file. You can then send the larger images in response to a direct request. This also helps you keep track of how your images are being put to use.

In general, keep image dimensions less than 9.5" tall or 12" wide, at a resolution of 72dpi. These dimensions are appropriate for a relatively common large-screen display, and still may be excessive.

The basic question you need to ask yourself: "How big does the image really need to be to communicate effectively?" . . . Make the image as small as you can without feeling uncomfortable.

IMAGE SIZES

Generally, 300k is about the maximum for a single image posted to SITO. Something in the range of 80-150k, or smaller, is preferred. Some images simply require more detail than others to be effective. And some complex images just don't compress well. These images should be the exceptions, the ones that might reach the 300k mark. Currently, there a few 500k+ images floating around; these will be scaled down. Do yourself a favor... keep the images small and more people will be tempted to download them.


EXPLANATION OF JPEG, & JPEG COMPARED TO GIF

The following four sections are quoted directly from the
JPEG FAQ, by Tom Lane, of the Independent JPEG Group (email: jpeg-info@uunet.uu.net if you have further questions.) The complete FAQ is 3-4 times bigger and includes information on much of the software available. Also check out the "alt.binaries.pictures.*" FAQ for a bigger list of image viewing and manipulation programs.

I. 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 like 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. Until recently, most publicly available JPEG code has adopted a best-possible-quality philosophy, but we are now starting to see the appearance of viewers that give up some image quality in order to obtain significant speedups.

II. 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 in section 4.)

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 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.

It's worth noting that when network or phone transmission is involved, the time savings from transferring a shorter file can be greater than the extra 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, 8-bit GIF will seem 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 display hardware. 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 technical meaning of "lossy" has nothing to do with this, though; it refers to loss of information over repeated compression cycles, a problem that you may or may not care about. (If you do, see section 9.)

III. When should I use JPEG or 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 (ray-traced scenes, for instance) 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).

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: 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.

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 7 and 8 if you want to convert GIFs to JPEG.

IV. How well does JPEG compress images ?

Very well, 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 (less than 3:1 overall compression). When a JPEG file is made from full-color 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.


COMPRESSION & FORMAT INFORMATION SOURCES:

Check out the
usenet directory for a full list of FAQs.

PLEASE! If you know of some other sources of information concerning either file format or compression, tell me! It would be nice to link FAQs concerning MPEG, movie/animation formats in general, and 3D-file formats. I personally can locate some of these FAQs, but I do not know of a stable place for a WWW link.


Chris Auxier, cba2@cornell.edu
page last updated 02-19-95
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