Conventional JPEG files store their data in left-to-right, top-to-down order. So when one third of a JPEG-compressed image of a rose has loaded, you see the top third of the image:
Progressive-display JPEG files store data in multiple passes. Early passes contain coarse-grain image information, and later passes contain finer detail. This means that a complete, but low-quality image can be displayed very soon. For example, when one third of a progressive-display version of the rose JPEG has loaded, you see:
(This is not a progressive-display JPEG; it is only a demonstration.)
As more data arrives, the image quality increases, as though coming into finer focus. When all of the image data has arrived, the entire image is visible at exactly the same quality as the sequential-display version:
(Look here for an actual progressive-display version of this image.)
Progressive-display JPEG files are usually slightly smaller than their sequential-display counterparts. However, many JPEG-viewing programs don't know how to handle progressive-display JPEG files yet, and will complain that the files are damaged. If you are going to put progressive-display JPEGs in WWW pages, you may wish to warn readers of this incompatibility.
You can also use it to convert progressive-display JPEGs into sequential mode, so that older browsers and viewers can display them. Double-click the Progressify icon and then click the sequential-display radio button. Now drag JPEG files or folders onto the Progressify icon.
In general, Progressify will run faster if you give it more memory. However, it should survive in as little as 500K, provided you have free disk space.
Progressify uses a free JPEG library written by the Independent JPEG Group; it's essentially a Macintosh port of the jpegtran program included in versions 6 and later of that library. (Note that Progressify copies embedded text comments; jpegtran does not.) Here, let me say it again: "this software is based in part on the work of the Independent JPEG Group. Permission for use of this software is granted only if the user accepts full responsibility for any undesirable consequences; the authors accept NO LIABILITY for damages of any kind."
Progressify is also based on DropShell 2.0 by Leonard Rosenthol, Marshall Clow and Stephan Somogyi.