A simple or "baseline" JPEG file is stored as one top-to-bottom scan of the
image.  Progressive JPEG divides the file into a series of scans.  The first
scan shows the image at the equivalent of a very low quality setting, and
therefore it takes very little space.  Following scans gradually improve the
quality.  Each scan adds to the data already provided, so that the total
storage requirement is roughly 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, one can see 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 fast compared to the communication link.
(If the data arrives quickly, a progressive-JPEG decoder can adapt by
skipping some display passes.  Hence, those of you fortunate enough to have
T1 or faster net links may not see any difference between progressive and
regular JPEG; but on a modem-speed link, progressive JPEG is great.)

Up until recently, there weren't many applications in which progressive JPEG
looked attractive, so it hasn't been widely implemented.  But with the
popularity of World Wide Web browsers running over slow modem links, and
with the ever-increasing horsepower of personal computers, progressive JPEG
has become a win for WWW use.  IJG's free JPEG software (see part 2, item
15) now supports progressive JPEG, and the capability is spreading fast in
WWW browsers and other programs.

Except for the ability to provide progressive display, progressive JPEG and
baseline JPEG are basically identical, and they work well on the same kinds
of images.  It is possible to convert between baseline and progressive
representations of an image without any quality loss.  (But specialized
software is needed to do this; conversion by decompressing and recompressing
is *not* lossless, due to roundoff errors.)

A progressive JPEG file is not readable at all by a baseline-only JPEG
decoder, so existing software will have to be upgraded before progressive
JPEG can be used widely.  See item 16 in part 2 for the latest news about
which programs support it.