>We are creating a digital collection by scanning transparencies onto CD. They
>are jpg's that become part of an educational collection accessed on-line from
>an Inmagic database.  Is there any loss of quality to the database copy when
>they are viewed on-line? If there is, would an archival master saved to CD be
>adequate back-up?

Well, the jpeg format is a 'lossy' one, meaning that it loses data
when initially created. The way it creates smaller files is by
averaging out little areas of pixels. However, there is a sliding
scale of quality that you can use when creating the initial jpegs -
it's a compromise on compression size versus quality. The more you
compress the file, the greater the loss in quality. If you're really
curious about the jpeg algorithm, fire up your favorite image editing
application (photoshop, paintshop, gimp, etc) and really zoom in on
the image. You'll start to see little chunks of blockiness which is
the joeg algorith at work.

Now, going back to your original question. It sounds like you're
asking if the jpeg will continue to lose quality through repeated
online viewings, and the answer is no. When someone looks at the jpeg
through a web browser, the web browser views an exact copy of the
jpeg at the quality at which it was originally saved - there's no
additional compression that would further degrade the quality of the
image. And, it shouldn't alter the jpeg image already in the
database; web browsers are read-only devices for what we're
describing.

It sounds like what you're doing is probably fine. However, if you're
concerned about losing some quality by creating jpegs in the first
place, a reasonable alternative approach is to save two versions of
the file. The first, a lower resolution version, would be the jpeg
that you're already creating, or maybe at an even higher level of
compression. The second version should be saved in tiff format with
compression. The tiff file format doesn't discard any data when it
compresses the file. It works through looking at the bits in the file
and looking for repeating 1s and 0s and mathematically crunches the
image. No pixels are touched in the process. Make both versions
available, with the realization that the higher resolution tiff is
meant for download rather than online viewing.

Of course, making higher resolution images available may not be
something that's desirable to you because it enables better
reproduction, but the discussion about copyright and digital
watermarking should be saved for another day.

Good luck. Hopefully this answers your question.

-bw.
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Bruce Wyman                                e: <[log in to unmask]>
Manager of Creative Development            v:  617.491.3184
Nearlife                                   f:  617.354.4191
147 Sherman Street, Cambridge, MA  02140   w: <http://www.nearlife.com/>

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