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RE: filmscanners: OT now? - JPEG Loss - File format

At 07:00 PM 30/03/01 -0500, you wrote:
>I was the first to answer this question and now I question myself for 
>letting this go on this long.

And I too apologise for dragging it on, but hopefully it will end with this
one.  The only reason I quote the following is that I still see some folk
under a false impression (even if they were only slightly wrong!).

The quote below is taken from the JPEG FAQ, from the JPEG.ORG site..  I
have edited it for brevity, but I don't think I have changed the context in
any way..  All capitalisation is as per the original, but all asterisks
except the last two are mine!

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

There are a few specialized operations that can be done on a JPEG file
without decompressing it, and thus without incurring the generational loss
that you'd normally get from loading and re-saving the image in a regular
image editor.  In particular it is possible to do 90-degree rotations and
flips losslessly...<SNIP>... *But you do need special software; rotating
the image in a regular image editor won't be lossless.*

It turns out that if you decompress and recompress an image at the same
quality setting first used, *relatively* little 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 still degrade, however.)  Counterintuitively, this works better the
lower the quality setting.  But you must use *exactly* the same setting,
or all bets are off.


I promise to shut up about jpg's now..



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