I believe you are not correct, here. I have read in several accounts, both
from people who have tried this experimentally and from people who
understand the theory of JPEG compression far better than I do, that opening
a JPEG, and then resaving it without making any changes to the file, at the
same compression ratio as it was originally saved, does not further degrade
the image. The degradation happens when the file is first saved as a JPEG,
and further degradation is likely to happen if you make changes to the
contents of the file and then resave it, but it won't degrade if you merely
open the file, make no changes, resave it, open it again, and resave it
making no changes, no matter how many times you repeat this process. (Though
why you would want to open a JPEG and then resave it unchanged, at the same
compression ratio, rather than simply closing it, is an open question.)
- David
-----Original Message-----
From: filmscanners_owner@halftone.co.uk
[mailto:filmscanners_owner@halftone.co.uk]On Behalf Of Robert Meier
Sent: Friday, February 08, 2002 9:31 PM
To: david_bookbinder@sprynet.com
Subject: [filmscanners] Re: JPEG Lossless mirror?
--- "Pat Cullinan, jr." <pcullinan@mindspring.com> wrote:
> I had been a believer in the proposition that multiple jpeg saves
> would
> degrade an image, but after reading a notice to the contrary in one
> of the
> trade mags, I did my own trials and now I save and resave jpegs which
> aren't even maximum quality without any qualms.
The trade magazine is wrong at least for the following common scenario.
If you save a picture as jpeg in PS, close the image, reload the image
and save the image again in jpeg you will lose data. The difference
noise like and very small. For a normal picture you won't see any
difference. Also it might be the additional loss gets smaller and
smaller with many additional savings (without editing) upto a point
where there is no change anymore. I have no mathematical proof for
that, though.
Now if you start with an image in PS, edit it, save it, edit it, save
it, etc. you are not losing any data. The reason is that PS only writes
the compressed image to a file but keeps the uncompressed image in
memory. So it does not compress it and then reload the compressed image
back into memory. In the later case you would lose data with each save
and it would be awfully slow.
One thing I wonder is if it is possible to do a lossless flipping of an
image that has not a multiple of 8 pixels in the direction you flip it.
Does anybody know about that?
Robert
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