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[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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