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[filmscanners] Re: JPEG Lossless mirror?





Robert Meier wrote:

> --- "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.

That's the conventional wisdom, but I just can't a difference.  I opened the
image, edited it (a little), saved it, closed it, opened it, etc...twenty 
times, and
I couldn't see a diference.

> 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.

That's correct.

>
> 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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