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RE: filmscanners: JPEG Loss - File format



>So the image DOES degrade with each iteration, whether or not it is
>changed.

Aside from the fact that this does not sound right to me nor has it been my
experience or in the things that I have read on the subject, such
degradation that you suggest in the case in question - if it takes place -
is not all that significant and would not become so until a very large
number of iterations had been gone through.  What one may see with greater
significance and frequency, which I did not explicitly refer to in my
previous comments, is the possible production of visible artifacts of the
compression recompression in terms of color artifacts and the like more than
actual additional loss or distortion of the file data.

-----Original Message-----
From: owner-filmscanners@halftone.co.uk
[mailto:owner-filmscanners@halftone.co.uk]On Behalf Of Mark Thomas
Sent: Friday, March 30, 2001 2:23 AM
To: filmscanners@halftone.co.uk
Subject: RE: filmscanners: JPEG Loss - File format


Maybe it's because it is Friday pm here:), but the explanation below
doesn't sound right to me.  The losses to the image occur when you *save*
it..  (So sure, if you just keep *re-saving* that same on-screen file, the
image won't degrade further - but why would you do that?)

But as soon as you close the image, and then re-open it, you will now be
looking at the 'lesser' image.  If you save it again, the JPEG algorithm
will try to further compress it (with little success as it has already
degraded the image to better 'suit' the compression).  But it *will* still
do it, and the image and filesize will change again (just checked it, and
it seems to go down in size a couple of times, but then starts to go back
up!).  So the image DOES degrade with each iteration, whether or not it is
changed.  Happy to post samples if anyone's interested..

The point is that 100% JPEG compression, while admirable, is not lossless,
and therefore can't be used if you wish to retain all original image detail.

Mark T.

At 10:57 PM 29/03/01 -0600, Laurie wrote:
>Not necessarily true. If you open and close ( or resave) the compressed
>file without changing the compression from one quality level to another in
>the case of .jog or without resampling the image prior to closing or
>resaving the file, there will be no more degradation than opening and
>closing or resaving a raw uncompressed file.
>
>When you open a compressed file you have uncompressed it, so resaving it
>with the same compression as before or without engaging in any resampling
>prior to resaving the file  should not result in any additional losses in
>data or quality.
>
>-----Original Message-----
>From: Larry Berman
>
>What would be the point of storing and reopening and saving the same image
>in a compressed format repeatedly. Each time there would be some
>generational loss. Store in an uncompressed native format to your graphics
>program. If you open a jpeg in Photoshop it automatically takes on the
>characteristics of a PSD. That's why you should save it as a PSD prior to
>working on it. Then use Photoshop's "Save for the Web" to create your
>compressed jpeg.
>
>Larry
>
>
> >Did any one knows whether is there any informations/details loses when
> >store in compressed JPG format in maximum quality 10 and while you keep
> >opening and saving the same file many times?
> >And how about the raw file TIFF?




 




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