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RE: filmscanners: File format

While I do not think that this effort was specifically directed at me, the
fact that my comments are included as part of the message gives me the right
to make a comment or two on my behalf, which in no way is related to your
message. :-)

As Hugo made me realize, I was using the terms "saved" and "resaved" when I
should have used "opened" and "reopened" in the remarks that you have cited.
I accept my error and apologize in hopes that it will help clear up the
apparent but not necessarily real differences that appear to have become the
center of debate and caused everyone to go to all this experimental trouble
testing out their positions. :-)

-----Original Message-----
From: owner-filmscanners@halftone.co.uk
[mailto:owner-filmscanners@halftone.co.uk]On Behalf Of Arthur Entlich
Sent: Tuesday, April 03, 2001 3:16 AM
To: filmscanners@halftone.co.uk
Subject: Re: filmscanners: File format

I decided to put my money where my mouth is ;-) and I just took an
image, which mainly consisted of some color areas and some typeface, and
ran it through Photoshop's jpeging.  With one set, I used level 4 and
the other set I applied level 2.  In each case, I did a series of ten
jpegs applications, each time starting with opening the previously
created file and applying the same jpeg level, and then saving that
file, then using the saved file for the next application.  Below are the
file sizes for each application

    LEVEL 4    LEVEL 2
1  170,700    122,197
2  168,292    120,947
3  167,927    120,761
4  167,865    120,702
5  167,862    120,692
6  167,802    120,698
7  167,801    120,698
8  167,787    120,698
9  167,785    120,698
10 167,789    120,698

So, what does this show?  Well, first of all, obviously something is
changing in these files, and I don't believe it is just Photoshop
tagging or thumbnails (which I admit, I should have probably turned off
before doing this, oh well).  BUT, more importantly, there appears to be
a point where JPEG reaches a saturation point in terms of its ability to
use the algorithms to compress.  In fact, it begins to very slightly
enlarge the files after that point.

Now, I'll admit, being Photoshop, perhaps it is recording something
beyond image info within the files, like history or who knows, but the
mechanism seems to be that the more drastic the jpeg compression, the
sooner it reaches maximum compressibility.  Also, I can't say that just
because the file sizes become stagnant that the pixels are identical,

Anyway, that's as far as I'm going with this.  I suspect that if I had
used a smaller jpeg compression, the file would have taken more
applications to be reduced to the minimum size.


Laurie Solomon wrote:

>> Each time there would be some generational loss.
> Not necessarily true. If you open and close ( or resave) the compressed
> 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.


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