JPEG degradation

Bert Hyman wrote:
In news wrote:

On Fri, 26 Jan 2007 00:43:26 +0000, Bert Hyman wrote:

In "TSKO"
wrote:

I was reading a photography book by Bryan Peterson and he says
something in it I never knew. He said that opening JPEG's over and
over........with time they will degrade to the point to where they
dont open anymore.

That's only possible if you re-save the file after changing it
somehow.
Matters not an iota if you change it or not. Saving it will always
cause additional degredation.

Why? If the app re-writes the same series of bytes back onto disk that
it read, there will be no change in the file or the resulting image.

I haven't tried to conduct the test, and the meta-data in the file might
make the test hard to run with stock software.

However, when you open a jpeg file, the file is translated into a full
bitmap as it's read into memory. If you now save the file, it has to be
re-compressed into a jpeg.

The question is, if you compress a file to jpeg (losing some data),
expand that jpeg to a full bitmap, and then compress that bitmap to jpeg
using exactly the same quality settings -- will the result be identical
to the first jpeg, or not? (I'm not talking about whether it looks
about the same to a quick glance on the screen; I'm talking about
whether the actual image bitmap represented is the same, or not). I
don't know the answer to this, and when I start thinking about how to
conduct the tests, I get either a very quick and simple answer of "yes",
or else the testing requirements get more and more complex very quickly.
So I haven't done it yet.

Just *copying* a jpeg file cannot cause further degradation; reading the
bytes from disk and writing them to another area of disk makes an exact
copy. (Well, except that there could be an undetected read error, or an
undetected write error.)

In article , ray wrote:
On Fri, 26 Jan 2007 00:43:26 +0000, Bert Hyman wrote:

In "TSKO"
wrote:

I was reading a photography book by Bryan Peterson and he says
something in it I never knew. He said that opening JPEG's over and
over........with time they will degrade to the point to where they
dont open anymore.


That's only possible if you re-save the file after changing it somehow.

Matters not an iota if you change it or not. Saving it will always cause
additional degredation.


BULLLL **** , that would only happen if you have your software set to change
the compression ratio or from standard to progressive jpeg or the like. I use
Thumbsplus, and have it set to not change anything from the original settings
of the source jpeg when i crop files.

On 26 Jan 2007 02:50:33 GMT, Bert Hyman wrote:

Matters not an iota if you change it or not. Saving it will always
cause additional degredation.

Why? If the app re-writes the same series of bytes back onto disk that
it read, there will be no change in the file or the resulting image.

Yes, that's possible, but . . . it's very unlikely that any app.
will be able to write the same bytes back to the disk because to do
that it would have to keep two copies of the image in memory, the
original as well as the expanded image, and I doubt that any apps.
do that. Checking one JPG picture produced by my camera shows that
after loading the 1.81MB file into memory it was expanded to
11.08MB. Using Irfanview to immediately save the jpg after opening
it, the size on disk increased to 2.19MB. All "Save" options were
chosen to try to prevent changes, such as not stripping EXIF data,
IPTC data, nor JPG comments. Choosing "Save Quality" was the
problem, as I had no way to determine the quality used by the
camera. Even if I had known what it was, the JPG file had to have
undergone expansion and compression, and from other recent threads
it should be apparent that even if the photo app. used the same
compression algorithm as the camera, it would be "lossy" and the
file would have changed.

On Thu, 25 Jan 2007 22:12:06 -0600, David Dyer-Bennet wrote:

Bert Hyman wrote:
In news wrote:

On Fri, 26 Jan 2007 00:43:26 +0000, Bert Hyman wrote:

In "TSKO"
wrote:

I was reading a photography book by Bryan Peterson and he says
something in it I never knew. He said that opening JPEG's over and
over........with time they will degrade to the point to where they
dont open anymore.

That's only possible if you re-save the file after changing it
somehow.
Matters not an iota if you change it or not. Saving it will always
cause additional degredation.

Why? If the app re-writes the same series of bytes back onto disk that
it read, there will be no change in the file or the resulting image.

I haven't tried to conduct the test, and the meta-data in the file might
make the test hard to run with stock software.

However, when you open a jpeg file, the file is translated into a full
bitmap as it's read into memory. If you now save the file, it has to be
re-compressed into a jpeg.

The question is, if you compress a file to jpeg (losing some data),
expand that jpeg to a full bitmap, and then compress that bitmap to jpeg
using exactly the same quality settings -- will the result be identical
to the first jpeg, or not? (I'm not talking about whether it looks
about the same to a quick glance on the screen; I'm talking about
whether the actual image bitmap represented is the same, or not). I
don't know the answer to this, and when I start thinking about how to
conduct the tests, I get either a very quick and simple answer of "yes",
or else the testing requirements get more and more complex very quickly.
So I haven't done it yet.

The answer is - no it will not be the same. In order for it to be the
same, you'd have to start with the same bitmap - but you're not, because
the jpeg was altered from the original image - when you read in the jpeg
you can't get what you had before it was saved.



Just *copying* a jpeg file cannot cause further degradation; reading the
bytes from disk and writing them to another area of disk makes an exact
copy. (Well, except that there could be an undetected read error, or an
undetected write error.)

On Fri, 26 Jan 2007 02:50:33 +0000, Bert Hyman wrote:

In news wrote:

On Fri, 26 Jan 2007 00:43:26 +0000, Bert Hyman wrote:

In "TSKO"
wrote:

I was reading a photography book by Bryan Peterson and he says
something in it I never knew. He said that opening JPEG's over and
over........with time they will degrade to the point to where they
dont open anymore.


That's only possible if you re-save the file after changing it
somehow.

Matters not an iota if you change it or not. Saving it will always
cause additional degredation.

Why? If the app re-writes the same series of bytes back onto disk that
it read, there will be no change in the file or the resulting image.

Because it won't be rewriting the same series of bytes. It will perform a
'fresh' compression on the image in memory.

I've just done a quick test in Photoshop.
Opened an NEF and saved to jpg. File size 3624KB
Open that jpg and "save as" with compression set to 12. File size
3669KB
Open that jpg and "save as" with compression set to 12. File size
3679KB
Open that jpg and "save as" with compression set to 12. File size
3685KB
Open that jpg and "save as" with compression set to 12. File size
3690KB

No editing to the files at any stage.

--
Joan
http://www.flickr.com/photos/joan-in-manly

"ray" wrote in message
news : On Fri, 26 Jan 2007 02:50:33 +0000, Bert Hyman wrote:
:
: Because it won't be rewriting the same series of bytes. It will
perform a
: 'fresh' compression on the image in memory.
:

On Fri, 26 Jan 2007 15:31:34 +1100, Joan wrote:

I've just done a quick test in Photoshop.
Opened an NEF and saved to jpg. File size 3624KB
Open that jpg and "save as" with compression set to 12. File size
3669KB
Open that jpg and "save as" with compression set to 12. File size
3679KB
Open that jpg and "save as" with compression set to 12. File size
3685KB
Open that jpg and "save as" with compression set to 12. File size
3690KB

No editing to the files at any stage.

Good definitive test. Unless . . . Photoshop has decided to
append an encrypted copy of the CPU's serial number every time it
saves. g

"Paranoia strikes deep. Into your life it will creep." -- For What
It's Worth / BS

TSKO wrote:
I was reading a photography book by Bryan Peterson and he says something in
it I never knew. He said that opening JPEG's over and over........with time
they will degrade to the point to where they dont open anymore.

My question is, if on a screen saver you are doing a slide show with all the
pictures that you took, does this count as 'opening' and 'closing' of a JPEG
and will those degrade over time?



Opening and closing does not affect them, so long
as you don't save a new version over the original.
So if you do edit one, best to save it with a
new name and keep your original intact.

Phil

Paul D. Sullivan wrote:
Matters not an iota if you change it or not. Saving it will
always cause additional degredation.
Why? If the app re-writes the same series of bytes back onto
disk that it read, there will be no change in the file or the
resulting image.

I think in almost all cases, it does cause degredation. At least
it seems that way to me based on my own experience.

I'm getting tired of this argument.

So, since I couldn't find a theoretical or rhetorical way to really
settle it, I finally sat down and ran the experiment. And documented it.

In the case I tested, resaving a JPEG caused image change, even though
no editing whatsoever was done.

I made a short article, exhibiting the two versions of the file and a
visual map of the differences between them (very small, but definitely
present). The article is at
http://dd-b.net/dd-b/Photography/Articles/resaving-jpeg/.

To be truly pedantic, of course, this only demonstrates that in one
particular case an image is changed by resaving a JPEG. However, there
should be detailed enough instructions there for anybody to reproduce
the experiment on various other images.

I haven't tried it with a pure white image, for example, but I think
it's quite possible that resaving that wouldn't result in degradation.

I just repeated the test but with the compression set to 6 and in each
case the file was 729KB.

--
Joan
http://www.flickr.com/photos/joan-in-manly

"Joan" wrote in message
...
: I've just done a quick test in Photoshop.
: Opened an NEF and saved to jpg. File size 3624KB
: Open that jpg and "save as" with compression set to 12. File size
: 3669KB
: Open that jpg and "save as" with compression set to 12. File size
: 3679KB
: Open that jpg and "save as" with compression set to 12. File size
: 3685KB
: Open that jpg and "save as" with compression set to 12. File size
: 3690KB
:
: No editing to the files at any stage.
:
: --
: Joan
: http://www.flickr.com/photos/joan-in-manly
: