jpg files

wrote in message
...
humiliatedgrapes wrote:
: I know that jpg loses detail every time you save it, but does it lose
detail
: every time you access a jpg file.

: Thank you,

No, it loses detail only when you resave it.

Ray

If the resaved jpg file is saved using the same quality settings, and the
resulting file size is the same as the original file, does this mean that
there is still a quality loss?

If a jpg does lose detail everytime it's saved, this means that there should
be a reduction in file size, which also means that at some point in the
future i.e. if a jpg is resaved enough times, it will eventually reach a
point where all detail is lost and the file ends up being 1 byte in size.
This doesn't make a whole lot of sense to me.

I would think there has to be a point where the compression scheme analyzes
an image and determines it cannot reduce the quality/size of the image any
further, dependent upon the actualy jpeg quality setting used.

Terence




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But it looses detail, if your harddisk breaks. - So make a backup. ;-)

On 2004-10-11 21:06, wrote:
humiliatedgrapes wrote:
: I know that jpg loses detail every time you save it, but does it lose detail
: every time you access a jpg file.

: Thank you,

No, it loses detail only when you resave it.

Ray

On Mon, 11 Oct 2004 15:17:22 -0400, Terence Gui wrote:

wrote in message
...
humiliatedgrapes wrote:
: I know that jpg loses detail every time you save it, but does it lose
detail
: every time you access a jpg file.

: Thank you,

No, it loses detail only when you resave it.

Ray

If the resaved jpg file is saved using the same quality settings, and the
resulting file size is the same as the original file, does this mean that
there is still a quality loss?

If a jpg does lose detail everytime it's saved, this means that there should
be a reduction in file size, which also means that at some point in the
future i.e. if a jpg is resaved enough times, it will eventually reach a
point where all detail is lost and the file ends up being 1 byte in size.
This doesn't make a whole lot of sense to me.

It looses real detail but acquires false detail - artifacts - each time it
is saved. The file size is not a measure of this, image quality gets worse
and worse.

--
John Bean

In a few minutes a computer can make a mistake so great that it would have
taken many men many months to equal it (Anon)

Terence Gui wrote:
wrote in message
...

humiliatedgrapes wrote:
: I know that jpg loses detail every time you save it, but does it lose

detail

: every time you access a jpg file.

: Thank you,

No, it loses detail only when you resave it.

Ray


If the resaved jpg file is saved using the same quality settings, and the
resulting file size is the same as the original file, does this mean that
there is still a quality loss?

If a jpg does lose detail everytime it's saved, this means that there should
be a reduction in file size, which also means that at some point in the
future i.e. if a jpg is resaved enough times, it will eventually reach a
point where all detail is lost and the file ends up being 1 byte in size.
This doesn't make a whole lot of sense to me.

I would think there has to be a point where the compression scheme analyzes
an image and determines it cannot reduce the quality/size of the image any
further, dependent upon the actualy jpeg quality setting used.

Terence



A number of factors, including the actual nature of the compression
algorithms, prevents that kind of 'entropy'. What really happens is
that the image quality degrades without much saving of file space until
the image is below acceptable quality.

In fact, resaving MAY not further reduce quality as many programs are
able to recognize that no editing was done, and they just resave the
input buffer.

"Bud" wrote in message
...
John Bean wrote:

It looses real detail but acquires false detail - artifacts - each time
it
is saved. The file size is not a measure of this, image quality gets
worse
and worse.

You're thinking analog, not digital. No loss. Of course there is always a
difference in digital editors, YMV.


John's right, quality gets worse every time you save the file.

"humiliatedgrapes" wrote in message
newsGAad.226631$D%.165144@attbi_s51...
I know that jpg loses detail every time you save it, but does it lose
detail
every time you access a jpg file.


As others pointed out: no it doesn't.
As for saving, I would suggest to use a lossless format like PNG or TIFF
while you're editing, and only convert to JPG when publishing the final
result. And even then, only if you need the smaller file size.
If you draw your images from a camera, you could use TIFF or RAW there too.

John Bean wrote:

It looses real detail but acquires false detail - artifacts - each time it
is saved. The file size is not a measure of this, image quality gets worse
and worse.

An example of how jpg files behave when resaved multiple times can be found on
the following web page:
http://www.cs.tut.fi/~leopold/AV/JPEG/

The text is in finnish but here's a resumé:
The first picture is the original. The next pictures have been saved 1, 10,
100 and 1000 times, using a random jpg quality between 85 and 94 percent.

--
Ola

"Ola Forsström" wrote in
:

An example of how jpg files behave when resaved multiple times can be
found on the following web page:
http://www.cs.tut.fi/~leopold/AV/JPEG/

The text is in finnish but here's a resum‚:
The first picture is the original. The next pictures have been saved
1, 10, 100 and 1000 times, using a random jpg quality between 85 and
94 percent.



That's a cool website -- I wish I could read it all, but your summary
explains the concept.

A similar and related experiment that I did was to open and "save as" a
..jpg many times in Photoshop. If you do not change the quality level, then
the image will not be altered, no matter how many times you save it.

Bob

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"Ola Forsström" wrote in
:

An example of how jpg files behave when resaved multiple times can be
found on the following web page:
http://www.cs.tut.fi/~leopold/AV/JPEG/

The text is in finnish but here's a resum‚:
The first picture is the original. The next pictures have been saved
1, 10, 100 and 1000 times, using a random jpg quality between 85 and
94 percent.



That's a cool website -- I wish I could read it all, but your summary
explains the concept.

A similar and related experiment that I did was to open and "save as" a
..jpg many times in Photoshop. If you do not change the quality level, then
the image will not be altered, no matter how many times you save it.

Bob

--
Delete the inverse SPAM to reply

"Ola Forsström" wrote in
:

An example of how jpg files behave when resaved multiple times can be
found on the following web page:
http://www.cs.tut.fi/~leopold/AV/JPEG/

The text is in finnish but here's a resum‚:
The first picture is the original. The next pictures have been saved
1, 10, 100 and 1000 times, using a random jpg quality between 85 and
94 percent.



That's a cool website -- I wish I could read it all, but your summary
explains the concept.

A similar and related experiment that I did was to open and "save as" a
..jpg many times in Photoshop. If you do not change the quality level, then
the image will not be altered, no matter how many times you save it.

Bob

--
Delete the inverse SPAM to reply