Does Renaming or rotating degrade jpegs?

Hi

1)When you rename and save a jpeg (in windowsxp or PSCS) is the image quality degraded?

2)I noticed that Irfanview has a "lossless" rotation function. Does this mean if I
rotate a jpeg in windowsxp or PSCS that there will be loss of image quality (presuming the
rotation is saved)?

3)How many separate (saved) changes can you make with a jpeg before significant
image degradation occurs?

Thanks

Sam

"Sam" wrote in message

Hi

1)When you rename and save a jpeg (in windowsxp or PSCS) is the image quality degraded?

Yes. If you just want to rename it, why not use Explorer? It doesn't touch the data in the file.

2)I noticed that Irfanview has a "lossless" rotation function. Does this mean if I
rotate a jpeg in windowsxp or PSCS that there will be loss of image

quality

(presuming the
rotation is saved)?

Yes. Remember, every time you save an image, PS really makes a new one with the same name (unless you made no changes at all).

3)How many separate (saved) changes can you make with a jpeg before significant
image degradation occurs?

Opinions differ... Perhaps 10 or so.
Jim

1)When you rename and save a jpeg (in windowsxp or PSCS) is the image
quality degraded?

Every time you save a JPEG from Photoshop, the image is degraded. Even if you just open the image and do Save As with a different name. If you just rename the file in Windows, though, you change its name without changing any of the data inside it.

2)I noticed that Irfanview has a "lossless" rotation function. Does this mean if I
rotate a jpeg in windowsxp or PSCS that there will be loss of image quality (presuming the
rotation is saved)?

Yes.

3)How many separate (saved) changes can you make with a jpeg before significant
image degradation occurs?

Define "significant." What I think is significant might be irrelevant to you. It also depends on the image itself, and on the quality setting when the JPEG is saved.

I try, whenever possible, never to re-save a JPEG, and always keep an original Photoshop-format file for any images I edit using Photoshop.

Some people can tell when a JPEG is resaved a couple of times; other people can re-save a JPEG a dozen times and not find the result objectionable.

--
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

Thanks for the reply.....

"Jim" wrote

If you just want to rename it, why not use Explorer? It doesn't touch the data in the file.

Because the batch naming convention sucks in windowsxp. Windows has an "intuitive"option to sort "11" after "9" but this is not respected if using
other photoeditors using the "09" for placing a pic ninth. Please see next post

Sam

Thanks for reply...see inline

"Tacit" wrote in message

If you just rename
the file in Windows, though, you change its name without changing any of the
data inside it.

Pl see previous post.
What about using the file browser in PSCS to rename one or more relevantly using the batch renaming function which works great?

2)I noticed that Irfanview has a "lossless" rotation function. Does this mean if I
rotate a jpeg in windowsxp or PSCS that there will be loss of image quality
(presuming the
rotation is saved)?

Yes.

There was always some myth that if you just rotated 90 to 180 degrees it didn't degrade the image but this didn't make sense to me. It had also been said that rotating within windowsxp/explorer doesn't degrade but not sure if this is true (may be analogous to changing name within explorer?)
I am puzzled how Irfanview offers lossless rotation (and PSCS does not)

I try, whenever possible, never to re-save a JPEG, and always keep an original
Photoshop-format file for any images I edit using Photoshop.

So as long as you make any new changes to the psd file and resave as a jpg there are no losses?

Thanks
Sam

Sam wrote:

There was always some myth that if you just rotated 90 to 180 degrees it didn't degrade the image but this didn't make sense to me. It had also been said that rotating within windowsxp/explorer doesn't degrade but not sure if this is true (may be analogous to changing name within explorer?)
I am puzzled how Irfanview offers lossless rotation (and PSCS does not)

Actually, this in entirely possible and very neat. As I understand it, the JPEG is never fully decompressed in the process. You can also losslessly crop JPEGs along 8 (I think it was..) pixel boundaries.

Cheers,
Nicholas Sherlock

"Sam" wrote in
news:gLQMd.147579$:

So as long as you make any new changes to the psd file and resave as a jpg there are no losses?

No.

Repeat after me:

You lose something every time you save in JPEG format

What Tacit is describing is the methodology for *minimizing* those losses by working in a format that doesn't degrade your image, then saving a copy in JPEG as a last step.

If you need to do corrections at a later date, you open your lossless working original, then again save a JPEG copy as the last step, never modifying any of the JPEGs you make.

Cool?

In article <gLQMd.147579$>,
"Sam" wrote:

<snip>

There was always some myth that if you just rotated 90 to 180 degrees it didn't degrade the image but this didn't make sense to me.

Assuming you mean "90 or 180", it's not a myth at all; the exact same pixels are in the transformed image as in the original, just in a different order. Any other rotation involves resampling, but the 90° multiples, like horizontal and vertical flips, certainly don't.

--
Odysseus

Assuming you mean "90 or 180", it's not a myth at all; the exact same pixels are in the transformed image as in the original, just in a different order. Any other rotation involves resampling, but the 90° multiples, like horizontal and vertical flips, certainly don't.

In some programs.

Using Photoshop always causes cumulative loss; Photoshop always uncompresses JPEGs to so anything, including rotation, then re-compresses the JPEG to save it.

There are programs which do lossless rotation, provided the rotation is in 90-degree increments; Photoshop isn't one of them.
--
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

Ahh thank you, this presumably explains why Irfanview offers lossless (presumably 90 degree multiples) rotations? I wonder if the same is true of windowsxp "pic and fax viewer" ?

Sam

"Tacit" wrote in message

Assuming you mean "90 or 180", it's not a myth at all; the exact same pixels are in the transformed image as in the original, just in a different order. Any other rotation involves resampling, but the 90

On Tue, 08 Feb 2005 08:07:45 GMT, "Sam" wrote:

Ahh thank you, this presumably explains why Irfanview offers lossless (presumably 90 degree multiples) rotations? I wonder if the same is true of windowsxp "pic and fax viewer" ?

I tried to find that answer by looking at a file before and after with a hex editor. The "fax viewer" did appear to change _all_ of the data, which makes me think it was not a lossless rotation. I believe lossless rotation is done by rewriting each 8x8 pixel block with identical values but in a different part of the file, and I could not find any non-trivial byte sequences shared by the before and after files. If the rotation was 90 degrees, "horizontal" sequences would now be "vertical". But the three RGB values for a particular pixel should be together somewhere in each file, and I couldn't even find that.

I even tried rotating the file back to the original orientation, which if lossless should have produced the same exact data, right? It didn't.

Was I wasting my time? Is there any way to prove by looking at the data whether a rotation was lossless or not?

I did just find a long list of programs that do lossless rotation: <http://sylvana.net/jpegcrop/losslessapps.html>

And those are just the ones based on IJG's jpegtran algorithm. MS "Image Preview" is listed, but not "Picture and Fax Viewer", FWIW.

Loren

In article ,
(Tacit) wrote:

Assuming you mean "90 or 180", it's not a myth at all; the exact same pixels are in the transformed image as in the original, just in a different order. Any other rotation involves resampling, but the 90° multiples, like horizontal and vertical flips, certainly don't.

In some programs.

Using Photoshop always causes cumulative loss; Photoshop always uncompresses JPEGs to so anything, including rotation, then re-compresses the JPEG to save it.

Sorry for being unclear; I was thinking of the general case, rather than JPEGs specifically. Once Photoshop opens a JPEG, some loss will inevitably occur on resaving it, even if nothing else at all is done to the image, but the rotations in question won't cause any *additional* degradation.

--
Odysseus

Thank You Loren for your efforts...I will therefore stick with Irfanview (lossless) to rotate pics.
Sam

"Loren Amelang" wrote in message

On Tue, 08 Feb 2005 08:07:45 GMT, "Sam" wrote:

Ahh thank you, this presumably explains why Irfanview offers lossless (presumably 90 degree multiples) rotations? I wonder if the same is true of
windowsxp "pic and fax viewer" ?

I tried to find that answer by looking at a file before and after with a hex editor. The "fax viewer" did appear to change _all_ of the data, which makes me think it was not a lossless rotation. I believe lossless rotation is done by rewriting each 8x8 pixel block with identical values but in a different part of the file, and I could not find any non-trivial byte sequences shared by the before and after files. If the rotation was 90 degrees, "horizontal" sequences would now be "vertical". But the three RGB values for a particular pixel should be together somewhere in each file, and I couldn't even find that.

I even tried rotating the file back to the original orientation, which if lossless should have produced the same exact data, right? It didn't.

Was I wasting my time? Is there any way to prove by looking at the data whether a rotation was lossless or not?

I did just find a long list of programs that do lossless rotation: <http://sylvana.net/jpegcrop/losslessapps.html>

And those are just the ones based on IJG's jpegtran algorithm. MS "Image Preview" is listed, but not "Picture and Fax Viewer", FWIW.
Loren

"Odysseus" wrote in message

Sorry for being unclear; I was thinking of the general case, rather than JPEGs specifically. Once Photoshop opens a JPEG, some loss will inevitably occur on resaving it, even if nothing else at all is done to the image, but the rotations in question won't cause any *additional* degradation.

Sorry to be still confused....Do You mean if you open the jpeg in PS AND make a change before closing .....as in
"Do you wish to save the changes?" Similarly if you choose "Save As" xyz , there will be a loss.

There will not be any loss by just opening a pic and closing it again, Correct?

Sam :-(

On 2/9/05 1:30 AM, in article
5_iOd.153534$, "Sam"
wrote:

There will not be any loss by just opening a pic and closing it again, Correct?

Sam :-(

Correct! I don't know how the myth ever started that just opening and closing a jpeg causes loss. If the file is not resaved, the original file remains unchanged and no loss can occur.
Chuck