JPEG Questions: Loss In Quality When "Saving"

X
Posted By
xtx99
Apr 8, 2004
Views
699
Replies
13
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Closed
I’m confused about quality deterioration when repeatedly opening and "saving" JPEG images. I’ve read that every time a JPEG image is "saved," some quality is lost due to the lossy compression format.
My questions are….

1) What is meant by "saving" a JPEG image?

2) If I simply "open" a JPEG image in Microsoft Photo Editor, Adobe Photoshop CS or any other image editor, View the JPEG image and then simply exit out of viewing the JPEG, am I actually "saving" it again and therefore loosing quality?

3) If in Windows I right click on a JPEG and make a duplicate copy of the JPEG, is the copy (with a different name) "saved" and therefore the quality of the copy diminished.

4) If I open a JPEG in a photo editor and "save as" (instead of simply exiting) and use the same name as the JPEG I just opened, is the quality diminished?

When I do all of the above things, I don’t notice any decrease in quality and the JPEG seems to be exactly the same size so I’m guessing I haven’t degraded the original. If I’m wrong, I’m guessing I should make each JPEG file "read only" or burn them to a CD or DVD so they can’t be changed when I view them.

If I open up a JPEG in a photo editing program and make any changes to the JPEG at all, I can understand that the quality degrades. Any advice on my questions are appreciated.

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XT
xalinai_Two
Apr 8, 2004
On 08 Apr 2004 04:58:43 GMT, (Xtx99) wrote:

I’m confused about quality deterioration when repeatedly opening and "saving" JPEG images. I’ve read that every time a JPEG image is "saved," some quality is lost due to the lossy compression format.
My questions are….

1) What is meant by "saving" a JPEG image?

Writing the flattened representation of the image in the memory allocated by your image processing software to a disk file using a JPEG compression algorithm.

2) If I simply "open" a JPEG image in Microsoft Photo Editor, Adobe Photoshop CS or any other image editor, View the JPEG image and then simply exit out of viewing the JPEG, am I actually "saving" it again and therefore loosing quality?

No. If you did not "save" the image as specified under 1.

3) If in Windows I right click on a JPEG and make a duplicate copy of the JPEG, is the copy (with a different name) "saved" and therefore the quality of the copy diminished.

No. Windows creates a binary duplicate of the file. It does not open the file and rewrite – the file copying routine does not care what kind of file it copies.

4) If I open a JPEG in a photo editor and "save as" (instead of simply exiting) and use the same name as the JPEG I just opened, is the quality diminished?

Save As is saving the file as specified under 1.

When I do all of the above things, I don’t notice any decrease in quality and the JPEG seems to be exactly the same size so I’m guessing I haven’t degraded the original. If I’m wrong, I’m guessing I should make each JPEG file "read only" or burn them to a CD or DVD so they can’t be changed when I view them.

Load a JPG image (Sharp, crisp, with detail in red…). Create a copy using "Save As".
Load that copy too.
Use image arithmetics to determine the difference between the images. Have lots of fun.

If I open up a JPEG in a photo editing program and make any changes to the JPEG at all, I can understand that the quality degrades. Any advice on my questions are appreciated.

The image has changed from the original on the way from memory to disk when it was saved. If you load it, you have a different image than before as a pixelmap in your computer memory – if you save that image, chances are that it will be changed again, exceptions are possible if everything that could get lot in the compression algorithm is alredy lost. If the shoe would have been presented to Cinderella’s sisters a second time there would have been no need to cut their toes again.

Michael
T
tacitr
Apr 8, 2004
1) What is meant by "saving" a JPEG image?

"Saving" means "opening, then using the Save or Save As command."

2) If I simply "open" a JPEG image in Microsoft Photo Editor, Adobe Photoshop CS or any other image editor, View the JPEG image and then simply exit out of viewing the JPEG, am I actually "saving" it again and therefore loosing quality?

No. You never save a file just by opening and then closing it. Saving means using the save or Save As command.

3) If in Windows I right click on a JPEG and make a duplicate copy of the JPEG, is the copy (with a different name) "saved" and therefore the quality of the copy diminished.

No. Windows does not open the JPEG. Windows just makes a precise, bit-for-bit identical copy, without opening and then saving.

4) If I open a JPEG in a photo editor and "save as" (instead of simply exiting) and use the same name as the JPEG I just opened, is the quality diminished?

Yes.

When I do all of the above things, I don’t notice any decrease in quality and the JPEG seems to be exactly the same size so I’m guessing I haven’t degraded the original.

If you open it and do a Save As, the quality will diminish, and the one you just saved will not be an exact copy of the original. You can verify this by opening both, placing one over the other, inverting it, and setting the Layer Mode to Difference.

If I open up a JPEG in a photo editing program and make any changes to the JPEG at all, I can understand that the quality degrades.

Correct–but you do not need to make any changes to degrade quality. All you need to do is use the Save or Save As command.


Biohazard? Radiation hazard? SO last-century.
Nanohazard T-shirts now available! http://www.villaintees.com Art, literature, shareware, polyamory, kink, and more:
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AA
Adelson Anton
Apr 9, 2004
Relative question, is it possible to crop a jpg image without recoding it? I mean lossless here.
BK
Brian K
Apr 9, 2004
Another question. What is the difference between (for jpg) Save for Web and Save as. They both have numerous "qualities".
If I open a jpg in Photoshop and make changes I Save for Web at 60 quality. It looks fine to me. I never use Save as. Any comments?

Brian

"Xtx99" wrote in message
I’m confused about quality deterioration when repeatedly opening
and
"saving" JPEG images. I’ve read that every time a JPEG image is "saved,"
some
quality is lost due to the lossy compression format.
My questions are….

1) What is meant by "saving" a JPEG image?

2) If I simply "open" a JPEG image in Microsoft Photo Editor, Adobe
Photoshop
CS or any other image editor, View the JPEG image and then simply exit out
of
viewing the JPEG, am I actually "saving" it again and therefore loosing quality?

3) If in Windows I right click on a JPEG and make a duplicate copy of the JPEG, is the copy (with a different name) "saved" and therefore the
quality of
the copy diminished.

4) If I open a JPEG in a photo editor and "save as" (instead of simply exiting) and use the same name as the JPEG I just opened, is the quality diminished?

When I do all of the above things, I don’t notice any decrease in quality and the JPEG seems to be exactly the same size so I’m guessing I haven’t degraded the original. If I’m wrong, I’m guessing I should make
each
JPEG file "read only" or burn them to a CD or DVD so they can’t be changed
when
I view them.

If I open up a JPEG in a photo editing program and make any changes
to
the JPEG at all, I can understand that the quality degrades. Any advice
on my
questions are appreciated.
T
tacitr
Apr 9, 2004
Another question. What is the difference between (for jpg) Save for Web and Save as. They both have numerous "qualities". If I open a jpg in Photoshop and make changes I Save for Web at 60 quality. It looks fine to me. I never use Save as. Any comments?

Save for Web lets you save slices with or without the HTML code to assemble them, lets you do 2-up and 4-up previews, lets you see interactively the results of different GIF and JPEG settings, and so on. Save As gives you less flexibility and less control.


Biohazard? Radiation hazard? SO last-century.
Nanohazard T-shirts now available! http://www.villaintees.com Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html
T
tacitr
Apr 9, 2004
Relative question, is it possible to crop a jpg image without recoding it? I mean lossless here.

Not in Photoshop. There are other programs which let you do this, but only if you crop in amounts divisible by the JPEG block encoding size–typically, 8 pixels. (For example, you can not crop 5 pixels off the left-hand side losslessly; the crop must be in 8-pixel increments.)


Biohazard? Radiation hazard? SO last-century.
Nanohazard T-shirts now available! http://www.villaintees.com Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html
BK
Brian K
Apr 10, 2004
The file size for equivalent "visual quality" seems higher with Save As. Any advantages with Save As?

"Tacit" wrote in message
Another question. What is the difference between (for jpg) Save for Web and Save as. They both have numerous "qualities". If I open a jpg in Photoshop and make changes I Save for Web at 60
quality.
It looks fine to me. I never use Save as. Any comments?

Save for Web lets you save slices with or without the HTML code to
assemble
them, lets you do 2-up and 4-up previews, lets you see interactively the results of different GIF and JPEG settings, and so on. Save As gives you
less
flexibility and less control.


Biohazard? Radiation hazard? SO last-century.
Nanohazard T-shirts now available! http://www.villaintees.com Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html
XT
xalinai_Two
Apr 10, 2004
On Sat, 10 Apr 2004 01:57:15 GMT, "Brian K" <iibntgyea4_
wrote:

The file size for equivalent "visual quality" seems higher with Save As. Any advantages with Save As?

If keepig the EXIF info is an advantage for you…
Save for web will drop the EXIF info.

Michael
"Tacit" wrote in message
Another question. What is the difference between (for jpg) Save for Web and Save as. They both have numerous "qualities". If I open a jpg in Photoshop and make changes I Save for Web at 60
quality.
It looks fine to me. I never use Save as. Any comments?

Save for Web lets you save slices with or without the HTML code to
assemble
them, lets you do 2-up and 4-up previews, lets you see interactively the results of different GIF and JPEG settings, and so on. Save As gives you
less
flexibility and less control.


Biohazard? Radiation hazard? SO last-century.
Nanohazard T-shirts now available! http://www.villaintees.com Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

AA
Adelson Anton
Apr 10, 2004
Well, obviously I’m going to ask "which programs?"

Tacit wrote:

Not in Photoshop. There are other programs which let you do this, but only if you crop in amounts divisible by the JPEG block encoding size–typically, 8 pixels. (For example, you can not crop 5 pixels off the left-hand side losslessly; the crop must be in 8-pixel increments.)
T
tacitr
Apr 10, 2004
The file size for equivalent "visual quality" seems higher with Save As. Any advantages with Save As?

Save fr Web creates smaller files because it removes certain information that has nothing to do withthe image itself–for example, it removes the image icon, the preview, (by default) the color profile, camera EXIF data, and so on.

Save As creates a JPEG which (by default) includes an icon and a preview, preserves EXIF data, includes a color profile, and so on.

The actual image is the same in both cases; the "extra" data (preview and so on) that gets included when you use Save As makes the file larger but does not improve the quality of the JPEG.


Biohazard? Radiation hazard? SO last-century.
Nanohazard T-shirts now available! http://www.villaintees.com Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html
T
tacitr
Apr 10, 2004
Well, obviously I’m going to ask "which programs?"

I haven’t used any lossless JPEG rotation/crop software myself, so I can’t really recommend any. A Google search for "lossless JPEG crop" produced 4,500 hits, including one for a freeware program at

http://www.snapfiles.com/get/jpegcrop.html

Looks like it should do the trick…


Biohazard? Radiation hazard? SO last-century.
Nanohazard T-shirts now available! http://www.villaintees.com Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html
BK
Brian K
Apr 10, 2004
Thanks, finally makes sense.

"Tacit" wrote in message
The file size for equivalent "visual quality" seems higher with Save As. Any advantages with Save As?

Save fr Web creates smaller files because it removes certain information
that
has nothing to do withthe image itself–for example, it removes the image
icon,
the preview, (by default) the color profile, camera EXIF data, and so on.
Save As creates a JPEG which (by default) includes an icon and a preview, preserves EXIF data, includes a color profile, and so on.
The actual image is the same in both cases; the "extra" data (preview and
so
on) that gets included when you use Save As makes the file larger but does
not
improve the quality of the JPEG.


Biohazard? Radiation hazard? SO last-century.
Nanohazard T-shirts now available! http://www.villaintees.com Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html
V
vizrosplugins
Apr 12, 2004
You can select quality when save jpeg.

Tony G. Smith
Vizros – Realistic 3D page curl plug-ins and more
Demo at http://www.vizros.com/gallery.html

(Xtx99) wrote in message news:…
I’m confused about quality deterioration when repeatedly opening and "saving" JPEG images. I’ve read that every time a JPEG image is "saved," some quality is lost due to the lossy compression format.
My questions are….

1) What is meant by "saving" a JPEG image?

2) If I simply "open" a JPEG image in Microsoft Photo Editor, Adobe Photoshop CS or any other image editor, View the JPEG image and then simply exit out of viewing the JPEG, am I actually "saving" it again and therefore loosing quality?

3) If in Windows I right click on a JPEG and make a duplicate copy of the JPEG, is the copy (with a different name) "saved" and therefore the quality of the copy diminished.

4) If I open a JPEG in a photo editor and "save as" (instead of simply exiting) and use the same name as the JPEG I just opened, is the quality diminished?

When I do all of the above things, I don’t notice any decrease in quality and the JPEG seems to be exactly the same size so I’m guessing I haven’t degraded the original. If I’m wrong, I’m guessing I should make each JPEG file "read only" or burn them to a CD or DVD so they can’t be changed when I view them.

If I open up a JPEG in a photo editing program and make any changes to the JPEG at all, I can understand that the quality degrades. Any advice on my questions are appreciated.

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