JPEG Questions: Loss In Quality When "Saving"

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.

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?

File- Save or Save As. Or in some software, File- Export, although
that generally exports a *copy* and won't overwrite the original unless
you tell it to by giving it the same file name in the same directory.


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 didn't save.


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. You haven't saved it, you've copied it.


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. You've overwritten the original file with a copy that is
additionally compressed. Whether you will be able to immediately see new
artifacts would depend, I guess, on what your compression is set to. If
it's compressing by only 1% each time, you probably wouldn't see it
until after you'd done so several times.


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.

Well, you could quit using .jpg as an archival file format. Use .tif,
..bmp, or your image editor's native file format. These should all be
lossless.


--
Angela M. Cable
Neocognition, digital scrapbooking source:
http://www.neocognition.com/

PSP Tutorial Links:
http://www.psplinks.com/

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http://www.fortunecity.com/westwood/alaia/354/

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?

Most software has a File/Save or File/Save As feature, this causes it to
re-compress the jpeg file.


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, your just viewing it.


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, your copying rather 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.
What happens is that when a Jpeg is saved, the compressor throws away
information it doesn't think it needs, in order to achieve it's high
compression ratios. You probably don't notice unless you load both the
before and after images and use high magnification.


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.

Jpeg is good when the ultimate goal is tiny files, where small details
are probably lost anyway. For example Jpeg is good on the web, where
tiny file size is most important. Other compression systems, that are
lossless don't compress as well. Saving as TIFF or PNG is better, when
your manipulating the files potentially several times over.

My preference is to work with TIFF and then compress using Bzip2, as
it's one of the better compressors, for cold storage on CD-R.

Paul

(Xtx99) wrote in
:

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....


Others have answered the previous questions adequately, so I'll skip
to the chase...


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?

Potentially. There's a lot of misunderstanding about what jpeg
compression does, and many people feel it's a lot worse than it is.

Most believe that opening an image than re-saving at the same
compression will degrade the image rapidly. It doesn't - degradation is so
small in cases like this that it can be difficult, if next to impossible,
to detect. Anyone that doubts it can try it out for themselves, which is
the best way to see what happens.

Jpeg compression is, roughly, a shortcut. Uncompressed or non-lossy
formats like TIFF typically record the info as "there's a pixel at x/y
coordinates, having a color register of 255/255/255". And they do this for
each pixel.

Jpeg uses groups. It may say "every pixel between 25/10 and 35/20
[x/y coordinates of a rectangle] has the color register of 255/255/255".
Can save a lot of space. Compression amounts actually refer to how
sensitive the 'Save' function is to color changes. Low compression (big
files, higher quality) means the function is very sensitive to minute
changes. High compression (smaller files, lower quality) means it will
consider a lot of color registers much the same, and run them all together.
This is what produces the 'blocky' jpeg artifacts. They become most
noticeable around contrasty details where the images go through the
greatest changes in small areas.

When opened by most editing programs, this info is read and converted
back into direct info, x/y pixel is color blahblah, for each pixel. You can
then save it as a lossless format like TIFF if you wanted. And the file
size will jump immensely. But if you lost color info in the initial jpeg
save (producing blocky artifacts, for instance), you don't get them back
with a TIFF save.

If you make minute changes, such as touching out dust, and save as
jpeg *at the same compression*, the program is performing the exact same
compression algorithm as before. Quality loss therefore becomes
exceptionally minor.

BUT - it needs to be noted... changing the color register, for
instance, can reset the 'shortcuts' that the compression takes. So can
converting the imbedded profile if you have a different color or monitor
profile active than the jpeg was originally saved within. So it can be very
easy to degrade the image. It really depends on where it came from and what
you do.

As others have said, master files such as original scans or digital
images are better in lossless formats like TIFF, but they're huge. If you
have no intentions of enlarging the image to 16x20 routinely, or inspecting
the image for fine details, then saving it as a jpeg at highest quality
reduces the size considerably and produces virtually no discernable detail
loss. If you take pictures for your own amusement, and don't sell them or
anything, TIFFs are a great way to jam up your harddrive with wasted space.

But with any image, anytime you edit it, save it under a different
filename. This prevents loss (even from TIFF format - if you change the
color register or contrast, that ain't gonna be recovered either). But more
importantly, I've had bad saves more times than I can count, and if you've
overwritten the original when you did it, you've trashed it forever.

By the way, the all-time worst format for bad saves and corrupted
files has been 'progressive jpegs', in my experience. I suggest avoiding
them at all costs.


- Al.

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