Go Back   Steve's Digicams Forums > Digital Cameras (Point and Shoot) > Panasonic / Leica

Reply
 
Thread Tools Search this Thread
Old Jan 23, 2005, 9:49 PM   #11
Senior Member
 
bobc's Avatar
 
Join Date: Sep 2004
Posts: 1,433
Default

NickTrop wrote:
Quote:
Hmmmmmmmm, so... if I open a jpg in Photoshop, resize it with the wizard (which makes another copy...), then post. Then, exit out of PS, I shouldn't re-save the image. If I don't re-save, the image I opened doesn't suffer degradation, I assume.

Thanks all... THIS STUFF REALLY HAS ME AT A LOSS
(Rim shot, please..... thank you)
Nick. The difference took place after I re-sized it and before I saved it.

bobc
bobc is offline   Reply With Quote
Old Jan 23, 2005, 9:58 PM   #12
Senior Member
 
Join Date: Nov 2004
Posts: 137
Default

Nothing wrong bobc ... i just want to understand clearly.
As you said,there is no BIG difference even saving over and over, unless you alter the image in some way. I think so.

Thanks
Basac,
Basac is offline   Reply With Quote
Old Jan 23, 2005, 11:14 PM   #13
Senior Member
 
Alan T's Avatar
 
Join Date: Jun 2003
Location: Chester, UK
Posts: 2,980
Default

NickTrop wrote:
Quote:
THIS STUFF REALLY HAS ME AT A LOSS
No great mystery, for practical purposes. Just don't keep resaving and reloading unnecessarily during an editingsession, except to give you something to return to in case of accidents. When you do save a jpeg as a backup, make sure it's of high quality, or at least of exactly the same quality as discussed above..

Then do your own experiments, as others in this thread have done, and see what you can put up with yourself. If in your image editor, you view a pair of images alongside each other at such a high magnification that you can see the pixels, you'll soon see the differences (if any) and learn what you can tolerate in the way of degradation.

Good luck,

Alan T
Alan T is offline   Reply With Quote
Old Jan 24, 2005, 8:25 AM   #14
Senior Member
 
thekman620's Avatar
 
Join Date: Feb 2004
Posts: 6,084
Default

To take away your worry, just duplicate the image you think you might want to do a lot of future editing to, and save the original untouched-just work on the copy. Cheers........thekman.
thekman620 is offline   Reply With Quote
Old Jan 24, 2005, 8:26 AM   #15
Member
 
arghman's Avatar
 
Join Date: Nov 2004
Posts: 45
Default

My understanding of JPEG (warning: although I've done quite a bit of research to try to understand this issue, I do not have full knowledge of the JPEG format) is that it breaks pictures up into 8x8 pixel blocks, then transforms each of those blocks into some coefficients (numbers) which describe the brightness & color of that area of the image. This is prior to any compression / loss.

Then to achieve the compression, it throws away some fraction of the coefficients that show the highest amount of detail (highest spatial frequency), encodes the remaining coefficients and stores those into the output JPG file. The "quality" number you get to pick controls how much of those coefficients get thrown away. (Theoretically you should be able to pick portions of the image that have more/less compression than others, in case you want only part of the image to stay high quality, though I don't know of any program which can do this.)

So my understanding is that if you were to use a program which can save JPGs (IrfanView, Photoshop, etc.) and open and save it repeatedly with the same "quality" setting, there wouldn't be any additional loss. Or if you used the same program to open/save it 5 times, with these quality settings: (50%, 30%, 90%, 75%, 40%) the resultant file would give you the same results as if you had just saved it once with the lowest quality setting (30% in this case).

What is less clear is what happens if you transform the image between saves, e.g. resize/crop/filter/whatever. (Or if you use different programs to open and save the same file in sequence, although if they're good programs they should use similar techniques and thereby not throw out much additional data) In that case you could get some additional degradation, although I don't think that by saving an image 100 times at 90% quality you're going to end up with a 10% quality image. It's not like the kind of degradation you get when you make a copy of a copy of a copy (etc.) on a copy machine.
arghman is offline   Reply With Quote
Old Jan 24, 2005, 8:40 AM   #16
Member
 
arghman's Avatar
 
Join Date: Nov 2004
Posts: 45
Default

p.s. here are some of the better sources I found, a bit on the technical side but they're useful.

http://www.maths.lth.se/na/staff/jose/jpeg/wallace.pdf
http://www.poynton.com/PDFs/Rehabilitation_of_gamma.pdf
http://stargate.ecn.purdue.edu/~ips/.../jpegtut1.html
arghman is offline   Reply With Quote
Old Jan 24, 2005, 8:44 AM   #17
Senior Member
 
NickTrop's Avatar
 
Join Date: Sep 2004
Posts: 2,249
Default

Exactly... thanks all. As far as testing, one would think that something like this would be "out there" but more scientific than a home test. I like lossless archival formats but then file management and disk space becomes an issue (lossless files are way larger) You get the original jpg, the archival version (tiff, raw, j2p), and however many altered versions (resized for web, sharpened, whatever) disk space also becomes an issue.... As you know image files multiply like rabbits.

I would just keep the original jpg and skip the whole archival step if the degredation of the original jpg is nominal or unperceptable after X saves... What I was after is the value of X.

If X is say, 20 or more saves (recompressions), then skip archiving. But, like anything else, there are lots of variables.

arghman - thanks, I'll check out those links!
NickTrop is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 4:31 PM.