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Old Jan 23, 2005, 10:55 AM   #1
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Okay, here's something I was never able to get a straight answer on. How lossy is lossy from a practical standpoint? We know that you lose some nfo each time you open and re-save a jpg. What I was never able to find out is, how many times to you have to open and close before the image degrades noticeabley? Is is 5X, 100X, 1000X?

I know that this is a bit subjective but I've never been able to get even a ballpark. Any thoughts anyone?
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Old Jan 23, 2005, 10:58 AM   #2
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It depends on the amount of compression used if you are talking about jpeg. From what I have read , which I can't seem to find the link to at the moment, it can be as few as 5 times. Using what the article said was a normal compression ratio.
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Old Jan 23, 2005, 11:16 AM   #3
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For what it is worth I did a "save as" of a jpg from Photoshop and deleted the one onscreen. Then I opened the one I had saved and repeated the process. With 20 saves at quality 12 I couldn't see any difference between the original from the camera and the one I had saved 20 times at full screen. There might have been slight degradation blown up.

My guess is that if you did the same thing at quality 4 the image would be unrecognizable. The compression quality of the original is probably significant as well since compression tends to enhance compression artifacts. I used a downloaded superfine jpg image from an A2 because someone claimed artifacts were so low you could probably save multiple times at high quality with no loss, so I thought I would try it.



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Old Jan 23, 2005, 11:36 AM   #4
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Hi Nick, I paste the informations i've read on other site

...if you decompress and recompress an image at the same
quality setting first used, relatively little further degradation occurs.
This means that you can make local modifications to a JPEG image without
material degradation of other areas of the image. (The areas you change
will still degrade, however.) Counter intuitively, this works better the
lower the quality setting. But you must use "exactly" the same setting,
or all bets are off. Also, the decompressed image must be saved in a
full-color format; if you do something like JPEG=>GIF=>JPEG, the color
quantization step loses lots of information.
......................

Here is the link: http://www.faqs.org/faqs/jpeg-faq/part1/

Hope this helps

Basac,


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Old Jan 23, 2005, 11:58 AM   #5
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The top is a crop from the original, and the bottom is the original resized to 50% then to 200% (back to the original size).


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Old Jan 23, 2005, 12:00 PM   #6
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Also... I took the original and saved it as another file about 10 times without altering, and there is absolutely no difference even zooming in to 1600x.

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Old Jan 23, 2005, 2:30 PM   #7
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bobc wrote:
Quote:
...... and the bottom is the original resized to 50% then to 200% (back to the original size).
Hi bobc,

just curious! ... I don't understand why you resized to 50% then back to 200% (?), can you explain?
Thanks

Basac,
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Old Jan 23, 2005, 3:38 PM   #8
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Another very interesting topic

Bob; I don't think resizing has got something to do with it; Nick means every time you open a jpg, and save it again, it will be compressed. (some, details will be removed, to accomplish a smaller filesize). So every time you open a file, you allready compressed, do something with it, and save it again, more loss of quality occurs. So how many times can we do this before a photo isn't nice anymore?

I can't tell; but I always start editing with the original file from the cam; even it's only a small touch; just to rule out every possibility of too much compression. And usually save it at almost the best jpg quality (usually 11 in photoshop). Unless it is for posting here ofcourse

Cheers, GB

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Old Jan 23, 2005, 4:26 PM   #9
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Basac wrote:
Quote:
Hi bobc,

just curious! ... I don't understand why you resized to 50% then back to 200% (?), can you explain?
Thanks

Basac,
First I tried saving the image without altering it in any way to a different file name using a quality of 12. I did this about 10 times. Each time, I compared it to the original at 1200x (zoomed in). There was no difference what so ever.

So I figured I had to alter the image in some way and then save it. I chose resize (with resample) because I did not want to alter the way the image looked (color, sharpness, filters, etc...).

So there is no difference even saving over and over, unless you alter the image in some way.

If this is wrong, please let me know.

bobc
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Old Jan 23, 2005, 6:48 PM   #10
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Wow, thank you all very much. I think it still remains a bit of a mystery but I have a better understanding of the variables. If you open a jpg, and save it at the same compression ratio... you could probably do this a lot and not notice artifacts and degredation... How much is a lot? Well, some folks here opened and closed files 10+ times and didn't see any visible difference. Whether that number remains 20, 50, 1000 still remains a mystery but under "normal use" it shouldnt be an issue.

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)
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