Urban Myth? jpeg's depreciate... do they?

My friend, a big MAC fan, and camera guru, tells me that jpegs depreciate. as a normal logigal guy, I find this hard to swallow. Is it true, or Urban Myth?
GordonRaeAsked:
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Bordeaux0113Connect With a Mentor Commented:
Yup.  It's all about editing or not editing.  Now you can tease your friend over this forever muahahah!  Friend's don't let friends use Mac :P
Jk hehe
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Bordeaux0113Commented:
JPEGs lose quality every time they are opened and/or saved.
False. Simply opening or displaying a JPEG image does not harm the image in any way. Saving a JPEG repeatedly during the same editing session (without ever closing the image) will not accumulate a loss in quality. Copying and renaming a JPEG will not introduce any loss, but some image editors do recompress JPEGs when the Save As command is used. To avoid more loss you should duplicate and rename JPEGs in a file manager rather than using "Save As JPEG" in an editing program.
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GordonRaeAuthor Commented:
Thanks,,,,
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critter42Commented:
If you do nothing but view a .jpg (ie open it read only), nothing happens.

However .jpg is a LOSSY format, which means information is discarded every time it is edited and resaved.  So, the more you edit and resave the .jpg, the more and more artifacts are introduced in the image:

see here for an example:

http://www.cywarp.com/faq_jpg_degradation.htm

But if the jpg just sits on a drive and nothing is done with it, it will not "degrade".
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Bordeaux0113Commented:
JPEGs lose quality every time they are opened, edited and saved.
True. If a JPEG image is opened, edited, and saved again it results in additional image degradation. It is very important to minimize the number of editing sessions between the initial and final version of a JPEG image. If you must perform editing functions in several sessions or in several different programs, you should use an image format that is not lossy (TIFF, BMP, PNG) for the intermediate editing sessions before saving the final version. Repeated saving within the same editing session won't introduce additional damage. It is only when the image is closed, re-opened, edited and saved again.
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GordonRaeAuthor Commented:
This is going to be a wee minefield..... (Scottish term)
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BloodCommented:
As critter42 said, it is a lossy compression format.  Any lossy compression format, regardless of type (mp3, jpg, wma) you will never generate the exact same file that you had before.  

It is, however, a misnomer to say that the same information is discarded -every- time and that the process is iterative.  You can actually open a JPEG (which has lost some of its original information) and ADD information to the picture while saving it on a lower compression level which was already set.   The loss of the original image information will be significantly different than if you were to save the same image over and over again with the same compression setting.   It will, obviously, compress the data again but you will have gained whatever changes or improvements to the original image (sharpness, colour correction, saturation levels...etc) - so it is a bit anecdotal.

If you don't want to deal with it, go with a lossless format like PNG.
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Bordeaux0113Commented:
What does that mean Gordon?  Hehehe
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Dave BaldwinFixer of ProblemsCommented:
To minimize problems, I keep the original image which is going to be the best quality available.  When I need a new but smaller image, I go back to the original and make a copy of it.  On the other hand, if you're making a little thumbnail, there is a lot of detail lost simply because of the reduction in size so it doesn't really matter that much.
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rafaperez3dCommented:
Probably your friend is not totally wrong.

Obviously, no digital file depreciate. This is the main idea from any digital file, since IBM punched cards!

Probably he's talking about lose quality after some editing. As critter42 showed us, after some open/save cycles, you get a blurry image.

This is normal, because JPEG is format that discards information to get small file sizes.

But if you dont compress your image so much between sessions, like using the Photoshop quality slider at 10 or 12, nobody can discern between a image saved a bunch of times from the original one.

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GordonRaeAuthor Commented:
OK, is this an accurate summary?

1-Simply opening and closing jpegs does NOT reduce quality. Ever.
2-If you open a jpeg and perform any editing, there may be a loss of quality.

How do I award points? Everybody is right?
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BloodCommented:
Gordon:

Yes, those two points are right.   Opening an image file without a save never does anything to the bits in the file.

You can split points however you wish.
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Dave BaldwinFixer of ProblemsCommented:
You know the difference between PC users and Mac users?  PC users have always been told their computer was crap so when something goes wrong, they blame the computer.  Mac users have always been told their computers are smarter than they are... so they blame themselves when things go wrong.
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rafaperez3dCommented:
And I just dont believe in nothing that a Mac fan tells me about how inner technology works. They usually don't know what they talking about....


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Bordeaux0113Commented:
LOL I agree with rafaperez3d...most Mac fans are just total n00b art kids...not saying anything is wrong with being artistic...but I hate that they are like "THIS IS HOW IT IS" when they have no complete knowledge of whatever the topic is...
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