JPEG compression loss

[david noone]

I was hoping someone could help me with an argument/ discussion/

learning experience I'm having with an instructor of mine, (there's a

pizza going to whom ever is correct, so this is serious business).

Simple question: does a JPEG file lose information every time it is

opened and closed? When opened in PhotoShop, it's uncompressed and

then re-compressed when it's closed...does that cause loss? Or is

there only loss of image information when it's first compressed; and

when there is an alteration done to it, only that change will be

affected? Also...is there any reference book I can find this answer

in so as to prove the point. Thanks for any help you can give us.

[mark_scheuern]

<P>You lose information every time the file is re-saved. Simply opening it and closing it (to view the image, for example) doesn't cause any loss since the file is not re-written.<P>

<P>See the <A HREF="http://www.faqs.org/faqs/jpeg-faq/">JPEG FAQ</A> for more info and, in particular, <A HREF="http://www.faqs.org/faqs/jpeg-faq/part1/section-10.html">Does loss accumulate with repeated compression/decompression?</A></P>

<P>Hope you win the pizza!</P>

[gordonr]

The most loss occurs the first time you convert to Jpeg, and subsequent times the loss may be so small as to be undetectable (if you use the same image editor and quality setting). The combination of editing, closing and re-opening produces more errors due to an accumulation of quantisation (rounding effects). There is no "single" answer to this question, since there are as many Jpeg standards as there are image editors, and at the highest settings the differences are barely detectable (at low settings the results can be terrible). There are already some threads on this subject in the archived forum.

[scott_eaton]

Opening and closing a JPEG inside of Photoshop does not degrade the contents of the file. The same algorithm simply gets applied with no additional, imaging degrading compression factors.

 

It's when you edit the image and resave it that a greater degree of image loss occurs.

[hal_bissinger]

When you just open a file it's read only. You do nothing to the file. Think of what happens if you open a .jpg file from a CD. You can only read it- you couldn't modifiy it if you wanted to.

[jim schwaiger]

Simply opening the file does not change it, but what if you open and then save it without modifications. If you use the exact same compression level, the size _should_ not change. But if you use a different (more or less) compression level, the file will more than likely change. Even if you save at the maximum quality, it is likely that it will degrade the image.

[dougb]

Re-saving does lose information, 'quitting' does not. This is documented in "Adobe Photoshop 6.0 for Photographers".

 

Doug.