Be sure to note that the highest quality level was used. Photoshop
defaults to quality #8 vs. the max of 10. I suspect degradation would
be apparent earlier. I posted this because, while I've heard of the
problem, I've never previously seen anything in the least bit quantitative.
Doug wrote:
> On Sunday, March 08, 2009 07:46, Chuck Norcutt wrote:
>> Taken from a note on the PWP forum this morning
>>
>> Chuck Norcutt
>> --------------------------------
>> I HAVE tested the loss (change) due to successive saves of JPEG files.
>> I opened a JPEG, did no editing and saved it under a new name
>> (generation #1). Then I opened Generation #1 and saved it as
>> Generation #2. Etc. Etc. [all the JPEG saves were at the highest
>> possible quality setting]
>>
>> After up to 3 generations I couldn't visually detect any difference
>> from the original JPEG. I used the COMPOSITE transformation with an
>> "Absolute Difference" setting to compare pixel-by-pixel any
>> differences and could see hardly anything but black in the comparative
>> result -- even with "brightness" highly exaggerated.
>>
>> But when I compared the original with Generation #10 -- WOW what a
>> difference. I could readily detect, visually, significant differences
>> from the original image. When I did an "Absolute Difference"
>> comparison, the differences were so dramatic that you could see an
>> actual image in black and white.
>
> I find this very interesting. My thought is that doing a very slight edit to
> the image and then saving it, would be a much better real world test. I may
> try it later today if I have the time using GIMP to edit and ImageMagick to
> compare. I don't know if the jpegs algorithms are implimented differently in
> different software.
--
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