I am curious to know what the subsequent file sizes of all your
'generations' were, Chuck. After all jpeg is only intended for
compressing the original file without to much quality loss for your
specific goal. You can always revert to raw, when quality is
important.
If the files stop getting smaller a further jpeg save would make no
sense, or am I wrong here?
Frank van Lindert
Utrecht NL.
On Sun, 08 Mar 2009 17:53:33 -0400, Chuck Norcutt wrote:
>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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