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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