Taken from a note on the PWP forum this morning
Chuck Norcutt
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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.
--
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