I'm going to preface all of this by saying that I'm always happy to be
proved wrong in things like this.
On 2/13/2009 7:22 PM, David W. Fenton wrote:
The usual method is to have, say, a 15% compression ratio. When you
open a file, your graphics editing progam knows what the compression
ratio that it was saved at is,
I don't believe that's true. Neither Paint Shop Pro nor GIMP appear to
display this information; I suppose they may know it internally, but
this is the first I've heard this assertion.
Actually, look at http://photo.net/learn/jpeg/#ijg . It discusses a
utility which allows the *estimation* of the quality settings for any
given JPG. It also says "It would be most useful for Jpeg writing
software to list the prior quality level so you could rewrite (if
necessary) at the same level."
I do lots of graphics editing and while I keep TIFs as my source
files, I do lots of editing in JPGs once the graphic has reaced a
certain point in the editing process. And that includes multiple
edits and multiple saves, and the quality does not decrease with each
save.
Well, see http://www.faqs.org/faqs/jpeg-faq/part1/section-10.html ,
which would seem to disagree with you. They do say that "relatively
little further degradation occurs", but that's not the same as no change
in quality.
Aaron.
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