(although this is as a reply to Marc's post I only noticed it in the
more recent replies, sorry if that screws up threading for anyone)

On Sat, Apr 01, 2000 at 07:46:03PM +0200, Marc Lehmann wrote:
> I don't think this really is so much of a problem. Saving a jpeg in the same
> quality as it was originally saved will do no good to your quality. The only
> thinvg it will ensure is that the file-size will be similar.

This isn't true, IJG documentation and home made tests show that, for
what little it is worth  JPEG(75) -> JPEG(75) -> JPEG(75) applied to
any set or subset of image tiles can do much LESS damage than e.g.
JPEG(75) -> JPEG(85) -> JPEG(75) -- even though the latter results in
the same size files, give or take.

This isn't guaranteed, but will usually work if you keep everything the
same (codec, compiler, libraries, CPU, direction of the wind...). In
any case it never does any harm. In fact, this improvement can be
enough for the difference between "obviously re-edited JPEG" and a
seamless fix to an image when you've lost the original.

The only trouble with this fact is that there's no usable mechanism
for divining which settings were used to compress the image, or even
whether the IJG codec was used at all. IMHO that's for the best. I
should not need to care which encoder was used to use the image.


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