On Thu, Apr 06, 2000 at 01:20:50AM +0200, Marc Lehmann wrote:
> As such, why save an image if you didn't change it?

There is no good reason why a PROFESSIONAL graphic designer should be
doing it, but lots of us are mere amateurs :)

JPEG works one tile at a time. The same behaviour I observe in one image
will be true on average for individual tiles, so if I alter only one
half of an image (or only touch up one word) and save with the same
quality as the original, the untouched tiles will be mostly unharmed.

A classic example (which I won't distribute because it's obviously
illegal) is a re-touched Matrix JPEG from the movie site, altered to
replace Keanu with Windy Miller from the old UK television shows.

By fiddling carefully with JPEG settings we can get the "right" setting
to make the hacked version look like the original, without additional
artifacts in the unaltered portions of the image. Looks great!

> Given that the problem is unsolvable in theory and almost impossible even
> to approximate in practise, I believt hat such a automatic detection
> scheme will fool people into thinking that saving at the same quality
> wouldn't destroy their image.

I don't want that, people shouldn't be using JPEG for works in progress
or as a common format moving between packages or ANYTHING like that, and
I agree that we don't want to give them false expectations.

> The problem here is that different encoders have different notions of
> "quality settings", and in most cases you can only approximate the
> quality setting of another encoder (quality settings are just a quick
> way to describe a 8x8 matrix, and setting up that matrix is very much
> decoder-dependent)

I think Marc and I agree on the realities of this situation, I just
wanted to make clear that "lossy" re-saving doesn't necessarily cause
any damage to the image. But that's NO REASON to be doing it, and
no reason for Gimp to encourage it either.

That was a public service announcement from the lossy compression posse


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