Re: [Gimp-developer] Save/export, option to go back to old behaviour

[Graeme Gill <graeme2 argyllcms com>]

drawoc wrote:
> If you simply open a jpeg, and then save it as a jpeg without
> modification, you will lose data. Jpeg is always lossy. More
> compression artifacts will be introduced in the saved image. There is
> no sane way to avoid this behavior.

Technically that's true (but see below), but have you actually
seen this ? Given a certain moderate compression ratio, how many open/slight edit/save
cycles does it take before you can visually notice that the quality is
worse than what you started with ?  2 ?  5 ?  10 ?

> If you allow the user to save back to jpeg, then you will not "honour
> the quality/file size trade-off" because the user will be slowly
> destroying their image every time they save and re-open.

It turns out that (typically) open/save of a jpeg does not cause a
steady decrease in quality, instead it asymptotes to a steady state.
That's because once DCT coefficients have been quantised, they
tend to get re-quantised to the same values. There is no
question that a lossily compressed format is not what you want
to use to create original artwork or do high quality editing or re-purposing,
but I think I can be confident that the vast majority of image use is
casual and non critical and in the jpeg format - all those
billions of digital photo's taken every day by ordinary people.

> We're not saying that you can't slowly destroy your image - you still
> have the freedom to do that. We just want you to click a single button
> that says, "yes, I want to destroy my image", if you really do like
> destroying images.

But you're saving to a lossy compressed file format - by doing that
you have made that choice. Having a nag telling you all the time
that "you do realise that jpeg is lossy, don't you?" seems fairly
pointless.

Graeme Gill.