On 7/8/07, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote:
> You clearly know more about the detail of this than I do but isn't there a
> direct one-to-one mapping once the original compression is done?

Nope.

> Any deviation from that must be errors in the decoding, so is what you
> posted a symptom of continual rounding errors in the decoding altering the
> image each time?
>
> Then the growing artifacts are a result of the limited colour resolution
> of 8 bits per channel used by gimp.

Also by the fact that in JPEG greyscale and color information is
decoupled and the color information is stored with lower spatial
resolution than the greyscale data. Thus there is additional rounding
that has to be done to get back to a RGB raster. The bottom line is
that JPEG, DV (for video) and other similar lossy compressions do
introduce generational loss, like mp3 and similar codecs do for audio.

/Øyvind K.
-- 
«The future is already here. It's just not very evenly distributed»
                                                 -- William Gibson
http://pippin.gimp.org/                            http://ffii.org/
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