On 7/8/07, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote:
> On Sat, 07 Jul 2007 21:38:57 +0200, Øyvind Kolås <[EMAIL PROTECTED]> wrote:
>
> >  the image used in
> > the "JPEG Generation Loss" figure in the example in the following text
> > uses an image that
> > shows how JPEG compression keeps different aspects of the image intact
> > across multiple encode/decode cycles:
> >  http://pippin.gimp.org/image_processing/chap_dir.html#id2526295
> >  /Øyvind K.
> > --
>
> yes there does seem to be an issue here. I snipped the "generation 0" part
> of that image did File | Save As...
>   then reopened the new version and repeated the save several times.
>
> There is continual degradation. This should not happen with an identical
> image. This seems to suggest that either there is a bug in the compression
> or that the decompression is not producing an identical image from the
> stored data.

This is not a bug but a consequence of how the lossy compression of
JPEG works, hence  you should NEVER should use JPEG as an intermediate
format in your workflow, but only for publishing the end result. It is
theoretically possible, to keep the compressed version of unchanged
parts of an image around, and only recompressing in the neighbourhood
of regions that have actually changed by comparing with JPEG that one
is saving over. But a full decode/encode/decode cycle of JPEG / DV /
MJPEG etc will accumulate more and more errors/artifacts. This
accumulated error would be smaller with a higher default quality
though.

/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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