> Facts are as I know them to be:
> - Jpeg is a lossy compression. That means you will lose some information
> when saving with Jpeg compression
> - When you open a jpeg file to Photoshop (for instance), it becomes just a
> raw pixels image in PS. If you then save it from that, it will apply the
> compression to the file again - after all, it assumes this is another
[..snip..]
> saves and openings. And after just a 3-4 rounds, the change was already
> quite small. This was surprising as I've never seen a mathematical proof
On one photoshop manual, the jpeg compression was discussed. The jpeg
algorithm is a lossy one. Depending on the compression level, the
algorithm
is used with cerain values. During the first save, there are some pixels
that are "on the edge" of having another value. After the save, some
nearby pixels have different values (because of the algorithm) and that
causes those pixels that were "on the edge" to go "over the edge"
during the next save. Naturally the number of pixels "on the edge"
degrades on every save, finally coming to a stable situation.
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