On 02 Feb 09 12:44 Alexander Rabtchevich 
<alexander.v.rabtchev...@iaph.bas-net.by> said:
> I guess lossless means the remained part ot the image hasn't been 
> re-encoded (it has the same jpg data as the original image).

That was what I understood when I made my initial reply. Hence I 
described the workings of a "jpeg file editor". Compare this with the 
GIMP, an "image editor".

In an image editor the image gets recreated from the data that is 
decompessed on loading the file. This reveals the imperfections of the
missing data thrown away when it was initially saved (as one expected 
with a JPEG file). The edges of that lossy image are then removed and 
then the file re-saved. Saving creates a new file and compressing the 
image once more, causing still more data loss to the image.

The jpeg file editor simply chops out the unwanted chunks of the 
existing compressed code from the file and rewrites that to the disk. 
It does NOT re-compress the file, hence the process is lossless (In as
much as it doesn't run a second compression on the original compressed
(and and admittedly lossy) data).

Greg Chapman

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