[Gimp-user] lossless cropping?

[Jeffrey Brent McBeth]

On Mon, Feb 02, 2009 at 01:37:24PM +0100, Claus Cyrny wrote:
> 
> I have to admit that, after following this thread, I still
> have no idea what Zhang Wei Wu means by 'lossless 'cropping'.
> I understand' lossless compression', but cropping is by definition
> lossy (you remove part of the original image), so I don't
> know what 'lossless' means in this context.

I'm going to take a wild guess on this (not having read the rest of
the thread).  If you were to take a lossy format (say, mp3 or jpeg),
and cut out portions of the data and then recompresses the remaining
data, the data that remains gets refiltered and suffers more loss
than that just due to the data cropped out.  On the other hand, both
mp3 and jpeg support something that could be termed lossless cropping
as long as you cut on the data boundaries (packet in mp3, 8x8 block on
jpeg) where the data you crop out obviously gets thrown out, but the
remaining data does not get refiltered with its resulting additional
informational loss.

Jeff

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