>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.

Yes, that's it. When people talk about lossless jpeg modifications, in 99%
they are referring to "elimination of cumulative losses introduced by repeated
compression and decompression". (And the other 1% ... there is a kind of
JPG-based format that is lossless, but there is almost not support for it in
common applications and it has poor compression ratio and hence negates the
advantage of classic JPG).

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