[Image-SIG] Manipulating JPEGs w/out decompressing?

Chris Cogdon chris at cogdon.org
Mon Jan 26 14:45:50 EST 2004

On Jan 26, 2004, at 09:54, Chris Barker wrote:

> My understanding of JPEG compression is that if you compress and 
> de-compress multiple times, more information is lost each time. If I 
> understand this right, it seems a shame to degrade an image just to 
> rotate or crop it.

The way that I understand JPEG is that the encoding for any particular 
pixel depends on 'state' that is set up from the previous pixels. In 
other words, if you were to crop off 1 or 2 pixels at the left hand 
side of the image, all the following pixels MUST be re-encoded... which 
means you get the re-compression that you were trying to avoid.

Doubly so for rotations.

In my view, JPEG images were never intended to be re-edited.

    ("`-/")_.-'"``-._        Chris Cogdon <chris at cogdon.org>
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