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