[Image-SIG] Manipulating JPEGs w/out decompressing?
smurf at smurf.noris.de
Mon Jan 26 15:35:05 EST 2004
> 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.
JPEG is based on 8x8 cells. (Hence the blocky artefacts.)
So you can in fact chop them by 8-pixel boundaries, and 90-degree rotate
them by swapping the coefficients in the individual blocks as well as
the blocks themselves, and probably some other minor stuff.
JPEG2000, now, that's a wholly different kettle of fish.
Matthias Urlichs | noris network AG | http://smurf.noris.de/
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