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

[Smurf]

Hi,

Chris Cogdon:
> 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/