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

[David Smith]

--- David Smith <areopagus125 at yahoo.com> wrote:
> > >> Doubly so for rotations.
> 
> If the image is rotated 90 degrees CCW, the new
> horizontal coefficients are the old vertical, while
> the new vertical coefficients have to encode a
> mirror
> reversal of the old horizontal coefficients.  I
> don't
> know whether this can be done without changing
> quantized coefficients.

Come to think of it, mirror reversal causes 1-d DCT
coefficients to be negated, suggesting rotation can be
done without further loss.  But 2-d coeffs, I'm still
not sure.

David Smith


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