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

[David Smith]

> >> 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.

True.

> >> 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.

> Secondly, there is SOME 'holistic information' kept
> from cell to cell, 
> or at least across the entire picture; I've seen
> corrupted JPEGs where 
> the 'base colour' after a particular point has gone
> wildly awry.

That's true of any losslessly compressed file that
gets corrupted in transmission.

Jpeg's information loss consists of quantizing and
thresholding the coefficients in the 8x8 block.  The
compression (entropy encoding) of those coefficients
(which uses context from previously encoded blocks)
proceeds without further loss.

HTH, but I don't claim to be an expert.  

David Smith


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