[filmscanners] Re: JPEG Lossless mirror?

["Robert Meier" <robert_meier_photo@yahoo.com>]

--- TonySleep@halftone.co.uk wrote:
> Re-encoding *unchanged* data at the same compression setting
> gives
> no additional loss.

It does give an additional loss. Nevertheless, the additional loss is
very small, much smaller then what you lose when you store a tiff image
with the highest jpeg quality in PS.

> The question is whether flipping constitutes
> changing,
> and I think it probably doesn't, but I don't know the JPEG spec well
> enough
> to be sure. I am sure someone will.

To flip an image you do not have to decode and code it again. You just
re-arrange the DCT coefficients. Considering the jpeg algorithm I
believe it will work only on images with a multiple of 8 pixels in the
flipping direction.

Robert

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