On Sat, Mar 22, 2008 at 10:48 AM, Patrick Shanahan <[EMAIL PROTECTED]> wrote:
> * norman <[EMAIL PROTECTED]> [03-21-08 13:06]:
> > I usually shoot in RAW and convert with UFraw plugged in GIMP. Then I
>  > have a choice of extensions to use for saving the file produced. I
>  > presume that if I use .xcf there is no compression and if I use .jpg
>  > there will be some compression. When working on the file I assume it is
>  > best to save as .xcf and not to use .jpg until the picture is finalised.
>  > I would like to know how the contents of the .jpg file so formed compare
>  > with the contents of a .jpg saved directly on the camera.
>  >
>  > The reason I ask is because I would like to use PTLens to remove CA but
>  > it seems that the software will only work with .jpg.
>   *compression* is not the problem as such.  "lossey" compression is
>  the problem, you *lose* some of the definition of your object/picture
>  when you *save* it as jpg.  jpg is a lossey compression scheme.
>  If ptlens is your *only* choice, you have no choice except to perform
>  that operation as the last operation.  You will still have a minimum
>  of two lossey saves.

What Patrick has said is true. It's also true that you can save your
jpeg with maximum quality in GIMP, and hopefully use a setting in
PTLens to save the processed jpeg with maximum quality*.. Given an
input picture of high resolution, the jpeg artifacts should remain
near imperceptible.

* if you can make PTLens use the same quantization tables as the input
jpeg, that would give best quality.

Another method that may work to minimize the effect of jpeg lossy
compression is to scale the image up (no interpolation) before
processing it, and scaling down afterwards.
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