On 10/5/2014 10:37 PM, Alan Browne wrote:
On 2014.10.05, 20:55 , PeterN wrote:
On 10/5/2014 6:57 PM, Alan Browne wrote:
On 2014.10.05, 14:42 , PeterN wrote:

We went through all this some many months ago. I demonstrated clearly
that the amount of 'loss' was negligible in practical terms.


I would use the terem "color change." anstead of loss.

Any change is a quality loss. Whether that is colour difference, tone,
brightness, sharpness ... whatever, it's a loss.


Then you are using a different definition of quality.

Not at all. A non lossy process would have:

RGB-A -- X-format -- RGB-B

with RGB-A identical to RGB-B

But - the fact is that with Lab

RGB-A -- Lab -- RGB-B

RGB-A =/= RGB-B, therefore there was quality loss.


It seems to me that the assumption in that logic is:
the quality of RGB-A quality of RGB-B.
LAB has a larger color gamut than RGB. If there is no processing in LAB
I would think that there would be no need for interpolation on the
return trip.

Do the round trip x + 10 times without processing and one might see a
difference. It is doubtful that there will be a noticable difference
from 10 round trips. Meanwhile there are color modification processes
that are easier to perform in LAB than RGB. I would think that if the
changes made in LAB created color outside the RGB gamut there would have
to be some interpolation. The interpolation coud mae a better image, or
it could make the changed image horrific.

In another area, I have found images to be fine with a color cast, but
when I remove the cast, to my eye the image looks horrific.

--
PeterN