On 2014.10.06, 11:33 , Martin Brown wrote:
On 06/10/2014 14:19, PeterN wrote:
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.

The problem is not one of interpolation but that there are unavoidable
minor rounding errors in the nonlinear transform from RGB to CIELAB and
also on the way back due to the finite representation of the results. See:


Excellent point.


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