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Re: Is RGB to Lab lossy? -...

Alan Browne
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SubjectRe: Is RGB to Lab lossy? - was(Re: Lenses and sharpening)
FromAlan Browne
Date10/06/2014 15:55 (10/06/2014 09:55)
Message-ID<K46dnXhaPvLZAq_JnZ2dnUU7-V-dnZ2d@giganews.com>
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Newsgroupsrec.photo.digital
FollowsPeterN
On 2014.10.06, 09:19 , PeterN wrote:

PeterN
On 10/5/2014 10:37 PM, Alan Browne wrote:

Alan Browne
On 2014.10.05, 20:55 , PeterN wrote:

PeterN
On 10/5/2014 6:57 PM, Alan Browne wrote:

Alan Browne
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.

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

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

PeterN
Then you are using a different definition of quality.

Alan Browne
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.

PeterN
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.

It's not about colour gamut since the "larger gamut" of LAB can't inherit anything better from the lesser gamut in the RGB image. If it was missing in the RGB it won't magically appear in the LAB. You'll just have more room to maneuver in the LAB version when you edit there.

It's not about "need for interpolation" - it's about what changed. Any change is loss. Whether it occurred in step RGB->Lab or Lab->RGB is not relevant to the end product.

If the end product changed then there was loss.

Any deviation from the original, whether more or less colour, lighter or darker tone, etc. is loss of information (quality) from the original. Period. There is no interpretation to do. It's a strictly technical thing. From a quality standpoint it is loss.

*to re-iterate: this is all quite pedantic as the actual amount of loss (change) is not visible in practical terms, edge cases aside.

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