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

Martin Brown
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SubjectRe: Is RGB to Lab lossy? - was(Re: Lenses and sharpening)
FromMartin Brown
Date10/08/2014 11:17 (10/08/2014 10:17)
Message-ID<qi7Zv.645894$ZX5.380005@fx32.am4>
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Newsgroupsrec.photo.digital
FollowsEric Stevens
On 08/10/2014 00:17, Eric Stevens wrote:

Eric Stevens
On Tue, 07 Oct 2014 17:26:23 -0400, nospam <nospam@nospam.invalid> wrote:

nospam
In article <slrnm38mt9.fqc.mr@irc.sandman.net>, Sandman <mr@sandman.net>wrote:

Martin Brown
Since CIELAB is a colour space intended to manage just noticeably colour differences more optimally than the naive RGB colour space it isn't too surprising that you cannot *see* a difference in the final JPG taken from RGB or via CIELAB. But they are very slightly different.

Eric Stevens
Agreed, but the question is, does the difference matter?

Sandman
That wasn't, however, "the question", Eric. You quote Dan saying this:

"RGB>LAB>RGB is damage free"

That is an incorrect statement, which nospam has corrected. That is all.

It does depend here rather critically on what you mean by damage free.

If you mean is it strictly lossless then the answer is no, but if you mean can you actually *see* the difference between them then the answer is yes. You would get a *much* larger image content variation if you had delayed pressing the shutter release by 1us.

Naive RGB has far too many irrelevant shades of not quite green. They are all numerically distinct but the human eye cannot tell them apart.

http://en.wikipedia.org/wiki/Color_difference#Tolerance

nospam
yep.

Eric Stevens
But you are also arguing that the damage is such that conversions to Lab should be completely avoided. You are heaping abuse on Dan Margulis who teaches how to use Lab. You are not just arguing that Lab conversions cause damage (no matter how infinitesmal that damage may be). You *hate* the idea of Lab conversion and froth at the mouth when the possibility is mentioned. Your opposition to Lab is totally irrational.

Why does that surprise you?

It is typical nospam sophistry.

-- Regards, Martin Brown