Re: PRMG gamut
Re: PRMG gamut
- Subject: Re: PRMG gamut
- From: Klaus Karcher <email@hidden>
- Date: Thu, 28 Feb 2008 21:49:40 +0100
Graeme Gill writes on Thu, 28 Feb 2008 13:49:10 +1100
The reference gamut is great if you like saturation rendering,
not much good if you want perceptual though.
(The implications of the ICC requirements that the
A2B table gamut mapping be the inverse of the B2A
>gamut mapping imply that the mapping be a saturation one.)
email@hidden wrote:
Hmmmm.... you're going to have to elaborate on this statement, Graeme. Its
not at all apparent to me why this would of necessity be the case.
The spec actually says its the A2Bx/B2Ax transforms that need to be
inverses of one another, not gamut mapping per se. As I see it, the PRMG
effectively reduces the size of the PCS volume of interest from the
humongous PCS Lab cube, down to a more manageable volume. If you're a v4
output profile builder and are tuning your profile to work with say an sRGB
v4 PRMG profile, you know that all colors delivered into the PCS will be on
or in the PRMG. You don't have to worry about the PCS volume outside that
region. You can then optimally re-position your B2A grid pts to take
advantage of this property. How you build your effective B2A output gamut
is in no way constrained by the PRMG and how you do the mapping onto or
into that gamut is also your choice. The presence of the PRMG, in this
respect, in no way limits the freedom we now have to do this in v2. You
still have to make sure that the A2Bx/B2Ax round trip, but we have to do
that under v2 too. The PRMG doesn't constrain the way we make choices to
differentiate Perceptual from Saturation, as I see it.
Your thoughts?
Well, I think it's getting exciting again ;-)
Here is Graeme's page with some notes about ICC v4 and the PRMG:
<http://argyllcms.com/doc8/iccgamutmapping.html>
It took a while until I understood and accepted his sobering conclusion.
I tried to illustrate the pitfalls schematically in this PDF:
<http://digitalproof.info/colorsync-users/GamutMapping.pdf>
Klaus Karcher
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