Re: Photoshop Gamut warning vs ColorThink
Re: Photoshop Gamut warning vs ColorThink
- Subject: Re: Photoshop Gamut warning vs ColorThink
- From: email@hidden
- Date: Wed, 27 Feb 2008 20:40:47 -0800
Chris Cox writes on Wed, 27 Feb 2008 10:00:03 -0800
> A less extreme example would have had people debating deltaE tolerances
...
The issue at hand was not delta-E's per se'. The issue had to do with
making sweeping and misleading assertions regarding the general nature of
the Perceptual transform.
> Perceptual intents do NOT map all colors into the target gamut -
>they map some reference gamut (say, sRGB) into the target gamut ...
That's simply not true. All Profile Rendering Intents map all colors onto
or into the effective (B2Ax) output device gamut. And there is no one way
to do it.
For SWOP-like or inkjet gamuts, the percentage of grid points in the B2Ax
table lying outside of the device (A2B1) gamut is typically between 85% or
90% of the total number. Each of these grid points receives an inking
which implies that they are all gamut mapped to the effective device gamut.
How this gamut mapping is done is not specified by the ICC and is vendor
specific.
Some gamut mapping algorithms (GMA) optimize for a particular RGB space
which entails an effort to connect B2A grid pts near the major RGBCMY hues
of the RGB gamut to the corresponding hues on the surface of the CMYK
gamut. Others GMA's have a more generic approach and are not tumed to a
particular RGB colorspace but leverage the innate desirable properties of a
given colorspace (i.e. CIECAM02). But all grid pts outside the gamut (and
hence "all colors") are gamut mapped onto or into the device gamut.
For the Relative Colorimetric Intent, the spec requires in-gamut grid
points to be inked colorimetrically which means that gamut mapping is to
the gamut surface - or clipping as it is referred to in this forum. Some
vendors maintain clipping in the Percep/Satn intents while others feather
in the edges by slightly desaturating. Its been my experience that the
former has been the more prevalent GMA being employed.
The one thing a Perceptual transform must do is a smooth tonal compression
of the entire L* range (100 to 0 for v2 and ~3.1 for v4) down to the tonal
range of the device (L* of the Black Pt typically on neutral). This
operation does not affect gamut extent - indeed, for some vendors, the B2A0
& B2A1 gamuts are effectively the same size in both tonal and chromatic
extent.
Harold
Harold Boll | 1.781.856.5174 | email@hidden
" The sunlights differ, but there is only one darkness. " Ursula K.
Leguin
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