Re: U.S. Web Coated (SWOP) v2 always produces blues that shift?
Re: U.S. Web Coated (SWOP) v2 always produces blues that shift?
- Subject: Re: U.S. Web Coated (SWOP) v2 always produces blues that shift?
- From: Harold Boll <email@hidden>
- Date: Thu, 28 Jun 2007 15:58:42 -0400
Andrew Rodney wrote:
> Here's one quote from an industry expert who believes that
> the TR001specification as reflected in the U.S. Web Coated
> (SWOP) v2 profile built byThomas Knoll in Photoshop always
> produces a blue shift. I was hoping some here with
> experience with this process could comment. .......
I can provide some indirect evidence that the above
profile is probably doing the right thing in this region
of colorspace.
The actual problem is Blues-going-Purple: colors that should be
blue have too much magenta and hence appear purple. This comes
about by doing a certain kind of gamut mapping in CIELAB
colorspace where you map along constant radial lines of Hue.
While CIELAB does a reasonable job of being perceptually linear
in many parts of the space, it falls flat on its face in the Blues.
CIELAB also suffers in the reds which go slightly orange, but not
to the degree that blues go purple. Doing this Hue preserving gamut
mapping in CIECAM02 eliminates these problems.
As a profile builder, I've spent considerable effort analyzing
US Sheetfed Coated (V2), a high quality profile if there ever was
one. Insights that I've gained on this Profile probably apply to
the Web Coated profile as well I should think. (Although Knoll
may not have built this profile - not sure). I've looked at the
gamut mapping vectors in this region and seen that they are not
Hue preserving and in fact smoothly bend toward Cyan which cures
the Blues-going-Purple problem. I've looked at the global
separations of both profiles and they are of a similar quality.
I would thus surmise that the same gamut mapping strategy was
used in the Web Profile as was effectively employed in US Sheefed
which cured the problem.
Harold
TOSHIBA AMERICA BUSINESS SOLUTIONS 2 Musick, Irvine, CA 92618
" The sunlights differ, but there is only one darkness. "
Ursula K. Leguin
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