[argyllcms] Re: fwa compensation unexpected results
- From: "Roberto Michelena" <colorsync@xxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 5 May 2006 06:54:36 -0500
I think you need to re-read your last sentence. I don't think it
makes a whole lot of sense. Modelling the spectra that reaches our
eyes is exactly what has to be done, to have a hope of modelling
how we see a color. If you don't have measurements to start with,
how can you go on ? I don't see that chromatic adaptation has
much to do with the subject at hand.
I agree that modelling the spectra is necessary to have a hope of
modelling perception. But that's not all, evidently, because I don't
see that paper as blue.
Case in point: my reference is Fogra or Gracol v7, with paper white
about 95,0,-2; moderate (usual) amount of FWA in the offset coated
paper is giving you that -2, while in fact we see it as perfectly
white (seen individually).
My proofing paper is b=-8.8 with L=95 (lots of FWA), which also if
seen individually looks white.
If we put both papers side by side, suddenly the offset paper looks
yellowish. Not that the proofing paper looks bluish, it's the other
way around. Perception here is the result of adaptation.
We make an abscol proof, and the proofer sees b=-2 reference, b=-8.8
proof, applies a lot of yellow ink. Put both the proof and the
reference together, and you see the proof is much yellower. Of couse,
the system 'saw' the proof paper as quite blue when we saw it as
white, in fact 'whiter' than the reference offset paper.
So we need to align the system with what we perceive. We then use a UV
filter to measure the proof characterization chart, resulting in b=-3
in white. Now the system applies just a tad of yellow to compensate,
we put both together, and they match.
When most commercial softwares talk about fwa compensation, they mean
simulating this effect. And this indeed improves the visual match in
proofing, as explained above.
And it's better than using a uv-cut filter; because there can be some
fluorescence in ink too, which should not be cut off because we don't
"adapt" to it, we do process it as color. UV-cut filters affect
saturated colors, while software compensation doesn't.
best regards,
-- Roberto Michelena
Infinitek
Lima, Peru
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