[argyllcms] Re: fwa compensation unexpected results
- From: "Roberto Michelena" <colorsync@xxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 5 May 2006 00:44:44 -0500
You have a omitted a critical piece of information :
what viewing illuminant did you tell profile you were going to use ?
<snip>
Consider: the instrument uses a tungsten light source, 'A' spectrum.
It has a moderate level of relative UV in it, and gives you a b = -8.8.
D50 has a slightly higher level of relative UV, so the FWA will get more
stimulation, and will get more blue. So b = -11.3 sounds like it's working
just as expected.
ok, understood; so your FWA compensation is actually more like a
simulation of the FWA behaviour under a different illuminant.
Usually when speaking of FWA compensation in commercial software
packages, it means aligning the measurements with human perception, in
other words simulating the effect of chromatic adaptation to the
substrate.
basically color management for proofing (assuming proofer gamut
completely contains reference gamut) can be reduced to three problems:
1) acquire reliable (low noise, repeatable) colorimetric or spectral
measurements of reference and proof eliminating influences from gloss,
translucency or other undesirable phenomena
2) generate a theoretical (read: numeric) metameric match between the
reference and the proof
3) ensure that those numbers do correlate with perception
#1 is instrument design and of course dependant on your proofer's
repeatability and noise levels
#2 is just the mathematic ability of the profiling algorithms and the
CMM; here there's debate between using a large patch number in a
single profile building step, or using iteration which effectively
amounts to a large patch number. And then also between using a PCS in
the conversion or using it only at the iterating stage but making the
conversion directly (link profile).
#3 is the real catch. Whereas #1 and #2 are totally objective and
measurable, #3 is where science still falls short. How many times
you've got a proof that verifies to less than 1dE from the reference,
and yet visually does not match. Different content of FWAs in proof
and reference is one common cause, and different illuminants are also
to blame.
So the way you're doing FWA adaptation... well, it's correct in some
theoretical aspect but it's not what's usually needed. You're
modelling the spectra that will reach our eyes, but in doing so you're
failing to model the color that we'll perceive after chromatic
adaptation occurs.
-- Roberto Michelena
Infinitek
Lima, Peru
Other related posts:
