[argyllcms] Re: [argyllcms] Re: FWA correction in XYZ space
- From: "Gerhard Fuernkranz" <nospam456@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Tue, 23 Aug 2005 18:06:23 +0200 (MEST)
> --- Ursprüngliche Nachricht ---
> Von: Graeme Gill <graeme@xxxxxxxxxxxxx>
> An: argyllcms@xxxxxxxxxxxxx
> Betreff: [argyllcms] Re: FWA correction in XYZ space
> Datum: Tue, 23 Aug 2005 09:48:16 +1000
> I guess it still leaves the issue of how to derive
> the transform parameters from a minimal set of
> measurements.
Graeme,
I've tested models with 3x3 matrix + offset, and 2nd order
polynomials. The former model has only 4x3 and the latter
only 10x3 model parameters, which could be dertermined
from 4 or 10 trichromatic measurement pairs. However, since
we're talking about data fitting, which eventually involves
measurement errors and also a residual systematic model error,
the "optimal" 4 or 10 patches should be picked to obtain best
overall model accuracy, or - even better - a least squares
regression with more than the minimum number of points needs
to be done.
As you see in my results, I have already tried to do the
regression with only 100 full-spread points, and to check
against 1700 points, and the error is nearly the same as
when the regression is done with all 1700 points (IMO no
statistically significant difference).
Sure, 100 points are probably not yet a "minimal set of
measurements" - I will also try the regression with even
fewer points and check the results.
Do you have any idea, how one could determine an "optimal"
small subset of my measurements for the regression (without
need to try e.g. 10^1700 combinations)?
> It also tends to validate Gretag's approach, which (I have
> been given the impression) is a XYZ_fwa_comp = f(XYZ) /
> Lab_fwa_comp = f(Lab) type of approach.
I don't know for sure, but I think Gretag's FWA compensation
is also only available in conjunction with *spectral*
measurements, is it? This would IMO suggest that they
rather use an approach like yours (or even use your
method?).
Regards,
Gerhard
--
Gerhard Fuernkranz
nospam456@xxxxxx
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