[argyllcms] Re: inter-instrument matching tables
- From: Roberto Michelena <colorsync@xxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 10 Nov 2011 09:27:49 -0500
Thanks Gerhard, great starting points... now that I think of it, it's
likely that the errors magnitude are to some extent a function of hue,
chroma and lightness (i.e., larger in saturated reds, smaller in dark
blues, so on..).. maybe a better correlation could be found with LCh
or HSB instead of XYZ or Lab, and that'd reduce the polynomial order
of the function. So I guess the first task would be to find the proper
space.
I was indeed thinking of the "refine" command, but I wonder if it'll
work properly when dealing with such small tweaks (mostly under 1dE),
or would do more damage than good. Do you know if the "refine"
algorithm has provisions for not overshooting?
-- Roberto
On Wed, Nov 9, 2011 at 8:17 PM, Gerhard Fuernkranz <nospam456@xxxxxx> wrote:
> Am 09.11.2011 20:24, schrieb Roberto Michelena:
>>
>> What should I use to create the "correction link profile" (Lab to Lab)
>> when I have the set of two measurements (master and individual) to be
>> corrected?
>> As long as it's restricted to one paper/inkset combo, will it be
>> useful even to uvcut instruments?
>
> I guess that this can be indeed feasible, at least for one particular
> paper/ink/printer/... combination.
>
> Good question what's the best model for this kind of correction. One would
> need to "play" a bit with the measurement data and evaluate different models
> in order to get a feeling which model works how well.
>
> I'd possibly start by investigating a NxM matrix model in spectral space
> (where N is the number of spectral bands reported by the first, and M the
> number of bands reported by the 2nd instrument), since this approach would
> even yield corrected _spectral_ readings (and not just tri-chromatic ones).
> As the rank of the reflectance spectra is supposed to be significantly lower
> than N or M, I would not derive the correction matrix directly in spectral
> space, but rather derive it in a dimensionality-reduced subspace of the
> spectral space (and possibly even then some additional regularization may be
> necessary).
>
> A correction in CIELAB or XYZ space would likely need to be a non-linear
> function. Since the inter-instrument difference is (hopefully) small, I'd
> possibly start trying a 3x3 matrix model (XYZ space only) and/or 2nd order
> multivariate polynomials (for both, XYZ and CIELAB). However, models which
> operate in tri-chromatic space (XYZ or CIELAB) are basically only valid for
> one particular black generation or separation (I don't have a feeling for
> the magnitude of the errors, when the same correction is nevertheless used
> for different separations).
>
> Non-parametric, "flexible" models (like splines, RBF, neural networks,...)
> could be used as well, given a sufficiently large number of readings, but
> IMO one must be careful to limit the number of effective parameters by
> proper regularization. Equally, I'd also avoid using polynomials with too
> high order for this use case.
>
> If you don't want to do any maths or programming yourself, then I think you
> could possibly "abuse" the the Argyll "refine" command for your purpose,
> which creates an abstract profile (i.e. CIELAB -> CIELAB) from two .ti3
> files (what you get here is a non-parametric smoothing spline model).
>
> Regards,
> Gerhard
>
>
>
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