[argyllcms] Re: noise and gamut, input profiles with B2A table

  • From: Gerhard Fuernkranz <nospam456@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 01 May 2008 00:02:16 +0200

Graeme Gill wrote:
> I was also debating as to the correct way to make scanner profiles.
> Technically I think I should do the opposite of what I'm currently
> doing: I should be making a rspl of the CIE -> device values, then
> inverting it to create the device -> CIE. I'm currently cheating
> (as I suspect almost all other profile software does) and simply
> applying the rspl to the device->CIE pairs.

Graeme, that's actually a good point. I was aready wondering the same
too for a while. Basically we're dealing with a measurement error model
(errors-in-variables (EIV) model) here, since neither the CIE nor the
RGB numbers are error-free, but both are measured numbers and thus
contain measurement errors. My understanding is that usually rather
methods based on total (orthogonal) least squares are used in such a
case. I've no idea though, whether it would be easily possible to adapt
RSPL so that it minimizes the orthogonal error between fitted curve and
the data points, instead of minimizing the error of the dependent
variable only. And yes, if we assume that the CIE measurement errors are
negligibly small compared to the RGB errors, then this leads to the
approach to do the fitting the other way round, i.e. treat the CIE
numbers as independent and the RGB numbers as dependent variable. But
I'm not sure, whether the assumption of negligible CIE measurement
errors is granted either, particularly for batch average targets, which
are not individually measured.

Regards,
Gerhard

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