[argyllcms] Re: CMP Digital Target 3 (not 003)

  • From: Gerhard Fuernkranz <nospam456@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sun, 08 Mar 2009 15:10:58 +0100

Pascal de Bruijn wrote:
> I've tried Multiple Shapers too, but got odd (and horrid) results. It seems 
> logical that a single shaper should be used, as there is no direct reason to 
> believe that the CMOS/CCD sensor should have a different response curve for 
> each of its color components.
>   

Most sensors (and their ADCs) have a pretty linear response per se, and
it's in many cases rather the raw converter which explicitly applies a
(hopefully well-know) gamma to the converted raw images. Therefore if
one deals with camera raw images then it may IMO even make sense to
assume fixed shapers [preferably gamma 1.0, as captured by the sensor,
or otherwise the well-know gamma value which was explicitly applied by
the used raw converter] and to estimate the matrix only [Graeme, I guess
it should not be too hard to add an option to colprof for prescribing a
fixed gamma for the shapers, instead of estimating them along with the
matrix].

[Btw, for instance, fitting the GSxx ramp of an IT8 target in order to
determine the three response curves of the R, G and B channels is
certainly subobtimal, IMO. While the reflectance spectrum of Dmin on an
IT8 target is still reasonably flat, it becomes more and more "bumpy"
for daker GSxx patches. As a result, the estimated sensor response
curves won't match the actual intensity response of the sensor (i.e. the
response to a _fixed_ spectrum at varying intensity). The deviation of a
linear sensor from a true linear response is likely smaller than the
error of the response curves estimated in this way. Thus simply assuming
gamma 1.0 for a sensor which is known to be linear may be more
appropriate than trying to estimate its response.]

Regards,
Gerhard


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