[argyllcms] Re: Camera matrix profile, adding ti3 perfect white data set

  • From: "Gerhard Fürnkranz" <nospam456@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 01 Feb 2010 23:44:08 +0100

-------- Original-Nachricht --------
> Datum: Mon, 01 Feb 2010 14:21:08 +1100
> Von: Graeme Gill <graeme@xxxxxxxxxxxxx>

> Hmm. I understand what you mean, but I'm really not sure how
> one would go about creating such a thing, since a cLUT inherently
> tries to fit in each local region.

Exactly - a nasty impact of CLUT profiles which fit the flare in the shot of 
the target is IMO that they are prone to clip dark colors when they are applied 
to other shots (of dark scenes, which suffer from less flare than the shot of 
the target).

[A matrix/gamma model may estimate a slightly too high gamma, but it still maps 
at least RGB=0 to XYZ=0, so that no image information is lost, but just a 
little bit of precision.]

Even more pronounced than with cameras is IMO the situation with film scanners. 
We have to deal with pretty high densities here. While the Dmax patch of 
well-made IT8 slide film targets has indeed a density of 3.5 or more, the 
scanner sensors "see" the Dmax patch with an apparent density of only say 
D=3...2.5 due to stray light from the neighbourhood. I.e. the magnitude of the 
actual signal (in linear RGB space) is well below the error here! Needless to 
say what's the implication when our goal is to accurately characterize the 
scanner over the full dynamic range of the target down to Dmax with a 
non-parametric model...

> Assuming the apparent gamma is caused by flare, then
> effectively it is necessary to remove the flare from the original
> readings so that the profile doesn't attempt to compensate for it,
> but what flare model should be used, and how is it determined?

For instance this approach

  http://graphics.stanford.edu/papers/glare_removal/glare_removal.pdf

sounds interesting, but it requires additional "equipment", and it's IMO beyond 
the scope of color profiling, but it's more an image processing task.

Regards,
Gerhard

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