[argyllcms] Re: Measuring a colorcheker on screen
- From: Kai-Uwe Behrmann <ku.b@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 24 Jan 2008 09:03:01 +0100 (CET)
Am 24.01.08, 18:16 +1100 schrieb Graeme Gill:
> Kai-Uwe Behrmann wrote:
>
> > The display in CinePaint, even with the ICC absolute rendering intent, is
> > not CIE absolute. The later would be in need to do a real light
> > comparision. You measurings are in need to scale up or down to match the
> > monitors white- and blackpoint.
>
> Black point can be recorded in profiles, but isn't (shouldn't!)
> be used in any way to change the profile data.
Indeed the black point tag for display profiles says almost nothing, like
what the ICC spec requires: map to 0,0,0 .
> > A option in Argyll to include the measurements would help in later veryfy
> > the profile by external tools as what Lars did want to achieve. LProf
> > does the measurement embedding already, at least on request.
>
> I've been meaning to add this, but haven't got around to it yet,.
Fine to know you have it on a TODO list.
> > Does Argyll support CIE absolute calculations?
>
> In terms of returning the instrument measurement numbers,
> no. It doesn't need much though, just scale the XYZ
> values returned by the Argyll CMM by the Luminance value
> tag value (which I notice is currently missing, but I will fix this.)
It is not enough. The black point must be taken into accout.
A RGB 0,0,0 smaple on screen will have a XYZ different from 0,0,0. CMM's
should give XYZ 0,0,0 for ICC relative intent as it is required for
display profiles.
Like the scaling by a lumi tag the blackpoint shift must be taken into
account as well. At least this is what ICC Examin does to match the
measurement tag to the actual display profile.
kind regards
Kai-Uwe Behrmann
--
developing for colour management
www.behrmann.name + www.oyranos.org
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