[argyllcms] Re: dispwin vs xcalib loading LUT

  • From: Kai-Uwe Behrmann <ku.b@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 1 Jan 2007 12:00:17 +0100 (CET)


Am 31.12.06, 11:36 +0100 schrieb Frédéric:

> I have a strange behaviour with xcalib vs dispwin when loading LUT. They do 
> not seem to read the vcgtag the same way, depending of the profile.
> Let me try to explain...
> I attached a profile coming with xcalib package. This profile has a 2.2 
> gamma. When loaded with xcalib, it does not change anything to my 
> uncalibrated screen (no previous profile loaded). But when loaded with 
> dispwin, the screen becomes very bright.

xcalib should change the grafics card gamma table as described in the vcgt 
tag. I would guess xcalib is not supporting the parametric entry (~2.2) in 
opposite to the most often used curves. 
The ICC Examin grafic cards gamma window (ctrl-t) window should show you 
the current state of the gamma table in your grafic card. A vcgt entry of 
2.2 is big and should affect the calibration and appearance of the 
monitor clearly.
> *But*, when I load an Argyll generated profile (also attached), they both 
> give the same result!

The argyll profile uses a curved vcgt tag.

> I had a look in the profiles, with iccexamin, and I saw differences; it 
> seems that the vcgtag is linear in Argyll generated profile, even if it is 
> a 2.2 gamma profile. But I also see that the [rgb]TRC tags are, in both 
> cases, the same (2.2 gamma curved).

The vcgt tag is independent of the TRC curves. Vcgt stores a calibration,
inside the profile. In opposite the TRC curves describe the 
colour space after the calibration or the appliance of the vcgt curves to 
the grafic card LUTs.

> Graeme, can you explain me how exaclty dispwin loads LUT? Do you know why 
> xcalib does not give the same results only on its sample file?

Kai-Uwe Behrmann
development for color management 
www.behrmann.name + www.oyranos.org + www.cinepaint.org

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