[argyllcms] Re: Understanding display profiles

  • From: Volker Sauer <vsauer@xxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sun, 20 Jul 2008 13:28:36 +0200

On So, 20 Jul 2008, Graeme Gill <graeme@xxxxxxxxxxxxx> wrote:
> Volker Sauer wrote:
> >To achieve an accurate output, do I always have to load the profile in
> >the software (in my case GIMP) *and* the LUT? Or do I have to load the
> >profile in the software *or* the LUT?
> You need to distinguish between the calibration and the profile.
> The calibration is optional, but if it is uses, it must always
> be used for the profile to remain valid, since they are in series,
> so the profile represents the calibrated state of the device.
> Typically the calibration is also stored in the profile in the vcgt tag.
> Assuming that GIMP just uses the profile information and doesn't
> load the calibration (vcgt) into the video hardware, then if you
> created a calibration and used that calibration during profiling,
> then you need to make sure that both the calibration is loaded
> and the application (GIMP) uses the profile.
> If you did not create and use a calibration during profiling,
> then (of course) there is nothing to load into the VideoLUT
> (or rather, they should be set to linear, just like they
> were during profiling), and just the profile can should be used
> with the application.

I haven't noticed GIMP changing the calibration of the display, so I
think it only loads the profile and doesn't use the vcgt tag.

Concerning calibration:
I though calibration was the process of bringing the display to a known
state... I didn't know that the vcgt tag of the profile which is loaded 
into the LUT is called calibration as well. 
You're writing "if you created a calibration" - I still do not
understand this completely and I'm confused:

what I basically did is:

dispcal -y l -o -t 6500 -O "G550 NEC1990 left" -qh -gs left

My display is set to:
- Brightness 50%
- Contrast 50%
- Black Point 50%
- Colortemp: sRBG (it offers that as a special setting)

which gives about 110cd/m^2 and 6400k temp. I have tried to set this to
120 and 6500, but still I'm confused how to use all the controls on the
displays - and if I really need them....??

If I examine the icc profile with iccexamin, I see the curves for the 
LUT in the vcgt tag.

Did I do a calibration?
What exactly needs to set up in the dispaly? If I specify -t 6500, do if
have to set the display to 6500 is will he calibration do that??
What's the difference between the .cal and the .icc file?

What can I do about my special situation - I cannot calibrate the second
screen and calibrating only the first gives an annoying difference in
colors - can I use colormanagment at all this way?


P.S.: I read the docs over an over. Maybe the whole thing with
calibration and profiling needs to be explained differently, since I
still don't get it.
  Volker Sauer  *  Poststrasse 1/601   *   64293 Darmstadt  *   Germany
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