[argyllcms] Re: The different color temperatures: questions: Please look again!

On Thu, 2009-04-16 at 18:27 +0200, "Gerhard Fürnkranz" wrote:

> In Calibration mode, the monitor uses its native (wide gamut) primaries, and 
> the luminance, white point chromaticity and gamma targets are established by 
> NCE (and eventually the resulting calibration is stored in the monitor).
> 
> In Emulation mode, the XL20 attempts to emulate the characteritics of a 
> different monitor. When you set up the Emulation mode with NCE then you have 
> to supply the ICC profile for the (other) monitor which should be emulated by 
> the XL20, and a luminance target. I.e. the major difference between 
> Calibration and Emulation mode is, that in Emulation mode the monitor won't 
> use it's native primaries, but emulate different ones.
> 
> sRGB and AdobeRGB mode are just special cases of Emulation mode, where the 
> XL20 emulates the sRGB color space or the AdobeRGB color space.
> 
> > Finally, I presume all this works only under Windows
> > (or MacOS), and not under Linux.
> 
> No, not under Linux...
> 
> You can of course calibrate the monitor once under Windows, and then use the 
> calibrated monitor subsequently under Linux.
> 
> You possibly still need to fine-tune it under Linux with dispcal (using the 
> graphics card LUTs), since the given gamma target and gray balance are not 
> established by NCE too accurately (at last not on my XL20). The white point 
> target on the other hand is established by NCE pretty accurately (thus 
> dispcal w/o specifying any WP target should suffice).
> 

Thanks for confirming what I thought and clarifying it further.

I am still a bit unclear about just exactly what happens in the monitor.

With the usual method using argyll, there is a vcgt tag which is loaded
in the video card.   I checked the default .icm file that came with the
monitor and one I made using the Huey under Windows.   (I did it in
custom mode, which was presumably wrong, but that doesn't matter for
what I say below).  Neither of these had a vcgt tag.   I presume this
means that what would normally go in that tag is (semi-)permanently
stored in the monitor and I don't get to see it.   Presumably the
two .icm files are meant to be used by Photoshop and other color aware
applications in the usual way.

Do I have that more or less right?  If so, using argyll under Linux,
assuming I can skip making the vcgt tag, I can produce a profile and be
in business.

By the way, The profiles appear to be matrix profiles.   At one time, I
understood exactly how they work, but I can't remember the function of
the TRC part of the profile.  As a mathematician, I have no problem with
the matrix part. 

> > If indeed there is some way to load data under Linux
> > directly into the monitor, I should know about it. Perhaps
> > ddcontrol, mentioned by Kurt, can do that, but so far I
> > haven't been able to get it to compile under Fedora 9
> > because of problems with the version of pciutils provided
> > by Fedora.
> 
> Don't waste your time, ddccontrol can't (at least not currently).
> 
> I've already spent some efforts to trace NCE's I²C communication with the 
> monitor. I already managed to get a hex dump of the packets, but it seems not 
> to be trivial to interpret the data (although there seem to be not so many 
> different commands involved, when a calibration is carried out).
> 
> Regards,
> Gerhard
>

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