[Openicc] Changing the display whitepoint using the VCGT

[Chris Murphy]

I'm guessing you mean more like 6000K+ whites if it's bluish, or 7000K whites if very bluish. 5000K is "yellowish" to most people.

I would categorize vcgt adjustment of the white point "unfortunate" but also a reality if it needs to be changed, that's your only option on these displays. Ideally the panel manufacturer would use analog adjustments to ensure a reasonable white point for the panel+backlight source used, so that video card LUT adjustment needed it minimal. So I'd say maybe not do it by default because humans do have a chromatic adaptation feature, but keep it as an option for those who expressly ask for white point adjustment? If you pull back on the blue channel, for example, you get a better white point, but then also get a luminance drop. And that luminance drop can be significant and a worse effect than a non-ideal white point.

But I think it's going to depend on the display. I look at my new Macbook Pro 2011 (high res matte screen) with a non-linear vcgt produced by the OS, which is something only Apple would know how to do, and with another profile that has a linear vcgt. With the linear vcgt the display is distinctly blue. I don't know that I'd adjust to this, maybe after a day or a week? Certainly not in a few minutes trying it out. The white point is OK, but tones below the white point (grays) are definitely very blue.

I don't know why Apple does it this way, rather than analog adjustments to the panel to make the entire TRC relatively neutral.

Chris


On Apr 26, 2011, at 8:48 AM, Richard Hughes wrote:

> Dear list,
> 
> My Dell mini display is very bluish by default (5000K whites), but
> when I calibrate this using argyll and a hardware calibration device
> the new profile sets a white temperature of 5700K and the display
> looks a lot more pleasant. A very blue native white is also a problem
> in some macbooks, and nearly all thinkpads.
> 
> I can easily create an RGB generated space using the EDID chromacity
> co-ordinates and the EDID gamma, and gnome-color-manager has been
> doing that for a few releases now. I'm wondering if it's sane to also
> adjust the whitepoint using the VCGT feature.
> 
> I've been playing about with cmsAdaptToIlluminant and using the VCGT
> which obviously does a linear conversion of white temperature, which
> seems to work fairly well. I've noticed that argyll-generated profiles
> seem to smooth to the maximum value a the top of the VCGT table and I
> wondered if that was a result of manual tweaking to ensure that
> 255,255,255 == display-white or may just be a peculiarity of my
> display.
> 
> I guess the real question is whether trying-to-be-helpful session
> software should try to change the whitepoint of a display to anything
> other than native or whether the VCGT table should just be left
> unmolested.
> 
> Advice very welcome,
> 
> Thanks.
> 
> Richard
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