[Openicc] Changing the display whitepoint using the VCGT

Richard Hughes hughsient at gmail.com
Wed Apr 27 07:12:01 PDT 2011

On 26 April 2011 23:27, Chris Murphy <lists at colorremedies.com> wrote:
> 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.

Yes, sorry. My T510 is D65 native.

> So I'd say maybe not do it by default because humans do have a chromatic adaptation feature

Yes, until they are comparing two screens...

> And that luminance drop can be significant and a worse effect than a non-ideal white point.


> 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?

Most people I talk to at conferences just think the blueish is the
"normal" and when calibrated everything looks too "gray". :-)

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

I think software is cheaper than hardware.

> ...they have particular information that they know about this display, which is how they automatically generate an ICC profile with such a radical.

The benefits of controlling the hardware.

> So that's a way to end up with a non-linear vcgt to try and correct for color temperature not just at the white point, but throughout the TRC.

Yup, and I don't think we can do it sensibly without a sensor (i.e. a
colorimeter or the human eye), certainly not just from 4
chroma-coordinates. I don't know if allowing the user to adjust the
linear color temperature using the VCGT is such a good idea.

I've also noticed that all the generated VCGT tables in existence map
1:1 at the display white, i.e. even when changing the blue channel
drastically, the VCGT still scales to linear past about 95%. I assume
it's to avoid loosing luminance at native white. Ideas welcome.


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