Re: Feature request: controlling the display white point

[Graeme Gill <graeme2 argyllcms com>]

Andrew Lutomirski wrote:

Hi,

> The last time I dealt with this (on my old Lenovo X200, which I barely
> use anymore) I got annoyed by how hard it was to see how well a given
> white point would work.  Some easy way to quickly estimate how much
> brightness I'd lose for a given white point (and even a way to preview
> a white point) would have been helpful. 

The dispcal interactive White Point adjustment would seem to be exactly
what you want then. Simply note the x,y you end up with,
and then restart dispcal with that as the target white point.

> On the other hand, aren't there some instruments that can quickly
> measure the ambient color?  For more accurate color work, I can
> imagine it being useful to be able to quickly adjust settings in a
> daylit office throughout the day as your eyes' adaptation changes.

That's asking a lot - you are expecting the display system to compensate
for viewing condition changes. While there are models for such things
(ie. CIECAM02), they are not perfect, and you risk getting odd results,
as well as demanding very thorough display characterisation and the full
cooperation of the operating system/desktop and the applications you use.

The usual approach is the opposite - eliminate changes in the lighting
of the environment where color is being critically judged, thereby
allowing simple fixed setup of the display and profiles, and ensuring
consistency.

> I suspect (or hope, anyway) that most decent displays are reasonably
> close to being additive.  (Isn't this necessary for matrix profiles to
> work right?) 

Yes, most are. LCD's tend to be less additive than other technologies,
but it's all about degree. Is additive within 5 delta E acceptable, or not ?

> If you assumed that a display were close to being
> additive and that the calibration, once applied, had the correct
> effect, you could try to measure (once) the response of each color
> channel to compute something like a matrix profile, only with an
> actual curve instead of just a gamma value.  Then you could, assuming
> the resulting profile were correct, choose a white point and calculate
> a gamma ramp and real matrix profile to apply it.
>
> Would this work reasonably well on modern LCD displays?

There is a long history of different models for profiling displays.
You can get an idea yourself simply by choosing different models
in colprof. A matrix/shaper profile has the form you describe above,
and the fit error report at the end of the profiling will give you
a measure of how well it fits the measurement. Then compare to an
XYZ cLUT profile type. Each model has tradeoffs of accuracy vs. smoothness.

> Hmm.  Hopefully it won't be too long until compositors color manage
> everything (maybe with 10 bit output), so this might be doable.  If
> that happened and people wanted to play with white points, it might
> pay to prefer a calibration for the native white point to maximize the
> gamut.

It seems hard enough to get the OS/GUI people to take color seriously
at all, nevermind add support for "cool color stuff". Traditionally
one is reduced to using whatever mechanisms are available, such
as Graphic card LUTs, because properly architected mechanisms
are simply not available...

Graeme Gill.