Re: Feature request: controlling the display white point

[Andrew Lutomirski <andy luto us>]

On Tue, Nov 19, 2013 at 11:31 PM, Graeme Gill <graeme2 argyllcms com> wrote:
> 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.

That's handy.  I'll try that next time.

> > 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.

True.  I don't have that luxury, though (I'm sometimes on a laptop and
sometimes on a computer in an office with a decent amount of natural
light).  I think there's some value in making it easier for people
without carefully configured workstations to get decent results.

> > 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 ?

Hmm.  Probably, since it's better than nothing.

> > 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...

That's why I cc-ed the g-c-m list.  It seems like Wayland may actually
end up with real systemwide color management :)

--Andy