[Openicc] Questions about color pickers and graphics libraries under LINUX

[Chris Murphy]

On Feb 12, 2008, at 9:18 PM, Hal V. Engel wrote:
> In addition X11 should to be extended to have LUT loader  
> functionality or at
> least to allow users to specify a LUT loader that X11 calls at  
> start up or
> when ever the ICC profile atom is changed so that it not only keeps  
> track of
> the profiles that have been setup but also automatically loads the  
> video card
> LUTs when needed.  I would like to see this support both normal  
> video card
> gamma tags in profiles and ArgyllCMS style calibration files.

Yes. It continues to baffle me why Microsoft refuses to support the  
vcgt tag in ICC profiles.

Now if only the "language" of communicating LUT information to  
displays themselves, for the higher end displays (where the video  
card LUT is always linear), could be standardized, and then it could  
also be possible for x11 to upload that LUT as well, if such a  
display is used. But as you point out later, those LUTs are not  
volatile. I don't know how long it remains resident.

> On the other hand unlike the LUTs in video cards which are volatile  
> the LUTs
> in LCD monitors appear to be persistant (if I am wrong about this  
> please
> correct me).  So I don't think X11 has a role in this area since I  
> think this
> should be handled by the monitor calibration software.  Unless you  
> are also
> proposing that X11 should have monitor calibration functionality.    
> But I
> don't think this is what you had in mind.

You'll need an explicit bypass of color management and calibration so  
that such applications can get at the "uncorrected" behavior of the  
display. These apps will have to know that they must use this bypass,  
which turns off display compensate *and* returns the video card LUT  
to linear.


> What I am getting at is that X11 needs to work into being fully  
> color managed
> in a systematic way by first supporting full management of the  
> video card
> LUTs (this would actaully be a step up from what current versions  
> of Windows
> Vista do in this area) and then later on putting in place basic  
> sRGB to
> display color management with an opt out option for applications  
> that are CM
> aware.

The opt out for CM aware apps could be as simple as application  
asking CMS what the current display profile is, and then application  
tags all data going to x11 with that profile as source.  
Source=Destination means null conversion.


> This would make X11 more advanced in this area than any other
> windowing system in existance.   Later still X11 could add hooks  
> for CM aware
> applications to have control over content color space and rendering  
> intent
> but with X11 handling the content to display color transformation  
> functions.

I agree with making the state of the video card LUTs the domain of  
x11 and take it away from applications. Application developers should  
get used to the idea of this not being something they should mess  
with directly.

Case in point, the shipping version of Windows Vista used video card  
LUT modification for a particular screen dimming effect when  
displaying security related dialogs for user input. When these  
dialogs went away, the LUT was actually *reset* not *restored* to  
previous values. So calibration is functionally wiped as soon as you  
experience one of these effects. Now, this is a case of some part of  
the multi-tentacled beast that is Windows development, going around  
their own window server and directly adjusting LUTs. Frequent  
offenders are game developers because they can use these for fast  
global display adjustments.

Chris Murphy