colour management and dual-head displays

Hal V. Engel hvengel at
Sun Jun 1 10:50:47 PDT 2008

On Sunday 01 June 2008 07:56:49 Marius Gedminas wrote:
> Somewhat tangentially to the colour management discussing currently
> going on: My desktop spans two different monitors (laptop's internal LCD
> + an external monitor).  These two monitors show the same RGB values
> differently.  What needs to be done to have applications look the same
> on either monitor?  What about windows spanning both monitors?
> Could this be an argument for the composition manager to handle
> color-space conversion, at least in some cases?  Figuring out which parts
> of which widgets fall on which monitor might be a bit too much of a
> complexity to require from the toolkits and applications.
> Or is this use-case unreasonably hard to support?  I also wonder about
> clone modes.
> Marius Gedminas

This can be a fairly "simple" calibration issue if your video drivers support 
the nessary features.  The monitors need to be setup with the same:

White point
black point
white level
black level

If you have a basic X11 setup that supports setting per device video card 
gamma tables and you have a supported measurement device this can be done 
with a little effort and once setup is not hard to support and maintain but 
the initial calibration can take a little effort.  With two monitors it is 
almost imposible to get all of the above calibration points the same on both 
devices.  For example on LCDs you can not adjust the black level or black 
point but all CRTs allow for adjusting the black level and some allow 
adjusting the black point.  But even with these limtations it is possible to 
get things "close enough" with the video card gamma table taking care of what 
ever can;t be handled via the monitor controls.  But keep in mind that the 
laptop LCD probably has few if any adjustments and this will affect how close 
of a match is possible.

Setups that are known to work:

Nvidia driver one card two monitors (perhaps other drivers as well but I don't 
know this for sure) using xinerama

Setups with two video cards using xinerama

Setups known not to work:

Drivers using MergeFB

Nvidia twinview setups

Problematic setups:

The ATI blob uses a non-standard API for setting video card gamma talbles 
(why?).  This is supported by xcalib but you must build a special version that 
is then specific to these cards.

There are reports that the video card gamma API in XRandR 1.2 has some issues 
with correctly retrieving the gamma tables (it returns all zeros) at least 
with the Intel drivers.    Other XRandR 1.2 capable drivers are currently 
untested as far as I know.

Also none of the currently release software in this area has XRandR 1.2 
support but work is now underway to add this.

The least costly measurement device for this is the X-Rite Huey and those who 
are using it are happy with the device.  If you shop around the net these can 
be had for about $50 plus shipping (you will find a ton of these on ebay).   
The difference between the stock Huey and the Huey Pro is software and you 
will not be able to use the software that is shipped with the device so don't 
spend the extra $ on a Huey Pro. 

The best available software at this point is ArgyllCMS 0.70Beta8.  It will 
take you a while to learn the software and to understand how this all fits 
together but it is possible to get two monitors to be a very close match.   
After the initial calibration effort you will probably want to recalibrate 
perhaps once or twice a month to keep things close to your calibration goals.  
The inital calibration will involve making fairly large adjustments to the 
devices and will take some time to get right.  Recalibration will be much 
quicker since these will amout to fine tuning for the devices drift.   CRTs 
generally have more drift than do LCDs and will need recalibration more often.

I am currently running a Samsung 245BW and a Dell that is based on the Sony 
G520.  One is a LCD and the other is a CRT and with a little effort these are 
now very well matched but overall the CRT (at leat this one) has better color 
characteristics than the LCD (for example the per channel gammas are much 
better matched and the white and black points are very well matched).

To sum up.  With a small investment in hardware and a little effort this can 
be done if you have X11 drivers and an X11 configuration that supports setting 
per device gamma tables.

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