Re: Does colour manager automatically modify the display

[Aravindh M <aravindh 2138 gmail com>]

Hi,

Thanks for the warm welcome.

Answer to your last question :-
I am doing a project at college in which I am running an experiment. In the experiment I am showing squares move horizontally, one by one, to different people (subjects).
These squares take one of four colours (159, 0, 0), (147, 147, 0), (0, 0, 255), (0, 129, 129). In the experiment I want the squares to always have the same brightness value but different chromatic coordinates. I came across the BCH colour model (Brightness, chroma and hue) which claimed to a better brightness model than luma, luminosity and HSV. I
started with colours (255, 0, 0), (255, 255, 0), (0, 0, 255), (0, 255, 255) and modified the colour codes so that based on the BCH model the
colours have the same B value. This is how I ended up with these colours.(159, 0, 0), (147, 147, 0), (0, 0, 255), (0, 129, 129) The issue with this was that the conversion from RGB to XYZ tristimulus values (the first step in conversion from RGB to BCH) requires me to do monitor calibration etc. This would be too hard and so I just stuck to these values (I got them using typical conversion matrix for wide gamut RGB based devices). Ultimately the perceived brightness varies subject to subject and no colour model can let me claim that the "perceived brightness" is same for all subjects and for all colours. Hence, I ignored this factor and am happy with the approximation I got based on "Wide gamut RGB" (Don't know what that means. But got the conversion matrix from http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html).



The query I had posted was just to make sure that the colour code sent to the monitor was the one I have specified and that the OS and 
related software were not manipulating it in any way.

I have not assigned any colour profile in the gnome colour manager. So
based on your reply I get that, the colour codes are not being transformed in any way.

Answered some of your questions here
   * If the display is color managed and "corrected"
I have not assigned any colour profile. But, in the defaults tab the checkbox "Apply display correction" is checked. This was the one that got me confused. I thought that it might manipulate the colours by default in an attempt to make some correction.



   * If the display is old, and the backlight is yellowing and dimmer
No, display is new.
   * The ambient light in the room
My light source is equidistant from all displays. All displays are identical (within acceptable error).
   * The operating system and the drivers you are using
Fedora 13. Not installed any custom drivers. Running on default drivers. No graphics card on system. It uses Intel® Graphics Media
Accelerator 3100. Intel Chipset G31. Desktop effects are set to "Standard" and not "Compiz".
   * If the application is "color managed"
Its a C code that I have written. It uses freeglut library. I did not do anything special to have colour management for the "a.out".

I have attached the screenshot for the "Default" tab just to make things non ambiguous.


Thanking you,
Best Regards,
M. Aravindh

On Mon, Sep 27, 2010 at 2:26 PM, Richard Hughes <hughsient gmail com> wrote:

On 25 September 2010 14:15, Aravindh M <aravindh 2138 gmail com> wrote:
> This is my first post to a linux/gnome related mailing list.

Welcome!

> I am new to the concept of colour management and colour models and still do
> not understand it. I tried reading about CIE1931 and tongue diagram but
> could not understand most of the wiki page. I am a computer science student
> and this is my first encounter with colour management.

Right, it's a pretty daunting subject for something as simple as "color" :-)

> I have written openGL code (using freeglut library) to display coloured
> squares with RGB values (159, 0, 0), (147, 147, 0), (0, 0, 255), (0, 129,
> 129)  {Using glColor function}. I have set colour manager to default
> settings.

Right. It really depends on lots of things what "color" you see:

* If the display is color managed and "corrected"
* If the display is old, and the backlight is yellowing and dimmer
* The ambient light in the room
* The operating system and the drivers you are using
* If the application is "color managed"

> Does this make the actual displayed colour independent of the monitor? Is
> the gnome colour manager influencing the actual displayed colour? If yes,
> what transformation does it do in default settings.

By default, gnome-color-manager does nothing. If you assign it a
profile, it loads some data into X to change the 2D tables of color
mapping (so called "gamma tables") -- this makes your display look
'different'.

If you assign a profile, GCM sets some settings that make color
managed applications take notice of the new profile, and the colors
that are sent to the monitor are changed accordingly.

> I do not plan to print or do anything else with the colours. I just want to
> render them.

What do you want to achieve? Perfect rendition of a specific color, or
just knowing what's going to affect the colors in use on random
computers? We're a pretty friendly group of people, so don't be afraid
to ask.

Richard.

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