GSoC CM collaboration

[Hal V. Engel]

On Sunday 02 March 2008 11:28:39 Maarten Maathuis wrote:
> On 3/2/08, Kai-Uwe Behrmann <ku.b at gmx.de> wrote:
> > Am 02.03.08, 19:56 +0100 schrieb Maarten Maathuis:
> > > On 3/2/08, Kai-Uwe Behrmann <ku.b at gmx.de> wrote:
> >  > > Am 02.03.08, 19:14 +0100 schrieb Maarten Maathuis:
> >  > >  > What kind of color correction do you have in mind? (besides the
> >  > >  > usual gamma adjustment)
> >  > >
> >  > > Many colour transformations are CLUT based. This is in case of X a
> >  > >  3 dimensional table to interpolate from input to output. Additional
> >  > >  gamma and matix operations can play a role. But for LCD's more and
> >  > > more CLUT's are used, as these devices are sometimes very non linear
> >  > > other than CRT's.
> > >
> > > If all you want is control of the LUT's, then i suggest you look at
> > >
> >  > randrproto. I'm not sure if it's perfect yet, but it should be
> >  > reasonable.
> >
> > You talk about the graphics card gamma tables, while I talk about 3
> >  dimensional tables. Usually the a 3*3*17 sized for monitors and
> >  additional one dimensional curves.
> >  A 3*3*17 cube needs some interpolation routine to get the final
> >  output from an argitrary input. A nearest match would not suffice.
>
> Why 3d tables if i may ask?

Because it is a 3D problem. For output devices like monitors color management 
maps from some absolute color space such as CIELab or CIEXYZ into the devices 
color space in a way that corrects all colors not just those along the 
neutral axis.  The most you can do with the video card LUT is to get the per 
channel gamma to be well behaved and the R=G=B axis to be close to neutral.   
You can not get colors correct for R!=G!=B.   This is a direct result of the 
1D limitation of the video card LUTs.   

An example, where this becomes very apparent is with the newer LED based wide 
gamut monitors.  Even with a well calibrated video card LUT the display 
colors where R!=G!=B, with out full color management, will be much too 
saturated to the point of being garish.  These monitors are starting to 
become fairly common so this will only become more of an issue going forward.

>
> >  The graphics card gamma tables are one dimensional, containing usual
> > 3*256 values.
>
> True.
>
> >  kind regards
> >  Kai-Uwe Behrmann
> >  --
> >  developing for colour management
> >  www.behrmann.name + www.oyranos.org
>
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