GSoC CM collaboration

[Maarten Maathuis]

On 3/2/08, Hal V. Engel <hvengel at astound.net> wrote:
> 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.

You're essentially assuming that the individual color channels are not
linearly independent?

>  >
>  > >  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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