[argyllcms] Re: dispwin vs xcalib loading LUT

  • From: Stefan Döhla <doe@xxxxxxxxx>
  • To: Gerhard Fuernkranz <argyllcms@xxxxxxxxxxxxx>
  • Date: Tue, 2 Jan 2007 12:01:56 +0100


> Stefan Döhla wrote:
>>>> Therefore I took the system gamma into account, which is 2.2 on Win32
>>>> and 2.222 on Unixes
>>> Hi Stefan, why 2.222 on Unix?
>>> According to which standard/specification?
>> I assumed this when reading Charles Poynton's GammaFAQ and some other
>> UNIX resources some time ago: As gamma usually 1/0.45

> IMO this magic number "0.45" is rather meant to be  0.454545... (i.e.
> 1.0 / 2.2), but written with only two decimal digits precision.

In Charles Poynton's FAQ it's 0.45 and 2.2222222... respectively.

> (But anyway, the difference is not very significant)

And therefore it doesn't matter at all.

> I'm wondering, which representative vcgt loaders for Unix do actually
> exist (besides xcalib and dispwin), which could be used for comparison?

None. Both dispwin and xcalib work under Win32 as well which can be
used to crosscheck.

> Is there actually a difference between vcgt loaders on Mac and Windows
> (i.e. is the gamma value of the VideoCardGammaFormula treated relative
> to a "system gamma" of 2.2 on Windows, but relative to 1.8 on Mac)?

Afaik, OS X uses 2.2 as well ... But as Graeme mentioned, on the Mac
it doesn't matter since there is no "system gamma" used - neither in
the spec nor in MacOS.

> Btw, do you (or somebody else) happen to know how the the "Gamma"
> parameter in the "Monitor" section of xorg.conf is related to this
> stuff? Does this parameter just set the initial graphic card LUTS (i.e.
> the state immediately after the start of the X server), which can be
> later overwritten by xcalib or dispwin, or is this parameter applied _in
> addition_ to the LUTS which are loaded with xcalib or dispwin?

xgamma (and the Monitor section in the config files) uses the same
LUTs that dispwin and xcalib use. So one app overwrites the settings
of the other app. (btw.: xcalib is a heavily modified version of

That's one of the things that are always asked:
Q: When two display loaders are used, are the changes adding up?
A: No, the only the changes of the last used display loader are in
  effect (not true anymore if the display itself is calibrateable -
  but they are still very rare).

xgamma -g 1.0 means "linear" as dispwin behaves (so without this extra
gamma that I use in xcalib currently).



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