Setting the Xorg gamma ramps

[Kai-Uwe Behrmann]

Am 12.03.2013 17:44, schrieb Richard Hughes:
> wondering how we should be doing this using wayland / weston. I
> appreciate we'll be doing the advanced sRGB->native gamut mapping as
> some kind of sub-surface, but this device calibration state is
> orthogonal to that. Most existing display ICC profiles encode a

*Additional* instead of *orthogonal* would be more precise, as the 
advanced calibration gamma curve adds to the ICC profile's internal 
gamma curve. Additional means in this context, the Xorg calibration 
curve reduces tonal range in a 8-bit per channel pixel pipeline, as it 
operates entirely in 8-bit precission. While a ICC colour transform can 
be applied to 16-bit data and then sent as 8-bit using all available 
bits and avoid banding artefacts.

> calibration RGB gamma ramp in them (the 'vcgt' tag) and the profile is
> not valid unless the device matches the calibration state.

Sad, but true. Better are ICC profiles, which omit the vcgt tag 
alltogether. That is much more straight forward IMO.

kind regards
Kai-Uwe