HDR support in Wayland/Weston

Graeme Gill graeme2 at argyllcms.com
Wed Mar 6 04:52:34 UTC 2019

Adam Jackson wrote:


> X kinda has three mechanisms for this. The first one, that nobody
> really uses, is setting the colormap for a DirectColor visual.

Actually this is something I check and set to linear before
calibration & profiling in the ArgyllCMS tools.

> The
> second, which games typically use, is setting per-channel gamma
> (implicitly for the whole screen) as single floating-point values with
> the xf86vidmode extension.

Typically this is too crude a control for color management use.
My assumption (which could be wrong) is that this is overridden
by the per-crtc LUT.

> The third, which desktop environments
> sometimes use to try to make distinct displays look similar, is setting
> per-crtc gamma as 256 (or whatever) stops per channel with the RANDR
> extension.

This is the avenue for implementing the Apple/ICC 'vcgt' calibration tag
under X11 (and analogous to the API's in other operating systems).

> All of these are effectively the program specifying its transformation
> to what it hopes is linear in device space. The sample server happens
> to implement all three as global state, but that's an implementation
> detail. It would be straightforward to give each Xwayland client the
> illusion of complete control if we wanted.

For the purposes of setting the display global color calibration state,
then this is not desirable.

	Graeme Gill.

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