HDR support in Wayland/Weston

Adam Jackson ajax at redhat.com
Wed Mar 6 16:00:44 UTC 2019

On Wed, 2019-03-06 at 15:52 +1100, Graeme Gill wrote:
> 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.

Yeah, I was corrected about this on IRC as well. Apparently DirectColor
really is a thing people use.

> > 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.

Technically I think what happens is the vidmode gamma is applied to
RANDR's "compat" output, which is chosen by some handwavey heuristic.

> > 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.

Sure, but one would not expect to control the display's global
calibration state from an X client in this model, for broadly the same
reasons that RANDR under Xwayland is read-only. The wayland server owns
that state, the Xwayland server is simply a very demanding wayland

- ajax

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