HDR support in Wayland/Weston
ppaalanen at gmail.com
Wed Feb 27 12:27:23 UTC 2019
On Fri, 22 Feb 2019 14:18:44 -0700
Chris Murphy <lists at colorremedies.com> wrote:
> On Fri, Feb 22, 2019 at 9:00 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:
> > Hi Chris,
> > that is some interesting background, but I feel like I didn't quite
> > catch the point.
> > If the CRTC color management pipeline (LUT-CTM-LUT + maybe more) is
> > programmed according to the monitor's color profile, where would those
> > "conflicting video card LUTs" arise from?
> A common offender were games. They'd try to access the video card LUT
> directly for effects, but then not reset it back to what it was,
> rather reset it to a hardwired assumption the game makes, e.g. all
> displays have gamma 2.2 so we'll just do that! A secondary offender
> were display calibration programs, user would upgrade or get a
> competing program and now you've got two startup applets that apply
> conflicting LUTs and it may be deterministically wrong or it may be
> subject to a race condition as to which applet applies.
> The later problem has mostly vanished with the advent of an OS API for
> reading the vcgt tag directly from the ICC profile set as the profile
> for a particular display.
> So who owns the pipeline? If it's shared, then anyone can use it and
> not set it back the way they found it. Or alternatively if they're
> going to mess with that pipeline, to have a kind of "reset" API for
> the thing that ought to be mostly responsible for such a thing, e.g.
there is a single, unambiguous answer on Wayland: the compositor owns
the pipeline. Therefore we won't have the kind of problems you describe
These are the very reasons I am against adding any kind of protocol
extension that would allow a client to directly touch the pipeline or
to bypass the compositor.
If we had to support such old games that insist on playing with the
video LUT (assuming that there is just one LUT, no CTM, etc.), we could
have a Wayland extension that allows attaching the LUT to a wl_surface,
and then the compositor would apply the LUT any way it wants but only
to that one window while at the same time all the other windows would
remain looking good regardless.
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