HDR support in Wayland/Weston

Chris Murphy lists at colorremedies.com
Fri Feb 22 21:18:44 UTC 2019

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.

Chris Murphy

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